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[[File:Solar System true color (captions).jpg|400px|thumb|[[Planet]]s and [[dwarf planet]]s of the Solar System. Compared with each other, the sizes are correct, but the distances are not]]

The '''Solar System''' is a group of space objects that are held together by [[gravity]], with the [[Sun]] in the center. The Sun is a huge ball of hot glowing [[gas]] that gives off [[light]] and [[heat]]. Everything else in the Solar System moves around the Sun. The Solar System formed about 4.6 billion years ago when a giant cloud of [[gas]] and [[dust]] collapsed and started to spin.  Scientists call this process the [[solar nebula theory]]. As the cloud collapsed, gravity caused it to spin faster and flatten into a disk. The Sun formed at the center, while the remaining material in the disk slowly stuck together to form planets through a process called accretion.

Most of the material went into forming the Sun, and the rest became planets and other objects. There are eight main [[planet]]s, including [[Earth]], and many moons that orbit them. There are also [[dwarf planet]]s like [[Pluto]], as well as [[asteroid]]s, [[comet]]s, [[meteoroid]]s, and tiny particles called interplanetary dust. Our Solar System is part of a [[galaxy]] called the Milky Way. It lies in a part of the galaxy called the [[Orion Arm]], about 27,000 light-years from the center of the [[Milky Way]].<ref name="hkt2004">
{{Cite book|last1=Hansen|first1=C.J.|last2=Kawaler|first2=S.A.|last3=Trimble|first3=V.|title=Stellar interiors: physical principles, structure, and evolution|pages=19–20|edition=2nd|publisher=Springer|date=2004|isbn=978-0-387-20089-7}}</ref><ref name="Connelly2012">{{cite journal|title=The absolute chronology and thermal processing of solids in the Solar protoplanetary disk|url=https://archive.org/details/sim_science_2012-11-02_338_6107/page/650|journal=[[Science (journal)|Science]]|first1=James N.|last1=Connelly|display-authors=etal|volume=338|issue=6107|pages=651–655|date=2012|doi=10.1126/science.1226919|bibcode=2012Sci...338..651C|pmid=23118187|s2cid=21965292|issn=0036-8075}}</ref><ref name=":15">{{Cite web|title=The Solar System: our celestial neighbourhood|url=https://www.australianenvironmentaleducation.com.au/education-resources/the-solar-system/|access-date=2025-07-03|website=Australian Environmental Education|language=en-AU}}</ref><ref>{{Cite journal|last1=Gillessen|first1=S.|last2=Plewa|first2=P. M.|last3=Eisenhauer|first3=F.|last4=Sari|first4=R.|last5=Waisberg|first5=I.|last6=Habibi|first6=M.|last7=Pfuhl|first7=O.|last8=George|first8=E.|last9=Dexter|first9=J.|last10=Fellenberg|first10=S. von|last11=Ott|first11=T.|date=2017-03-01|title=An Update on Monitoring Stellar Orbits in the Galactic Center|journal=The Astrophysical Journal|volume=837|issue=1|pages=30|doi=10.3847/1538-4357/aa5c41|doi-access=free|issn=0004-637X}}</ref>

There are eight planets in our Solar System. Starting from the closest to the Sun, they are: [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], [[Mars]], [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]]. Scientists often group them into two main types. [[Terrestrial planet]]s (Mercury, Venus, Earth, and Mars) are small and made mostly of [[rock]] and [[metal]].<ref name=":1">{{Cite web|date=2020-10-22|title=Terrestrial - NASA Science|url=https://science.nasa.gov/exoplanets/terrestrial/|access-date=2025-07-03|language=en-US}}</ref> Giant planets include [[gas giant]]s (Jupiter and Saturn), made mostly of [[hydrogen]] and [[helium]],<ref name=":2">{{Cite web|date=2020-10-22|title=Gas Giant - NASA Science|url=https://science.nasa.gov/exoplanets/gas-giant/|access-date=2025-07-03|language=en-US}}</ref> and [[ice giant]]s (Uranus and Neptune), which have more icy materials like [[water]], [[methane]], and [[ammonia]].<ref name=":3">{{Cite web|title=The Realm of the Ice Giants|url=https://www.planetary.org/articles/simon-the-realm-of-the-ice-giants|access-date=2025-07-03|website=The Planetary Society|language=en}}</ref><ref>{{Cite web|date=2024-07-21|title=Ice Giant Resources - NASA Science|url=https://science.nasa.gov/solar-system/resources/resource-packages/ice-giant-resources/|access-date=2025-07-03|language=en-US}}</ref> Each planet moves around the Sun in an oval-shaped path called an [[ellipse]].<ref>{{Cite web|title=Ellipses and Elliptic Orbits|url=http://hyperphysics.phy-astr.gsu.edu/hbase/Math/ellipse.html|access-date=2025-07-03|website=hyperphysics.phy-astr.gsu.edu}}</ref><ref name=":4">{{Cite web|date=2015-11-07|title=1.4: Elliptic Orbits - Paths to the Planets|url=https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Supplemental_Modules_(Astronomy_and_Cosmology)/Astronomy/Gravity/1.4%3A_Elliptic_Orbits_-_Paths_to_the_Planets|access-date=2025-07-03|website=Physics LibreTexts|language=en}}</ref> These paths are all mostly in the same flat area of space, called the [[Ecliptic|ecliptic plane]].<ref name=":5">{{Cite web|title=What Is the Plane of the Ecliptic? - NASA|url=https://www.nasa.gov/image-article/plane-of-ecliptic/|access-date=2025-07-03|language=en-US}}</ref>  The ecliptic plane is the flat, disk-shaped region formed by the rotation of the original cloud of gas and dust. Because the planets formed from this rotating disk, they [[orbit]] the Sun in nearly the same plane, with only slight variations in their tilt.

Many planets have [[Natural satellite|moon]]s, which are natural objects that orbit them.<ref name=":16">{{Cite web|date=2023-05-31|title=Moons|url=https://science.nasa.gov/solar-system/moons/|access-date=2025-07-03|language=en-US}}</ref> Some planets, like Saturn and Jupiter, also have rings made of ice and dust particles that circle around them.<ref>{{Cite web|date=2016-10-07|title=12.5: Planetary Rings|url=https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Astronomy_1e_(OpenStax)/12%3A_Rings_Moons_and_Pluto/12.05%3A_Planetary_Rings|access-date=2025-07-03|website=Physics LibreTexts|language=en}}</ref>

Far beyond the planet Neptune is a region called the [[Kuiper Belt]]. This area is filled with icy objects, including small worlds called dwarf planets, like [[Pluto]], [[Haumea]], and [[Makemake]]. These objects are made mostly of [[rock]] and [[ice]] and orbit the Sun just like the planets, but they are much smaller.<ref>{{Cite web|date=2017-11-14|title=Kuiper Belt: Facts - NASA Science|url=https://science.nasa.gov/solar-system/kuiper-belt/facts/|access-date=2025-07-03|language=en-US}}</ref> Even farther out is a zone called the scattered disk, and beyond that is a mysterious, faraway area called the [[Oort cloud|Oort Cloud]]. Scientists think the Oort Cloud is a huge, invisible shell of icy objects surrounding the Solar System. It may be where long-period [[comet]]s come from, those that take hundreds or thousands of years to travel around the Sun. These distant regions are at the edge of the Sun’s power.<ref name=":6">{{Cite web|date=2017-11-14|title=Oort Cloud: Facts - NASA Science|url=https://science.nasa.gov/solar-system/oort-cloud/facts/|access-date=2025-07-03|language=en-US}}</ref> The Sun’s [[gravity]] and [[solar wind]] stretch out to form a giant bubble called the [[heliosphere]]. This marks the boundary of the Sun’s influence, even farther than where the planets orbit.<ref name=":7">{{Cite web|title=Components of the Heliosphere - NASA|url=https://www.nasa.gov/image-article/components-of-heliosphere/|access-date=2025-07-03|language=en-US}}</ref>

The Solar System has more than just planets and moons. It also has thousands of space rocks called [[asteroid]]s.<ref>{{Cite web|date=2017-10-26|title=Asteroids|url=https://science.nasa.gov/solar-system/asteroids/|access-date=2025-07-03|language=en-US}}</ref> Most of these are found in a big area between the planets Mars and Jupiter. This area is called the [[asteroid belt]].<ref>{{Cite web|date=2017-11-14|title=Asteroid Facts|url=https://science.nasa.gov/solar-system/asteroids/facts/|access-date=2025-07-03|language=en-US}}</ref> Some asteroids, called [[Trojan asteroid]]s, share their paths around the Sun with planets.<ref>{{Cite web|title=What are the Trojan Asteroids? We Asked a NASA Scientist: Episode 8 - NASA|url=https://www.nasa.gov/solar-system/what-are-the-trojan-asteroids-we-asked-a-nasa-scientist-episode-8/|access-date=2025-07-03|language=en-US}}</ref> There are also [[Comet|comets]]. Comets are made mostly of ice and dust. They move in long, stretched-out paths around the Sun. When a comet gets close to the Sun, the heat makes it grow a glowing tail that points away from the Sun.<ref name=":8">{{Cite web|date=2017-11-14|title=Comet Facts|url=https://science.nasa.gov/solar-system/comets/facts/|access-date=2025-07-03|language=en-US}}</ref> Smaller pieces of rock and metal, called [[meteoroid]]s, also float around in space. If one of these enters Earth’s atmosphere, it burns up and creates a bright streak in the sky. That’s what we call a [[meteor]] or a shooting star. If part of it survives the trip and lands on the ground, it is called a [[meteorite]].<ref name=":9">{{Cite web|date=2017-11-30|title=Meteors and Meteorites: Facts - NASA Science|url=https://science.nasa.gov/solar-system/meteors-meteorites/facts/|access-date=2025-07-03|language=en-US}}</ref>

The Sun is by far the biggest and most powerful object in the Solar System. It makes up more than 99.8% of all the mass, which means almost all the [[weight]] in the Solar System comes from the Sun. The Sun is a type of [[star]] that produces [[energy]] in its center through a process called [[nuclear fusion]]. This means it turns a gas called [[hydrogen]] into another gas called [[helium]]. When it does this, it releases a huge amount of energy. This energy spreads out in all directions as [[light]], [[heat]], and other types of [[radiation]].<ref name=":10">{{Cite web|date=2017-11-14|title=Sun: Facts - NASA Science|url=https://science.nasa.gov/sun/facts/|access-date=2025-07-03|language=en-US}}</ref> The Sun also sends out a flow of tiny, charged particles called the solar wind. This wind can hit the [[magnetic field]]s and atmospheres of planets like [[Earth]]. When this happens, it can cause things like the auroras, the colorful lights in the sky near the [[North Pole|North]] and [[South Pole]]s. The solar wind also affects space weather, which can sometimes interfere with [[satellite]]s and power systems on Earth.<ref name=":11">{{Cite web|date=2024-11-19|title=What Is the Solar Wind? - NASA Science|url=https://science.nasa.gov/sun/what-is-the-solar-wind/|access-date=2025-07-03|language=en-US}}</ref>

Scientists believe the Solar System formed from a big spinning cloud of gas and dust. This idea is called the [[nebular hypothesis]]. Over time, the cloud began to collapse and spin faster, forming a flat disk. Most of the material gathered in the center to form the Sun, while the rest started forming planets. Closer to the Sun, it was very hot, so only rocky materials could stick together. That’s why the inner planets like [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]] are made of rock. Farther out, it was much colder, so gases and ices could join together to form the giant planets like [[Jupiter]] and [[Saturn]]. Over billions of years, the planets moved around, crashed into other objects, and were affected by gravity. These changes helped shape the Solar System into the way we see it today.<ref>{{Cite web|date=2019-11-04|title=8.2: Origin of the Solar System—The Nebular Hypothesis|url=https://geo.libretexts.org/Bookshelves/Geology/Book%3A_An_Introduction_to_Geology_(Johnson_Affolter_Inkenbrandt_and_Mosher)/08%3A_Earth_History/8.02%3A_Origin_of_the_Solar_SystemThe_Nebular_Hypothesis|access-date=2025-07-03|website=Geosciences LibreTexts|language=en}}</ref><ref>{{Cite web|title=How Our Solar System Formed|url=https://www.khanacademy.org/humanities/big-history-project/solar-system-and-earth/earth-and-form-solar-system/a/how-our-solar-system-formed|access-date=2025-07-03|website=www.khanacademy.org|language=en}}</ref>

People have been studying the Solar System for thousands of years. Ancient civilizations watched the [[sky]] and noticed how the planets and stars moved. They used this knowledge to make [[calendar]]s and guide their lives.<ref>{{Cite web|last=wpadmin|date=2018-11-26|title=7 Ancient Cultures and How They Shaped Astronomy|url=https://astro4dev.org/7-ancient-cultures-and-how-they-shaped-astronomy-guest-blog/|access-date=2025-07-03|website=IAU Office of Astronomy for Development|language=en-US}}</ref> A big change came in the 1500s, when a man named [[Nicolaus Copernicus]] said that the Sun, not the Earth, was at the center of the Solar System. This idea is called the [[Heliocentrism|heliocentric model]], and it replaced the older belief that everything moved around the Earth. It was a major turning point in how people understood space.<ref>Draper J.W. 1874. ''History of the conflict between religion and science''. New York: D. Appleton 1874, p172–173. ISBN 978-1-59102-533-7</ref><ref>{{Cite web|title=Heliocentrism {{!}} Definition, History, & Facts {{!}} Britannica|url=https://www.britannica.com/science/heliocentrism|access-date=2025-07-03|website=www.britannica.com|language=en}}</ref> Later, in the 1600s, [[Isaac Newton]] came up with the laws of [[motion]] and [[gravity]]. These laws helped explain how the planets move and why they stay in orbit around the Sun.<ref>{{Cite web|date=2025-06-20|title=Newton's law of gravitation {{!}} Definition, Formula, & Facts {{!}} Britannica|url=https://www.britannica.com/science/Newtons-law-of-gravitation|access-date=2025-07-03|website=www.britannica.com|language=en}}</ref> In more recent times, the 1900s and 2000s, we have sent robotic spacecraft to explore the Solar System. Missions like [[Voyager program|Voyager]], [[Cassini–Huygens|Cassini]], [[Juno (spacecraft)|Juno]], and the [[Mars rover]]s have taught us a lot about the planets, moons, and even the chance of finding life somewhere else in space.<ref>{{Cite web|date=2024-04-09|title=Mission Overview - NASA Science|url=https://science.nasa.gov/mission/voyager/mission-overview/|access-date=2025-07-03|language=en-US}}</ref><ref>{{Cite web|date=2018-08-21|title=Cassini: About the Mission - NASA Science|url=https://science.nasa.gov/mission/cassini/about-the-mission/|access-date=2025-07-03|language=en-US}}</ref><ref>{{Cite web|date=2023-06-15|title=Juno - NASA Science|url=https://science.nasa.gov/mission/juno/|access-date=2025-07-03|language=en-US}}</ref><ref>{{Cite web|date=2024-03-22|title=Mars Exploration Rovers: Spirit and Opportunity - NASA Science|url=https://science.nasa.gov/mission/mars-exploration-rovers-spirit-and-opportunity/|access-date=2025-07-03|language=en-US}}</ref>
== Formation and Evolution of the Solar System ==
{{main|Formation and evolution of the Solar System}}
== Formation of the Sun and Protoplanetary Disk ==
[[File:Soot-line1.jpg|thumb|upright=1.35|Diagram of the early Solar System's [[protoplanetary disk]], out of which Earth and other Solar System bodies formed]]
The formation and [[evolution|evolution]] of the Solar System began 4.6 [[1,000,000,000|billion]] years ago with the [[gravitation]]al collapse of a small part of a giant [[molecular cloud]].<ref>{{citation|first1=Audrey|last1=Bouvier|first2=Meenakshi|last2=Wadhwa|date=2010|title=The age of the Solar System redefined by the oldest Pb–Pb age of a meteoritic inclusion. |doi=10.1038/ngeo941|journal=Nature Geoscience|volume=3|issue=9|pages=637–641|bibcode=2010NatGe...3..637B}}<br>Date based on oldest inclusions found to date in [[meteorite]]s, thought to be among the first solid material to form in the collapsing solar nebula.</ref> The [[solar nebula]] was the cloud of [[gas]] and [[dust]] that eventually formed the [[Sun]] and all the [[planet]]s. This cloud did not just appear out of nowhere. It came from the [[interstellar medium]], which is the thin, spread-out stuff that fills the space between [[star]]s. The part of space that formed our [[solar system]] was probably in a giant [[molecular cloud]].<ref>{{Cite web|title=Solar Nebula|url=https://www.lpi.usra.edu/education/timeline/gallery/slide_1.html|access-date=2025-07-04|website=www.lpi.usra.edu}}</ref> These are cold, thick areas in space where new stars are often born. That’s why scientists sometimes call them stellar nurseries. These clouds were made mostly of [[hydrogen]] and [[helium]], the two lightest [[Chemical element|element]]s. But they also had tiny amounts of heavier elements like [[carbon]], [[oxygen]], [[silicon]], and [[iron]].<ref>{{Citation|last=Irvine|first=William M.|title=The Molecular Composition of Dense Interstellar Clouds|date=1991|url=https://doi.org/10.1007/978-94-009-0695-2_4|work=Chemistry in Space|pages=89–121|editor-last=Greenberg|editor-first=J. M.|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-009-0695-2_4|isbn=978-94-009-0695-2|access-date=2025-07-04|editor2-last=Pirronello|editor2-first=V.}}</ref> These heavier elements were made by older stars that had exploded in huge blasts called [[supernova]]s. When these stars exploded, they sent these elements out into space, mixing them into the cloud.<ref>{{Cite web|title=Origin of the Chemical Elements - T. Rauscher & A. Patkos|url=https://ned.ipac.caltech.edu/level5/Sept16/Rauscher/Rauscher4.html|access-date=2025-07-04|website=ned.ipac.caltech.edu}}</ref><ref>{{Cite web|date=2023-12-07|title=Extreme conditions in stars produce the universe's heaviest elements|url=https://news.umich.edu/extreme-conditions-in-stars-produce-the-universes-heaviest-elements/|access-date=2025-07-04|website=University of Michigan News|language=en-US}}</ref> Because of these ingredients, our part of the cloud had everything needed to form [[Terrestrial planet|rocky planet]]s like [[Earth]] and even the building blocks of life.<ref>{{Cite web|title=Origin of the Universe and the Earth|url=https://www.columbia.edu/~vjd1/solar_nebula.htm|access-date=2025-07-04|website=www.columbia.edu}}</ref>

The formation of our Solar System began with a triggering event, something that caused part of a giant cloud of gas and dust to start collapsing. Scientists are not completely sure what started it, but there are a couple of good guesses. One idea is that a nearby [[supernova]], a huge [[explosion]] at the end of a [[star]]’s life, sent out a powerful [[Shock wave|shockwave]]. This shockwave may have hit the cloud, pushing and squeezing the gas and dust until it became dense enough for [[gravity]] to take over.<ref>{{Cite journal|last1=Schramm|first1=David N.|last2=Clayton|first2=Robert N.|date=1978|title=Did a Supernova Trigger the Formation of the Solar System?|url=https://www.jstor.org/stable/24955827|journal=Scientific American|volume=239|issue=4|pages=124–139|doi=10.1038/scientificamerican1078-124 |jstor=24955827 |issn=0036-8733}}</ref> Another possibility is that small instabilities inside the cloud caused it to start collapsing on its own. Once gravity became stronger than the pressure trying to push the gas outward, the center of the cloud started to collapse inward. As it collapsed, it began to spin and flatten, forming a thicker, spinning area called the [[Formation and evolution of the Solar System|protosolar nebula]]. This was the early stage of what would eventually become the Sun and planets.<ref>{{Cite web|title=The Origin of the Solar System|url=https://www.pas.rochester.edu/~blackman/ast104/nebular.html|access-date=2025-07-04|website=www.pas.rochester.edu}}</ref>

As the center of the giant cloud of gas and dust collapsed, it started to spin faster. This happened because of a rule in [[physics]] called [[conservation of angular momentum]], kind of like how a figure skater spins faster when they pull their arms in. As the spinning increased, the cloud also flattened into a disk. In the middle of this spinning disk, more and more gas and dust gathered, making the center hotter and denser. As this cloud got smaller, most of the material started gathering in the center, forming a growing ball of gas called a [[protostar]]. This was the very early stage of what would become the Sun.<ref>{{Cite web|title=Sky Tellers - Sun|url=http://www.lpi.usra.edu/education/skytellers/sun/|access-date=2025-07-04|website=Lunar and Planetary Institute (LPI)|language=en}}</ref> As more gas and dust fell into the center, the protostar got hotter and denser. The particles inside began to crash into each other more often, causing the [[pressure]] to rise. When the core reached a temperature of about 10 million [[Kelvin]], a powerful reaction called [[nuclear fusion]] started. During fusion, [[Hydrogen|hydrogen atom]]s joined together to form [[helium]], and this released a huge amount of [[energy]]. At this moment, the Sun officially came to life as a [[main-sequence star]].<ref>{{Citation|title=chapter 47 - BEGINNINGS – SOLAR SYSTEM, SUN, UNIVERSE……LIFE?|date=1966-01-01|url=https://www.sciencedirect.com/science/article/pii/B9780080121536500349|work=Abridged Science for High School Students|pages=47–1|editor-last=Messel|editor-first=H.|publisher=Pergamon|doi=10.1016/b978-0-08-012153-6.50034-9|isbn=978-0-08-012153-6|access-date=2025-07-04}}</ref> Once fusion began, the Sun started giving off strong [[light]], [[heat]], and [[solar wind]], a stream of tiny, charged [[particle]]s. This wind helped blow away the extra gas and dust in the inner Solar System. This clearing-out process is called [[photoevaporation]]. It helped stop more material from falling into the Sun and made room for the planets to start forming.<ref>{{Cite journal|last1=Ercolano|first1=Barbara|last2=Picogna|first2=Giovanni|date=2022-12-19|title=Modelling photoevaporation in planet forming discs|url=https://doi.org/10.1140/epjp/s13360-022-03515-8|journal=The European Physical Journal Plus|language=en|volume=137|issue=12|pages=1357|doi=10.1140/epjp/s13360-022-03515-8|issn=2190-5444}}</ref> Even though the Sun was still young, it now had strong gravity and energy that controlled everything around it. The Sun had become the center of the Solar System.<ref>{{Citation|last=Kaula|first=William M.|title=Formation of the Sun and its Planets|date=1986|url=https://doi.org/10.1007/978-94-009-5255-3_1|work=Physics of the Sun: Volume III: Astrophysics and Solar-Terrestrial Relations|pages=1–32|editor-last=Sturrock|editor-first=Peter A.|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-009-5255-3_1|isbn=978-94-009-5255-3|access-date=2025-07-04|editor2-last=Holzer|editor2-first=Thomas E.|editor3-last=Mihalas|editor3-first=Dimitri M.|editor4-last=Ulrich|editor4-first=Roger K.}}</ref>

Surrounding the young Sun was a large, spinning disk of gas and dust called the [[protoplanetary disk]]. This material was left over from the cloud that formed the Sun. The disk was flat like a pancake and rotated around the Sun. This disk was not the same all over, it had different temperatures in different places. The part closest to the Sun was very hot because of the Sun’s strong [[light]] and [[heat]]. In this hot area, only tough materials like [[metal]]s and [[rock]]s could stay solid. These materials came together to form the [[Terrestrial planet|rocky planet]]s like [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]. Farther away from the Sun, the disk was much cooler. In this colder area, things like [[water]], [[ammonia]], and [[methane]] could freeze into ice. These icy and gassy materials helped form the [[Gas giant|gas]] and [[ice giant]]s, planets like [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]]. So, the different temperatures in the disk helped decide what kind of planets would form in different parts of the Solar System.<ref>{{Cite web|title=The Outer Planets: How Planets Form|url=https://lasp.colorado.edu/outerplanets/solsys_planets.php#:~:text=The%20temperature%20across%20this%20protoplanetary,is%20called%20the%20frost%20line.|access-date=2025-07-04|website=lasp.colorado.edu}}</ref> This model, known as the [[nebula]]r [[hypothesis]], was developed in the 18th (1700s) century by [[Emanuel Swedenborg]], [[Immanuel Kant]], and [[Pierre-Simon Laplace]]. It has been adjusted by scientific disciplines such as [[astronomy]], [[physics]], [[geology]], and [[planetary science]]. As our knowledge of space has grown, the models have been changed to account for the new observations.<ref>{{Cite web|date=2025-06-19|title=Solar nebula {{!}} Formation, Accretion, Protoplanetary Disk {{!}} Britannica|url=https://www.britannica.com/science/solar-nebula|access-date=2025-07-04|website=www.britannica.com|language=en}}</ref>

One important part of the protoplanetary disk was something called the [[Frost line (astrophysics)|snow line]] (also known as the ice line). This was an invisible boundary in the disk that marked the point where it was cold enough for water and other [[gas]]es to freeze into ice. Inside the snow line, closer to the Sun, it was too hot, so only [[rock]] and [[metal]] could stick together and form solid objects. That’s why the inner planets, like Earth and Mars, are made mostly of rock. Beyond the snow line, it was much cooler, and there were lots of ices in addition to rock. These ices made it easier for large planet cores to grow quickly. Once these big cores formed, they had strong enough [[gravity]] to pull in lots of gas from the disk around them. This is how the giant planets like [[Jupiter]] and [[Saturn]] were able to form.<ref>{{Cite web|title=Planets and the Snow Line {{!}} Center for Astrophysics {{!}} Harvard & Smithsonian|url=https://www.cfa.harvard.edu/news/planets-and-snow-line|access-date=2025-07-04|website=www.cfa.harvard.edu}}</ref>
== Formation of the Planets ==
[[File:NASA-ExocometsAroundBetaPictoris-ArtistView.jpg|thumb|Formation of plantets in the early solar system]]
[[File:Artist's concept of collision at HD 172555.jpg|thumb|Artist's conception of the [[Giant impact hypothesis|giant impact]] thought to have formed the [[Moon]]]]

The process of forming [[planet]]s began inside the spinning disk of [[gas]] and [[dust]] around the young [[Sun]]. It all started with tiny dust grains made of things like [[rock]], [[metal]], and [[ice]]. These grains were spread all throughout the disk. At first, the dust grains were tiny. But as they floated in the gas, they bumped into each other and started to stick together. This happened because of [[electrostatic force]]s, the same kind of [[static electricity]] that makes a person's [[hair]] stand up. Over time, the grains grew bigger and turned into [[pebble]]s, then clumps, and finally into objects that were about a kilometer wide. These larger bodies are called [[planetesimal]]s, and they were the first real building blocks of planets. As the planetesimals (small space rocks) kept growing, something important started to happen, gravity became a big deal. The larger planetesimals had stronger [[gravity]], which let them pull in more nearby dust and rocks. This made them grow even faster. This fast-growing phase is called [[Accretion (astrophysics)|runaway accretion]]. During runaway accretion, the biggest planetesimals kept getting bigger and bigger, leaving the smaller ones behind. These big ones started to take over certain parts of the disk, becoming the main objects in their areas.<ref>{{Citation|last=Raymond|first=Sean|title=Runaway Growth|date=2011|url=https://link.springer.com/rwe/10.1007/978-3-642-11274-4_1397|encyclopedia=Encyclopedia of Astrobiology|pages=1485–1486|publisher=Springer, Berlin, Heidelberg|language=en|doi=10.1007/978-3-642-11274-4_1397|isbn=978-3-642-11274-4|access-date=2025-07-04}}</ref> After most of the smaller pieces were gone, the growth slowed down. This slower stage is called [[Formation and evolution of the Solar System|oligarchic growth]]. In this phase, several large bodies, called planetary embryos, formed. These were about the size of the [[Moon]] or even [[Mars]]. Each embryo controlled a small area of the disk. Over millions of years, the large space rocks called planetary embryos began to crash into each other, join together, or pull on each other with [[gravity]]. These actions helped shape the final layout of the planets in the Solar System.<ref>{{Cite journal|last1=Dencs|first1=Z.|last2=Regály|first2=Zs|date=2021-01-01|title=Oligarchic growth in a fully interacting system|url=https://www.aanda.org/articles/aa/abs/2021/01/aa39567-20/aa39567-20.html|journal=Astronomy & Astrophysics|language=en|volume=645|pages=A65|doi=10.1051/0004-6361/202039567|arxiv=2012.07683 |issn=0004-6361}}</ref> In the inner part of the Solar System, where there were mostly rocky materials, these giant collisions eventually formed the rocky planets like [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]. In the outer part, beyond the snow line where it was much colder, some embryos grew so big that their gravity could pull in thick layers of gas, mainly hydrogen and helium. These became the huge gas and ice giants like [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]].<ref>{{Cite web|date=2020-09-01|title=How Do Planets Form? - NASA Science|url=https://science.nasa.gov/exoplanets/how-do-planets-form/|access-date=2025-07-04|language=en-US}}</ref><ref>{{Citation|last=Williams|first=I. P.|title=Planetary Formation|date=1974|url=https://doi.org/10.1007/978-94-010-2206-4_1|work=Exploration of the Planetary System|pages=3–12|editor-last=Woszczyk|editor-first=A.|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-010-2206-4_1|isbn=978-94-010-2206-4|access-date=2025-07-04|editor2-last=Iwaniszewska|editor2-first=C.}}</ref>

One of the most important events in the story of how planets formed is the creation of Earth’s [[Moon]]. Scientists believe the Moon was formed by a giant impact. This idea is called the [[giant impact hypothesis]]. According to this theory, about 4.5 billion years ago, a space object about the size of Mars, which we call [[Theia (planet)|Theia]], crashed into the young Earth. The crash was so powerful that it melted parts of both worlds and sent a huge amount of debris into space around Earth. This debris, made mostly of rock from Earth’s outer layers, slowly came together because of [[gravity]]. Over time, the pieces joined to form the Moon. Scientists believe this theory is true because the Moon is made of materials very similar to Earth’s outer layers, especially the [[Mantle (geology)|mantle]]. Also, the Moon does not have many gases or a large [[Planetary core|iron core]], which supports the idea that it formed from Earth’s surface material, not from a separate object that was captured.<ref>{{Cite web|title=How did the Moon form? {{!}} Natural History Museum|url=https://www.nhm.ac.uk/discover/how-did-the-moon-form.html|access-date=2025-07-04|website=www.nhm.ac.uk|language=en}}</ref><ref>{{Citation|last=Genda|first=Hidenori|title=Giant Impact Hypothesis|date=2018|url=https://link.springer.com/rwe/10.1007/978-3-319-39312-4_338|encyclopedia=Encyclopedia of Geochemistry|series=Encyclopedia of Earth Sciences Series |pages=617–620|publisher=Springer, Cham|language=en|doi=10.1007/978-3-319-39312-4_338|isbn=978-3-319-39312-4|access-date=2025-07-04}}</ref>

As young planets were forming, something important called [[planetary differentiation]] began to happen. This is the process where the inside of a [[planet]] becomes layered, with heavier materials sinking and lighter ones rising. As the planets grew bigger, they got hotter inside. This heat came from [[radioactive decay]], big impacts, and the pressure of [[gravity]] squeezing the planet. The heat was so strong that parts of the inside of the planet began to melt. When a planet is melted, the materials inside can separate by weight. Heavy materials like [[iron]] and [[nickel]] sank to the center to form the core. Lighter materials, such as [[Silicate mineral|silicate rock]]s, floated upward to form the mantle and crust.<ref>{{Citation|last1=Sohl|first1=Frank|title=Differentiation, Planetary|date=2014|url=https://link.springer.com/rwe/10.1007/978-3-642-27833-4_430-2|encyclopedia=Encyclopedia of Astrobiology|pages=1–5|publisher=Springer, Berlin, Heidelberg|language=en|doi=10.1007/978-3-642-27833-4_430-2|isbn=978-3-642-27833-4|access-date=2025-07-04|last2=Breuer|first2=Doris}}</ref> Differentiation helped planets develop many important features. One major result was the formation of metallic cores inside planets like Earth. These cores moved and flowed, creating [[magnetic field]]s that protect the planet from harmful [[Outer space|space]] [[radiation]].<ref>{{Cite web|date=2013-12-16|title=How does the Earth's core generate a magnetic field? {{!}} U.S. Geological Survey|url=https://www.usgs.gov/faqs/how-does-earths-core-generate-a-magnetic-field|access-date=2025-07-04|website=www.usgs.gov|language=en}}</ref> At the same time, [[volcano]]es released gases from inside the planet in a process called [[outgassing]]. These gases, along with materials delivered by space rocks, helped form the planet’s [[atmosphere]]. On [[Earth]], this made it possible for life to eventually develop.<ref>{{Cite web|date=2010-04-14|title=The Origin of the Atmosphere|url=https://volcano.oregonstate.edu/origin-atmosphere|access-date=2025-07-04|website=Volcano World|language=en}}</ref> The leftover heat from the planet’s formation, along with moving materials inside the planet, caused ongoing geological activity. This includes things like [[volcano]]es, [[earthquake]]s, and [[Tectonic plate|tectonic plate movement]]. These processes helped shape the planet’s surface and kept the planet active.<ref>{{Cite web|title=How volcanoes form|url=https://www.bgs.ac.uk/discovering-geology/earth-hazards/volcanoes/how-volcanoes-form-2/|access-date=2025-07-04|website=British Geological Survey|language=en-GB}}</ref>

The [[gas giant]]s, like [[Jupiter]] and [[Saturn]], formed in a different way than the smaller rocky planets like Earth. This is because they were born in the colder, outer parts of the Solar System, beyond a boundary called the [[Frost line (astrophysics)|snow line]], where ices could form and last. Scientists have two main ideas about how these big planets formed. The most popular one is called the [[Core accretion (planet formation)|core accretion model]]. According to this idea, the process began with icy space rocks, called [[planetesimal]]s, slowly sticking together to form a large solid core. This core had to be about 10 times the size of Earth to be strong enough. Once the core got that big, its gravity became powerful enough to pull in large amounts of gas, especially [[hydrogen]] and [[helium]], from the surrounding disk. This made the planets grow very quickly and gain their thick, gassy outer layers. But this had to happen fast, within just a few million years, before all the gas in the [[protoplanetary disk]] disappeared. If the gas was gone, the planet could not grow into a gas giant.<ref>{{Cite web|title=Core accretion|url=https://faculty.ucr.edu/~krice/coreacc.html|access-date=2025-07-04|website=faculty.ucr.edu}}</ref><ref>{{Citation|last=Hubickyj|first=Olenka|title=Core accretion—gas capture model for gas giant planet formation|date=2008|url=https://www.cambridge.org/core/books/decade-of-extrasolar-planets-around-normal-stars/core-accretiongas-capture-model-for-gas-giant-planet-formation/2C8FD98F276C0DA52A451F34178EC475|work=A Decade of Extrasolar Planets around Normal Stars: Proceedings of the Space Telescope Science Institute Symposium, held in Baltimore, Maryland May 2–5, 2005|pages=138–152|editor-last=Valenti|editor-first=Jeff|series=Space Telescope Science Institute Symposium Series|place=Cambridge|publisher=Cambridge University Press|isbn=978-0-521-89784-6|access-date=2025-07-04|editor2-last=Sahu|editor2-first=Kailash|editor3-last=Livio|editor3-first=Mario}}</ref> Another idea about how gas giants like Jupiter and Saturn formed is called the [[Disk instability (planet formation)|disk instability model]]. Instead of starting with a solid core, this theory suggests that parts of the gas disk around the Sun became unstable and collapsed under their own gravity. These collapsing clumps of gas could have turned directly into gas giants, without needing a rocky center first. This model could explain how gas giants formed very quickly, which is something the core accretion model struggles with. However, scientists do not have much direct evidence that this happened in our Solar System, so the core accretion model is still the more popular idea.<ref>{{Cite journal|last=Jang-Condell|first=H.|date=2007|title=Disk Instability vs. Core Accretion: Observable Discriminants|url=https://ui.adsabs.harvard.edu/abs/2007lyot.confE..18J/abstract|journal=In the Spirit of Bernard Lyot: The Direct Detection of Planets and Circumstellar Disks in the 21st Century|language=en|pages=18 |bibcode=2007lyot.confE..18J }}</ref>

The [[ice giant]]s, [[Uranus]] and [[Neptune]], formed differently from both the rocky planets and the gas giants. They were born farther from the Sun, in a place where there was less material to build planets. According to the core accretion model, these planets started out by forming solid cores, just like Jupiter and Saturn. But because there was not as much gas around, or because the gas did not last very long, they could not pull in as much [[hydrogen]] and [[helium]]. So, they ended up with only thin layers of gas.<ref>{{Cite journal|last1=Boss |first1=Alan P. |title=Formation of gas and ice giants|url=https://adsabs.harvard.edu/full/2003ESASP.539..143B|access-date=2025-07-04|journal=Earths: Darwin/TPF and the Search for Extrasolar Terrestrial Planets |date=2003 |volume=539 |page=143 |bibcode=2003ESASP.539..143B }}</ref> Instead of having thick, gassy atmospheres, ice giants are made mostly of [[volatiles]], substances like [[water]], [[ammonia]], and [[methane]]. Even though we call them “ice” giants, much of this material is actually in a hot, thick, fluid form deep inside the planets because of the high [[pressure]]. This is why Uranus and Neptune are smaller than Jupiter and Saturn and have a different makeup.<ref>{{Cite web|title=The Realm of the Ice Giants|url=https://www.planetary.org/articles/simon-the-realm-of-the-ice-giants|access-date=2025-07-04|website=The Planetary Society|language=en}}</ref>  At the same time as the planets were forming, the outer edges of the Solar System were creating [[dwarf planet]]s and other icy objects. Far beyond Neptune, there’s a region called the [[Kuiper Belt]].<ref name=":17">{{Cite web|date=2017-11-14|title=Kuiper Belt: Facts - NASA Science|url=https://science.nasa.gov/solar-system/kuiper-belt/facts/|access-date=2025-07-04|language=en-US}}</ref> In this area, leftover pieces from the early Solar System came together to form objects like [[Pluto]], [[Eris (dwarf planet)|Eris]], and [[Haumea (dwarf planet)|Haumea]]. These icy bodies are like frozen [[time capsule]]s. They have not changed much since they first formed, so they give scientists clues about what the outer Solar System was like long ago. Their icy make-up and stable orbits tell us about the cold, quiet conditions far from the Sun. Some of these objects might not have started out in the Kuiper Belt. They may have formed closer to the Sun and were later pushed outward by the strong [[gravity]] of the moving gas giants, like Jupiter. This idea supports theories like the [[Nice model]], which explains how the planets and other objects ended up where they are today.<ref name=":18">{{Cite web|title=Kuiper Belt Objects (KBOs) {{!}} EBSCO Research Starters|url=https://www.ebsco.com/research-starters/astronomy-and-astrophysics/kuiper-belt-objects-kbos|access-date=2025-07-04|website=www.ebsco.com|language=en}}</ref><ref>{{Cite web|date=2018-12-14|title=10 Things to Know About the Kuiper Belt - NASA Science|url=https://science.nasa.gov/solar-system/kuiper-belt/10-things-to-know-about-the-kuiper-belt/|access-date=2025-07-04|language=en-US}}</ref>
== Evolution of the Solar System ==
[[File:Lunar cataclysm.jpg|thumb|upright=1.1|Artist's impression of the [[Moon]] during the Late Heavy Bombardment (above) and today (below)]]
One of the most important steps in the Solar System's history was the clearing of the [[protoplanetary disk]]. This was the spinning cloud of gas and dust around the young Sun where planets were forming. As the [[Sun]] got older and became a full [[main-sequence star]], it started sending out strong [[solar wind]]s, streams of tiny, charged particles. It also gave off intense [[Ultraviolet|ultraviolet radiation]]. Together, these forces began to blow away the leftover gas and dust in the disk. This process is called [[photoevaporation]]. Photoevaporation was especially strong in the inner part of the Solar System, where the Sun's [[heat]] and [[light]] were most powerful.<ref>{{Cite journal|last1=Owen|first1=James E.|last2=Ercolano|first2=Barbara|last3=Clarke|first3=Cathie J.|date=2011-03-21|title=Protoplanetary disc evolution and dispersal: the implications of X-ray photoevaporation: Disc evolution and dispersal|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=412|issue=1|pages=13–25|doi=10.1111/j.1365-2966.2010.17818.x |doi-access=free }}</ref> Once the gas was gone, planets could not grow much more, especially [[gas giant]]s, because the materials they needed had disappeared. Scientists believe this clearing happened within about 10 million years after the Solar System began. We know this by looking at other young stars and their disks in space today. The clearing of the disk marked the end of major planet formation and shaped the Solar System as we know it.<ref>{{Cite journal|last=C. Lin|first=Douglas N.|date=2008|title=The Genesis of Planets|url=https://www.scientificamerican.com/article/the-genesis-of-planets-2008-05|journal=Scientific American|volume=298|issue=5|pages=50–59|doi=10.1038/scientificamerican0508-50|doi-broken-date=12 July 2025|pmid=18444325|issn=0036-8733}}{{Dead link|date=August 2025 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref>{{cite journal | author=B. G. Elmegreen| title=On the disruption of a protoplanetary disc nebula by a T Tauri like solar wind | journal=Astronomy & Astrophysics |year=1979 | volume=80 | issue=1 | pages=77 | bibcode=1979A&A....80...77E }}</ref><ref>{{cite web |date=24 November 2004 |author=Heng Hao |publisher=Harvard University |title=Disc-Protoplanet interactions |url=http://cfa-www.harvard.edu/~kstanek/astro200/disk-protoplanet.pdf |access-date=2006-11-19 |url-status=dead |archive-url=https://web.archive.org/web/20060907170907/http://cfa-www.harvard.edu/~kstanek/astro200/disk-protoplanet.pdf |archive-date=7 September 2006 }}</ref>

After the planets finished forming and started to settle into their [[orbit]]s, the Solar System went through a chaotic time called the [[Late Heavy Bombardment|Late Heavy Bombardment (LHB)]]. This happened about 4.1 to 3.8 billion years ago and was a period when the inner planets, like [[Earth]], [[Moon|the Moon]], [[Mars]], and [[Mercury (planet)|Mercury]], were hit by tons of [[asteroid]]s and [[comet]]s.<ref>{{Cite web|date=2024-12-19|title=What is the Late Heavy Bombardment? - NASA Science|url=https://science.nasa.gov/moon/lunar-craters/what-is-the-late-heavy-bombardment/|access-date=2025-07-05|language=en-US}}</ref> Scientists know this happened because of rock samples from the Moon that were brought back by the [[Apollo mission]]s. Scientists studied the [[Impact crater|crater]]s on the Moon and used special techniques to figure out how old the rocks were. They found that many of the Moon’s biggest craters, like the [[Mare Imbrium|Imbrium]] and [[Mare Orientale|Orientale basin]]s, were all made around the same time. This shows that there was a sudden increase in [[Impact event|space impact]]s, not just a slow decline over time. The same thing probably happened on Earth and other nearby planets. [[Meteorite]]s found on Earth and information from Mars also support the idea that this was a Solar System-wide event with a huge number of collisions happening in a short period.<ref>{{Citation|last1=Claeys|first1=Philippe|title=Late Heavy Bombardment|date=2015|url=https://link.springer.com/rwe/10.1007/978-3-662-44185-5_869|encyclopedia=Encyclopedia of Astrobiology|pages=1365–1369|publisher=Springer, Berlin, Heidelberg|language=en|doi=10.1007/978-3-662-44185-5_869|isbn=978-3-662-44185-5|access-date=2025-07-05|last2=Morbidelli|first2=Alessandro}}</ref><ref name=":0">{{Cite journal|last1=Gomes|first1=R.|last2=Levison|first2=H. F.|last3=Tsiganis|first3=K.|last4=Morbidelli|first4=A.|date=2005|title=Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets|url=https://www.nature.com/articles/nature03676|journal=Nature|language=en|volume=435|issue=7041|pages=466–469|doi=10.1038/nature03676|pmid=15917802 |issn=1476-4687}}</ref>

Scientists are still trying to figure out exactly what caused the Late Heavy Bombardment (LHB), but one popular idea is called the [[Nice model]] (named after a city in France). This model suggests that the [[giant planet]]s, like [[Jupiter]] and [[Saturn]], did not always stay in the same [[orbit]]s. They might have moved. At some point in the early Solar System, Jupiter and Saturn may have crossed a special point in their orbits called a [[Orbital resonance|resonance]]. This happens when their orbits line up in a way that their gravitational pulls reinforce each other. In this case, it created a powerful gravitational effect that caused destabilized the orbits of many icy bodies in the [[Kuiper Belt]] and rocky objects in the asteroid belt. These objects were either flung out of the Solar System or sent crashing inward toward the inner planets, like [[Earth]], [[Mars]], and [[Venus]]. This sudden movement of so many objects led to the Late Heavy Bombardment, when the inner planets were hit by lots of [[asteroid]]s and [[comet]]s.<ref name=":0" /> These impacts helped shape the surfaces of the planets and may have even brought [[water]] and [[Organic compound|organic material]]s to Earth, ingredients that later made [[life]] possible.<ref>{{Cite web|last=bgray|date=2016-12-24|title=Late Heavy Bombardment may not have happened|url=https://geologyglasgow.org.uk/headlines/late-heavy-bombardment-may-not-have-happened/|access-date=2025-07-05|website=Geological Society of Glasgow|language=en-GB}}</ref>

One idea in [[planetary science]] is the [[Grand Tack hypothesis]]. This theory tries to explain some strange things about our Solar System by suggesting that Jupiter did not always stay where it is now. Instead, it says that Jupiter once moved inward, getting much closer to the [[Sun]], about 1.5 times the distance from Earth to the Sun, before turning around and moving back outward to where it is today. This big change in direction, or "tack" (like a [[sailboat]] changing course), may have happened because of [[Saturn]]. After Jupiter formed, Saturn also started forming, and the gravity between the two may have caused Jupiter to change direction.<ref>{{Cite web|title=Jupiter's Youthful Travels Redefined Solar System|url=https://www.nasa.gov/topics/solarsystem/features/young-jupiter.html|access-date=2025-07-05|website=www.nasa.gov|archive-date=2017-03-01|archive-url=https://web.archive.org/web/20170301170530/https://www.nasa.gov/topics/solarsystem/features/young-jupiter.html|url-status=dead}}</ref> As Jupiter moved inward, it would have pushed away or scattered a lot of [[dust]], [[rock]]s, and [[planetesimal]]s. This would have cleared out space in the Solar System and changed how the rocky planets, like Earth and Mars, formed. When Jupiter moved back outward, it may have pulled in a new mix of material, adding both rocky and icy objects into the [[asteroid belt]]. This helps explain why the asteroid belt today has a mix of different types of [[asteroid]]s, some rocky, some icy.<ref>{{Cite web|author1=Charles Q. Choi|date=2015-03-23|title=Jupiter's 'Smashing' Migration May Explain Our Oddball Solar System|url=https://www.space.com/28901-wandering-jupiter-oddball-solar-system.html|access-date=2025-07-05|website=Space|language=en}}</ref><ref>{{Cite web|title=How Did Jupiter Shape Our Solar System?|url=https://www.universetoday.com/articles/how-did-jupiter-shape-our-solar-system|access-date=2025-07-05|website=Universe Today|language=en}}</ref>

One big area scientists study is how the [[orbit]]s of [[planet]]s, [[Natural satellite|moon]]s, [[asteroid]]s, and [[comet]]s change over time. These changes happen slowly and are often caused by [[gravity]] pulling on objects in special ways. One important idea is called a [[Orbital resonance|resonance]]. This happens when two objects go around the Sun in a regular pattern, like one going around twice for every three times the other one goes around. These simple ratios (like 2:1 or 3:2) can keep orbits stable or sometimes make them unstable.<ref>{{Cite web|title=Orbital resonance|url=https://www.aanda.org/glossary/175-orbital-resonance|access-date=2025-07-05|website=www.aanda.org}}</ref><ref>{{Cite journal|last=Christopher|first=Lee|title=The effect of orbital resonance on the stability of a planetary system|url=https://www.the-iyrc.org/uploads/1/2/9/7/129787256/iyrc2021_14_final.pdf|journal=The International Young Researchers' Conference}}</ref> Resonances help explain why the asteroid belt looks the way it does, why certain moons are in just the right spots, and why icy objects beyond Neptune tend to cluster in certain areas.<ref>{{Cite web|title=Planetary orbits: Couplings and resonances {{!}} EBSCO Research Starters|url=https://www.ebsco.com/research-starters/history/planetary-orbits-couplings-and-resonances|access-date=2025-07-05|website=www.ebsco.com|language=en}}</ref><ref>{{Cite journal|last=Malhotra|first=Renu|date=2019-11-09|title=Resonant Kuiper belt objects: a review|journal=Geoscience Letters|volume=6|issue=1|article-number=12|doi=10.1186/s40562-019-0142-2|doi-access=free |issn=2196-4092|pmc=7067275|pmid=32215241}}</ref> Another idea is called [[secular variation]], which means slow, long-term changes in things like a planet’s orbit shape, tilt, or direction. These changes happen because planets gently pull on each other over millions of years.<ref>{{Cite web|title=Secular evolution of planetary orbits|url=https://farside.ph.utexas.edu/teaching/celestial/Celestial/node91.html#:~:text=78)-,for%20.,to%20the%20ecliptic%20plane,%20is|access-date=2025-07-05|website=farside.ph.utexas.edu}}</ref><ref>{{Citation|last=Rimrott|first=Fred P. J.|title=Secular Variations of the Orbital Elements|date=1989|url=https://doi.org/10.1007/978-3-322-90338-9_7|work=Introductory Orbit Dynamics|pages=101–106|editor-last=Rimrott|editor-first=Fred P. J.|place=Wiesbaden|publisher=Vieweg+Teubner Verlag|language=en|doi=10.1007/978-3-322-90338-9_7|isbn=978-3-322-90338-9|access-date=2025-07-05}}</ref> For example, Earth’s orbit and tilt slowly shift because of the gravity from Jupiter and Saturn. These changes affect Earth’s climate over a long time, in something called [[Milankovitch cycles]].<ref>{{Cite web|date=2020-02-27|title=Milankovitch (Orbital) Cycles and Their Role in Earth's Climate - NASA Science|url=https://science.nasa.gov/science-research/earth-science/milankovitch-orbital-cycles-and-their-role-in-earths-climate/|access-date=2025-07-05|language=en-US}}</ref> Sometimes, gravity can even throw objects off course or capture them. This is called [[gravitational scattering]].<ref>{{Cite web|title=Gravitational Dynamics {{!}} Center for Astrophysics {{!}} Harvard & Smithsonian|url=https://www.cfa.harvard.edu/research/topic/gravitational-dynamics|access-date=2025-07-05|website=www.cfa.harvard.edu}}</ref> For example, Neptune’s moon [[Triton (moon)|Triton]] orbits backward, which suggests it did not form around Neptune but was a Kuiper Belt object that got caught by Neptune’s gravity.<ref>{{Cite journal|last1=Farinella|first1=P.|last2=Milani|first2=A.|last3=Nobili|first3=A.M.|last4=Valsecchi|first4=G.B.|date=1980|title=Some remarks on the capture of Triton and the origin of Pluto|url=https://linkinghub.elsevier.com/retrieve/pii/0019103580901487|journal=Icarus|language=en|volume=44|issue=3|pages=810–812|doi=10.1016/0019-1035(80)90148-7}}</ref><ref>{{Cite web|date=2017-11-21|title=Triton - NASA Science|url=https://science.nasa.gov/neptune/moons/triton/|access-date=2025-07-05|language=en-US}}</ref> Mars’s tiny moons, [[Phobos]] and [[Deimos]], may also be captured asteroids because of their strange shapes and materials.<ref>{{Cite web|date=2017-11-11|title=Mars Moons - NASA Science|url=https://science.nasa.gov/mars/moons/|access-date=2025-07-05|language=en-US}}</ref>

Another important force that helps shape the Solar System is tidal interaction, especially between planets and their moons. [[Tidal force]]s happen because of gravity. Planets pull on their moons, and moons pull back on their planets. This tugging creates bulges, which causes [[energy]] to be lost and leads to slow changes in [[orbit]]s and [[rotation]].<ref>{{Cite web|last=Plait|first=Phil|title=Tides Move Heaven and Earth More Than You Know|url=https://www.scientificamerican.com/article/how-do-tides-shape-earth-and-the-solar-system/|access-date=2025-07-05|website=Scientific American|language=en}}</ref><ref>{{Cite web|date=2016-10-18|title=13.7: Tidal Forces|url=https://phys.libretexts.org/Bookshelves/University_Physics/University_Physics_(OpenStax)/Book%3A_University_Physics_I_-_Mechanics_Sound_Oscillations_and_Waves_(OpenStax)/13%3A_Gravitation/13.07%3A_Tidal_Forces|access-date=2025-07-05|website=Physics LibreTexts|language=en}}</ref> One famous example is the [[Earth]] and [[Moon]]. Because of tidal forces, the Moon always shows the same side to Earth. This is called being [[tidally locked]].<ref>{{Cite web|date=2023-06-06|title=Tidal Locking - NASA Science|url=https://science.nasa.gov/moon/tidal-locking/|access-date=2025-07-05|language=en-US}}</ref> Also, Earth’s rotation is slowly slowing down, and the Moon is very slowly moving farther away from us.<ref>{{Cite book|title=Time: from earth rotation to atomic physics|last1=McCarthy|first1=Dennis D.|last2=Seidelmann|first2=P. Kenneth|date=2009|publisher=Wiley-VCH|isbn=978-3-527-62795-0|location=Weinheim}}</ref><ref>{{Cite web|date=2017-11-15|title=Moon Facts - NASA Science|url=https://science.nasa.gov/moon/facts/|access-date=2025-07-05|language=en-US}}</ref> Tidal forces can also create heat inside moons, especially those that orbit big planets like Jupiter and Saturn. For example, [[Europa (moon)|Europa]] (a moon of Jupiter) and [[Enceladus (moon)|Enceladus]] (a moon of Saturn) are squeezed and stretched as they move around their planets.<ref>{{Cite web|title=Europa Tidal Heating|url=http://astronomy.nmsu.edu/candaceg/Europa/Tidal_Heating.html|access-date=2025-07-05|website=astronomy.nmsu.edu}}</ref><ref>{{Cite journal|last1=Meyer|first1=Jennifer|last2=Wisdom|first2=Jack|date=2007|title=Tidal heating in Enceladus|url=https://linkinghub.elsevier.com/retrieve/pii/S0019103507001054|journal=Icarus|language=en|volume=188|issue=2|pages=535–539|doi=10.1016/j.icarus.2007.03.001}}</ref> This squeezing, called [[Tidal heating|tidal flexing]], creates heat inside them.<ref>{{Citation|last=Lunine|first=Jonathan I.|title=Tidal heating|date=1997|url=https://link.springer.com/rwe/10.1007/1-4020-4520-4_408|encyclopedia=Encyclopedia of Planetary Science|series=Encyclopedia of Earth Science |pages=828|publisher=Springer, Dordrecht|language=en|doi=10.1007/1-4020-4520-4_408|isbn=978-1-4020-4520-2|access-date=2025-07-05}}</ref><ref>{{Cite web|title=tidal heating|url=https://evolution.berkeley.edu/glossary/tidal-heating/|access-date=2025-07-05|website=evolution.berkeley.edu}}</ref> That heat might keep liquid [[ocean]]s under their icy surfaces.<ref>{{Cite web|last=Owen|first=Rebecca|date=2024-01-10|title=Scientists Investigate How Heat Rises Through Europa's Ocean|url=https://eos.org/research-spotlights/scientists-investigate-how-heat-rises-through-europas-ocean|access-date=2025-07-05|website=Eos|language=en-US}}</ref> These hidden oceans could have [[geyser]]s or even [[cryovolcanoe]]s (volcanoes that erupt [[ice]] instead of [[lava]]).<ref>{{Cite web|date=2020-08-04|title=Cryovolcanism in the Solar System|url=https://www.skyatnightmagazine.com/space-science/cryovolcanism-in-the-solar-system|access-date=2025-07-05|website=BBC Sky at Night Magazine|language=en}}</ref> Because of this, Europa and Enceladus are interesting places to look for life beyond Earth.<ref>{{Cite web|last=Velasco|first=Rebecca Boyle, Juan|date=2023-05-01|title=The Six Moons Most Likely to Host Life in Our Solar System|url=https://www.scientificamerican.com/article/the-six-moons-most-likely-to-host-life-in-our-solar-system/|access-date=2025-07-05|website=Scientific American|language=en}}</ref>
== The Present-day Conditions of the Solar System ==
[[File:Solar System graphic by NASA (wide).png|thumb|The Solar System]]
At the very center of the Solar System is the [[Sun]], which is a middle-aged [[star]] called a [[G-type main-sequence star]]. The Sun is very massive, it holds more than 99.8% of all the [[mass]] in the Solar System. The Sun gives off [[light]] and [[heat]], which make [[life]] on [[Earth]] possible. It also creates the [[gravity]] that keeps all the [[planet]]s, [[Natural satellite|moon]]s, [[asteroid]]s, and [[comet]]s moving in their [[orbit]]s.<ref name=":10" /> Even though the Sun looks calm in the sky, it is actually very active. It gives off [[radiation]] and sends out a stream of charged particles called the [[solar wind]].<ref name=":11" /> This wind stretches far out into space and helps create the edge of a giant bubble called the [[heliosphere]], which surrounds the entire Solar System.<ref name=":7" />

Surrounding the Sun are the eight major planets, which are split into two main groups. The first group is the terrestrial planets, [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]. These are small, [[Terrestrial planet|rocky planet]]s made mostly of [[rock]] and [[metal]].<ref name=":1" /> The second group includes the [[Gas giant|gas]] and [[ice giant]]s, [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]]. These planets are much bigger and made mostly of gases like [[hydrogen]] and [[helium]], and ices such as [[water]], [[ammonia]], and [[methane]].<ref name=":2" /><ref name=":3" /> All the planets move in oval-shaped orbits (called [[elliptical orbit]]s) around the Sun, and they mostly stay in the same flat area called the [[Ecliptic|ecliptic plane]].<ref name=":4" /><ref name=":5" /> Most of them spin in the same direction as the Sun, but there are a couple of exceptions. Venus spins backward, and Uranus spins on its side.<ref>{{Cite web|last=Franzen|first=Harald|title=Why Venus Spins the Wrong Way|url=https://www.scientificamerican.com/article/why-venus-spins-the-wrong/|access-date=2025-07-05|website=Scientific American|language=en}}</ref><ref>{{Cite web|date=2017-11-10|title=Uranus: Facts - NASA Science|url=https://science.nasa.gov/uranus/facts/|access-date=2025-07-05|language=en-US}}</ref> Besides the planets, the Solar System also has lots of smaller objects. Between Mars and Jupiter is the [[asteroid belt]], which contains millions of space rocks. Some as small as dust, and others as big as the [[dwarf planet]] [[Ceres]].<ref name=":19">{{Cite web|date=2025-05-14|title=Asteroid Belt: what is it and how did it form?|url=https://www.skyatnightmagazine.com/space-science/asteroid-belt-facts-formation|access-date=2025-07-05|website=BBC Sky at Night Magazine|language=en}}</ref> Even farther out is the Kuiper Belt, which is full of icy objects like [[Pluto]], [[Haumea (dwarf planet)|Haumea]], and [[Makemake (dwarf planet)|Makemake]]. These are called [[Trans-Neptunian object|trans-Neptunian objects (TNOs)]], and they are leftovers from the Solar System's early days. They never became [[planet]]s because the gravity from the big planets, like Jupiter, kept pulling things around and disturbing them.<ref>{{Cite web|title=Trans-Neptunian object {{!}} EBSCO Research Starters|url=https://www.ebsco.com/research-starters/astronomy-and-astrophysics/trans-neptunian-object|access-date=2025-07-05|website=www.ebsco.com|language=en}}</ref> Way beyond that, scientists think there is something called the [[Oort cloud|Oort Cloud]]. It is a giant, round shell of icy objects that surrounds the Solar System. Scientists have not seen it directly, but they think it is there because it is likely the home of long-period comets. The ones that take hundreds or thousands of years to orbit the Sun. The Oort Cloud is believed to be the very edge of the Sun’s gravitational reach.<ref name=":6" />

The Solar System is always changing. It is not just made of planets and moons, there are lots of smaller objects moving around too, like [[comet]]s, [[meteoroid]]s, and [[Centaur (planetoid)|centaur]]s. Centaurs are unusual because they have features of both asteroids and comets.<ref name=":12">{{Cite web|last=Metcalfe|first=Tom|date=2021-10-07|title=Our solar system's centaurs are half-asteroid, half-comet|url=https://www.astronomy.com/science/our-solar-systems-centaurs-are-half-asteroid-half-comet/|access-date=2025-07-05|website=Astronomy Magazine|language=en-US}}</ref> Sometimes, comets from faraway places like the Kuiper Belt or the Oort Cloud travel into the inner part of the Solar System. As they get closer to the Sun, the heat causes their ice to melt, creating bright tails of gas and dust. These tails can be really beautiful and are often visible from [[Earth]].<ref name=":8" /> [[Meteoroid]]s are small rocks that sometimes enter Earth’s atmosphere. When they burn up in the sky, we see them as [[meteor]]s or "shooting stars." If a bigger piece makes it all the way to the ground, it is called a [[meteorite]].<ref name=":9" /> While big impacts are rare, scientists still keep an eye on [[Near-Earth object|near-Earth objects (NEOs)]] just in case.<ref>{{Cite web|title=Near-Earth Object Program - NASA|url=https://www.nasa.gov/stem-content/near-earth-object-program/|access-date=2025-07-05|language=en-US}}</ref> Centaurs are icy objects with unstable orbits between Jupiter and Neptune.<ref name=":12" /> They might one day become short-period comets that visit the inner Solar System more often.<ref>{{Citation|last1=Peixinho|first1=Nuno|title=Chapter 14 - From Centaurs to comets: 40 Years|date=2020-01-01|url=https://www.sciencedirect.com/science/article/pii/B978012816490700014X|work=The Trans-Neptunian Solar System|pages=307–329|editor-last=Prialnik|editor-first=Dina|publisher=Elsevier|doi=10.1016/b978-0-12-816490-7.00014-x|isbn=978-0-12-816490-7|access-date=2025-07-05|last2=Thirouin|first2=Audrey|last3=Tegler|first3=Stephen C.|last4=Di Sisto|first4=Romina P.|last5=Delsanti|first5=Audrey|last6=Guilbert-Lepoutre|first6=Aurélie|last7=Bauer|first7=James G.|arxiv=1905.08892 |editor2-last=Barucci|editor2-first=M. Antonietta|editor3-last=Young|editor3-first=Leslie A.}}</ref> In recent years, astronomers even discovered objects from outside our Solar System, like ‘[[Oumuamua]] and Comet [[2I/Borisov]]. These interstellar visitors came from far beyond the Sun’s reach, giving us clues about how other star systems might form.<ref>{{Cite web|date=2017-11-29|title='Oumuamua - NASA Science|url=https://science.nasa.gov/solar-system/comets/oumuamua/|access-date=2025-07-05|language=en-US}}</ref><ref>{{Cite web|date=2020-04-17|title=Comet 2I/Borisov - NASA Science|url=https://science.nasa.gov/solar-system/comets/2i-borisov/|access-date=2025-07-05|language=en-US}}</ref>

There are over 800 known [[Natural satellite|moon]]s in our Solar System, and they come in all shapes and sizes.<ref>{{Cite web|date=2023-05-31|title=Moons|url=https://science.nasa.gov/solar-system/moons/|access-date=2025-07-05|language=en-US}}</ref> Some moons are fiery and active, like [[Io (moon)|Io]], which has [[volcano]]es constantly erupting.<ref>{{Cite web|date=2017-11-17|title=Io|url=https://science.nasa.gov/jupiter/jupiter-moons/io/|access-date=2025-07-05|language=en-US}}</ref> Others, like [[Europa (moon)|Europa]] and [[Enceladus (moon)|Enceladus]], are covered in ice but may have liquid [[ocean]]s hidden underneath.<ref>{{Cite web|last=US Department of Commerce|first=National Oceanic and Atmospheric Administration|title=Are there oceans on other planets?|url=https://oceanservice.noaa.gov/facts/et-oceans.html|access-date=2025-07-05|website=oceanservice.noaa.gov|language=EN-US}}</ref> These icy moons are interesting because they might even have the right conditions for [[life]].<ref>{{Cite web|last=Barbuzano|first=Javier|date=2023-08-14|title=Subsurface Oceans Could Boost Exoplanet Habitability|url=https://eos.org/articles/subsurface-oceans-could-boost-exoplanet-habitability|access-date=2025-07-05|website=Eos|language=en-US}}</ref> Some planets, like Saturn and Neptune, have lots of moons, creating huge moon systems that scientists are still exploring and learning about.<ref>{{Cite web|date=2025-03-17|title=Which planet has the most moons?|url=https://www.skyatnightmagazine.com/space-science/which-planet-has-most-moons|access-date=2025-07-05|website=BBC Sky at Night Magazine|language=en}}</ref> Besides natural moons, the Solar System also has many [[spacecraft]] and [[Satellite (artificial)|satellite]]s that humans have sent into space to learn more about our Solar System. Some of these orbit Earth, while others go much farther.<ref>{{Cite web|date=2025-06-18|title=Space exploration - Solar System, Probes, Missions {{!}} Britannica|url=https://www.britannica.com/science/space-exploration/Solar-system-exploration|access-date=2025-07-05|website=www.britannica.com|language=en}}</ref> For example, [[Voyager 1]] has traveled so far that it has entered interstellar space, beyond the edge of the Solar System.<ref>{{Cite web|date=2024-03-10|title=Where Are Voyager 1 and 2 Now? - NASA Science|url=https://science.nasa.gov/mission/voyager/where-are-voyager-1-and-voyager-2-now/|access-date=2025-07-05|language=en-US}}</ref> Other missions like [[Juno (spacecraft)|Juno]] (orbiting Jupiter), [[Perseverance (rover)|Perseverance]] (exploring Mars), the [[James Webb Space Telescope]], and [[New Horizons]] (which flew past Pluto) are helping us study planets, moons, and stars in detail.<ref>{{Cite web|date=2023-06-15|title=Juno - NASA Science|url=https://science.nasa.gov/mission/juno/|access-date=2025-07-05|language=en-US}}</ref><ref>{{Cite web|date=2017-12-05|title=Mars 2020: Perseverance Rover - NASA Science|url=https://science.nasa.gov/mission/mars-2020-perseverance/|access-date=2025-07-05|language=en-US}}</ref><ref>{{Cite web|title=Mission Timeline|url=https://webbtelescope.org/home/news/milestones/mission-timeline|access-date=2025-07-05|website=Webb|language=en}}</ref><ref>{{Cite web|date=2017-12-05|title=New Horizons|url=https://science.nasa.gov/mission/new-horizons/|access-date=2025-07-05|language=en-US}}</ref>

Over billions of years, the planets in our Solar System have settled into stable orbits. This means that even though their paths can slowly change, they mostly stay the same over time. These changes happen very slowly and are caused by [[gravity]] pulling on the planets from other planets and the [[Sun]]. The way the planets are arranged now is called the [[Epoch (astronomy)|current epoch]], and it is expected to stay stable for billions more years. The [[orbit]]s might change a little in shape or tilt, but not in a way that causes big problems. This stability is really important because it means we can predict how [[planet]]s move far into the future. It also helps keep [[Earth]] safe and habitable, making [[life]] possible for a long time.<ref>{{Citation|last=Laskar|first=Jacques|title=Solar system: Stability|date=1997|url=https://link.springer.com/rwe/10.1007/1-4020-4520-4_382|encyclopedia=Encyclopedia of Planetary Science|series=Encyclopedia of Earth Science |pages=762–766|publisher=Springer, Dordrecht|language=en|doi=10.1007/1-4020-4520-4_382|isbn=978-1-4020-4520-2|access-date=2025-07-05}}</ref>
== Future of the Solar System ==
[[File:Sun red giant.svg|thumb|The current Sun compared to its peak size in the red-giant phase]]
The future of the Solar System depends a lot on what happens to the [[Sun]], since it is the [[star]] at the center and controls everything with its [[gravity]] and [[energy]]. Right now, the Sun is in the main part of its life, called the main sequence. During this time, it is turning [[hydrogen]] into [[helium]] through a process called [[nuclear fusion]], which creates the [[light]] and [[heat]] [[Earth]] gets. But this would not last forever. In about 5 billion years, the Sun will start to run out of hydrogen fuel. When that happens, the core will shrink, and the outer layers will puff up, making the Sun grow much bigger. This next stage is called the red giant phase. As a [[red giant]], the Sun will become so large that it might swallow up [[Mercury (planet)|Mercury]] and [[Venus]], and it could even reach [[Earth]], depending on how much it expands.<ref name=":13">{{Cite journal|last1=SchrÃ¶der|first1=K.-P.|last2=Connon Smith|first2=Robert|date=2008-05-01|title=Distant future of the Sun and Earth revisited|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=386|issue=1|pages=155–163|doi=10.1111/j.1365-2966.2008.13022.x |doi-access=free }}</ref>

When the Sun becomes a red giant, it will also start to lose a lot of its [[mass]]. This happens because strong [[solar wind]]s will blow material off the Sun and into space. As the Sun gets lighter, its gravity will weaken, and the planets that survive will slowly move into wider [[orbit]]s. Even if Earth is not swallowed by the Sun, it will still be in big trouble. The intense [[heat]] and [[radiation]] will likely destroy [[Atmosphere of Earth|Earth's atmosphere]] and [[ocean]]s, making it impossible for [[life]] to survive. Earth will become dry and lifeless. The inner Solar System will turn into a very dangerous and hostile place. The outer planets, like [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]], will probably survive, but their [[Natural satellite|moon]]s and [[Ring system|ring]]s might change.<ref name=":14">{{Cite web|date=2018-12-11|title=Chapter 6: Aging Into Gianthood - NASA Science|url=https://science.nasa.gov/exoplanets/resources/life-and-death/chapter-6/|access-date=2025-07-07|language=en-US}}</ref> At some point, the core of the Sun will get hot enough to start burning helium. This process is called [[Triple-alpha process|helium fusion]]. However, the Sun will only be able to burn helium for a short time compared to how long it burned hydrogen. The Sun is not big enough or heavy enough to fuse heavier elements like [[carbon]] or [[oxygen]]. So after the helium runs out, the [[nuclear reaction]]s in the core will stop. Without these reactions, the Sun would not be able to hold up its outer layers.<ref name=":14" />

After the Sun finishes its red giant phase, it will shed its outer layers and leave behind a small but very hot core called a white dwarf. A [[white dwarf]] is about the size of Earth, but it still holds about half the mass of the original Sun. Even though it would not make energy anymore, it will still be very hot and dense.<ref name=":13" /> When the Sun's outer layers float away into space, they might create a cloud of glowing gas called a [[planetary nebula]]. This is a bright shell of gas that shines in space for a short time. These outer layers are made of material that once belonged to the Sun, but now they contain heavier elements like [[carbon]] that were made inside the Sun over its lifetime. This gas will go back into the [[interstellar medium]], the space between stars, where it can help form new [[star]]s and [[planet]]s one day.<ref>{{Cite web|title=Planetary Nebulas {{!}} Center for Astrophysics {{!}} Harvard & Smithsonian|url=https://www.cfa.harvard.edu/research/topic/planetary-nebulas|access-date=2025-07-07|website=www.cfa.harvard.edu}}</ref><ref>{{Cite journal|last1=Gesicki|first1=K.|last2=Zijlstra|first2=A. A.|last3=Miller Bertolami|first3=M. M.|date=2018-05-07|title=The mysterious age invariance of the planetary nebula luminosity function bright cut-off|url=https://www.nature.com/articles/s41550-018-0453-9|journal=Nature Astronomy|language=en|volume=2|issue=7|pages=580–584|doi=10.1038/s41550-018-0453-9|hdl=11336/82487 |issn=2397-3366}}</ref>

Over time, the white dwarf will cool down slowly, taking trillions of years to lose its [[heat]]. It would not shine like the Sun does now, but it will still have gravity strong enough to keep the [[outer planet]]s and moons orbiting around it. However, over a very long time, the orbits of those planets might change slightly because there will be less [[gravity]] and other [[force]]s acting on them. Over an even longer period of time, tens or even hundreds of billions of years, the Solar System may start to break apart. Tiny changes, like the pull from passing [[star]]s or forces from the [[galaxy]], can slowly change the orbits of [[planet]]s and other space objects. Some planets or moons might get knocked out of the Solar System and drift off into deep space, becoming free-floating objects. Others might spiral inward and crash into the [[white dwarf]] that used to be the [[Sun]]. Eventually, the Solar System could completely fall apart, with its planets and moons scattered across the galaxy. This slow breakup is a natural part of how gravity works over a long time, and it would be the end of the Solar System.<ref>{{Cite journal|last=Dyson|first=Freeman J.|date=1979-07-01|title=Time without end: Physics and biology in an open universe|url=https://link.aps.org/doi/10.1103/RevModPhys.51.447|journal=Reviews of Modern Physics|language=en|volume=51|issue=3|pages=447–460|doi=10.1103/RevModPhys.51.447|issn=0034-6861}}</ref>
== General Characteristics of the Solar System ==

== Overall Structure ==
[[File:The Solar System (without scale representation and extra facts).svg|thumb|Overall structure of the Solar System]]
The Solar System is held together by [[gravity]], with the [[Sun]] at the center. The Sun is so huge that it makes up more than 99.8% of all the [[mass]] in the entire Solar System. Because of its strong gravity, all the major objects, like [[planet]]s, [[Natural satellite|moon]]s, [[asteroid]]s, and [[comet]]s, orbit around the Sun. This setup is called the [[Heliocentrism|heliocentric system]], which means “Sun-centered.” The Solar System is divided into different regions, and each region has its own kinds of space objects and special features.<ref name=":15" />

Closest to the Sun are the [[terrestrial planet]]s: [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]. These planets are small, rocky, and heavy, made mostly of [[rock]] and [[metal]]. This part of the Solar System is called the inner Solar System, and all the planets here are packed close together.<ref name=":1" /> Just past Mars is the [[asteroid belt]]. This is a doughnut-shaped area filled with many rocky objects. These space rocks did not come together to form a planet, mostly because Jupiter’s strong gravity kept pulling things apart. The asteroid belt is like a border zone between the inner rocky planets and the outer giant planets.<ref name=":19" />

Farther from the Sun are the [[giant planet]]s. First come [[Jupiter]] and [[Saturn]], known as [[gas giant]]s because they are made mostly of [[hydrogen]] and [[helium]].<ref name=":2" /> Then come [[Uranus]] and [[Neptune]], called [[ice giant]]s because they have more icy materials like [[water]], [[ammonia]], and [[methane]].<ref name=":3" /> These outer planets are very big, have many moons, and also have [[ring system]]s, though not all are as big as Saturn’s rings.<ref>{{Cite web|date=2024-08-13|title=A guide to rings around planets and across the Solar System|url=https://www.skyatnightmagazine.com/space-science/rings-of-the-solar-system|access-date=2025-07-07|website=BBC Sky at Night Magazine|language=en}}</ref> Beyond Neptune is the [[Kuiper Belt]], a cold, flat region full of icy objects, including [[dwarf planet]]s like [[Pluto]].<ref name=":17" /><ref name=":20">{{Cite web|date=2023-07-05|title=Pluto & Dwarf Planets|url=https://science.nasa.gov/dwarf-planets/|access-date=2025-07-07|language=en-US}}</ref> There are also many tiny frozen bodies out there.<ref name=":18" /> Even farther away is something called the [[Oort cloud|Oort Cloud]]. It is a giant, round cloud of icy objects that surrounds the Solar System. Scientists think this is where long-period comets come from. It is also where the Sun’s gravity starts to fade.<ref name=":6" />

The objects in the Solar System are grouped into different categories based on how they move and what they are made of. First, there are the [[planet]]s, like [[Earth]] and [[Jupiter]]. These are big enough to control their [[orbit]]s by pulling in or pushing away other objects nearby.<ref>{{Cite web|date=2023-07-10|title=About the Planets|url=https://science.nasa.gov/solar-system/planets/|access-date=2025-07-07|language=en-US}}</ref> Next are [[dwarf planet]]s, like [[Pluto]], [[Eris (dwarf planet)|Eris]], and [[Haumea (dwarf planet)|Haumea]]. These are round like planets, but they do not clear their orbits, which means other objects still share their paths around the Sun.<ref name=":20" /> Then we have [[natural satellite]]s, which are moons that orbit planets.<ref name=":16" /> Finally, there are [[Small solar system body|small solar system bodies]] like [[asteroid]]s, [[comet]]s, and [[meteoroid]]s. These are smaller chunks of rock, metal, or ice that travel through space in all kinds of orbits.<ref>{{Cite web|title=Small body {{!}} Asteroids, Comets & Meteors {{!}} Britannica|url=https://www.britannica.com/science/small-body|access-date=2025-07-07|website=www.britannica.com|language=en}}</ref>
== Distances and scales ==
[[File:Solar System distance to scale.svg|center|thumb|upright=2.5|Comparison of the distances between planets, with the white bar showing orbital variations. The size of the planets is not to scale.]]
[[File:Orbital distances in the solar system linear scale.png|thumb|Relative orbital distances in the Solar System visualized as a condensed rectangle]]

The Solar System is huge, way bigger than what humans can easily imagine. The [[distance]]s and [[size]]s of the planets and other space objects are so big that it is hard to picture them using everyday ideas.<ref>{{Cite web|title=The Solar System to Scale: Dynamic 2D Model|url=https://solartoscale.com/|access-date=2025-07-08|website=solartoscale.com}}</ref> The [[Sun]] is the biggest thing in the Solar System. It is so massive that it makes up almost all the [[mass]] in the entire Solar System. The Sun’s [[radius]] is about 700,000 kilometers (or 400,000 miles). Even though [[Earth]] feels big to us, about 1.3 million Earth's would fit inside the Sun.<ref name=":10" /> [[Jupiter]] is the largest planet. It is 5.2 times farther from the Sun than Earth and is so big that over 1,300 Earths could fit inside it. Its [[diameter]] is more than 11 times that of Earth.<ref>{{Cite web|date=2017-10-05|title=Jupiter Facts|url=https://science.nasa.gov/jupiter/jupiter-facts/|access-date=2025-07-08|language=en-US}}</ref> On the other hand, [[Mercury (planet)|Mercury]], the smallest planet, is just a little bigger than [[Moon|Earth’s Moon]].<ref>{{Cite web|title=Mercury|url=https://observatory.astro.utah.edu/Mercury.html|access-date=2025-07-08|website=observatory.astro.utah.edu}}</ref> And even smaller are [[dwarf planet]]s like [[Pluto]] and tiny space rocks like [[comet]]s and [[asteroid]]s, which can be just a few kilometers wide.<ref>{{Cite web|date=2025-04-22|title=Planet Sizes and Locations in Our Solar System|url=https://science.nasa.gov/solar-system/planets/planet-sizes-and-locations-in-our-solar-system/|access-date=2025-07-08|language=en-US}}</ref>

Because the Solar System is so big, scientists use a special unit called the [[Astronomical unit|Astronomical Unit (AU)]] to measure distances in space. One AU is the average distance between the Earth and the Sun, which is about 149.6 million kilometers (or 93 million miles). Using AU makes it easier to talk about how far planets are from the Sun.<ref>{{Cite web|date=2025-05-29|title=Astronomical unit (AU, or au) {{!}} Definition, Conversion, & Facts {{!}} Britannica|url=https://www.britannica.com/science/astronomical-unit|access-date=2025-07-08|website=www.britannica.com|language=en}}</ref> For example, [[Venus]] is about 0.72 AU from the Sun, [[Mars]] is about 1.52 AU, and [[Neptune]], the farthest main planet, is about 30 AU away. The [[Kuiper Belt]], which contains icy objects like Pluto, stretches from around 30 AU to 50 AU.<ref>{{Cite web|title=Scale of the Solar System|url=https://web.pa.msu.edu/people/frenchj/solarsystemscale/|access-date=2025-07-08|website=web.pa.msu.edu}}</ref> Even farther out is the [[Oort cloud|Oort Cloud]], a mysterious area that might reach 100,000 AU from the Sun.<ref name=":6" /> Usually, the farther a planet is from the Sun, the bigger the gap between it and the next planet. For instance, Venus is only about 0.33 AU farther from the Sun than Mercury, but Saturn is 4.3 AU farther out than Jupiter, and Neptune is 10.5 AU beyond Uranus.<ref>{{Cite web|title=Comparing the Distance Between Planets|url=https://www.msnucleus.org/membership/html/k-6/uc/solar_system/2/ucss2_2a.html|access-date=2025-07-08|website=www.msnucleus.org}}</ref> In the past, scientists tried to find patterns in these distances, like the [[Titius–Bode law]] or [[Mysterium Cosmographicum|Kepler's ideas]], but as more planets and objects were discovered, those ideas did not hold up.<ref>{{Cite journal|last=Jaki|first=Stanley L.|date=1972-07-01|title=The Early History of the Titius-Bode Law|url=https://pubs.aip.org/ajp/article/40/7/1014/1045433/The-Early-History-of-the-Titius-Bode-Law|journal=American Journal of Physics|language=en|volume=40|issue=7|pages=1014–1023|doi=10.1119/1.1986734|issn=0002-9505}}</ref><ref>{{Cite journal|last=Phillips|first=J. P.|date=1965|title=Kepler's Echinus|url=https://www.journals.uchicago.edu/doi/10.1086/349957|journal=Isis|language=en|volume=56|issue=2|pages=196–200|doi=10.1086/349957|issn=0021-1753}}</ref><ref>{{Cite web|last=Staff|first=Astronomy|date=2006-10-01|title=Is it a coincidence that most of the planets fall within the Titius-Bode law's boundaries? {{!}} Astronomy.com|url=https://www.astronomy.com/science/is-it-a-coincidence-that-most-of-the-planets-fall-within-the-titius-bode-laws-boundaries/|access-date=2025-07-08|website=Astronomy Magazine|language=en-US}}</ref>

There is a lot of empty space between the planets in our Solar System. Even though the planets follow paths around the Sun, the distances between them are huge compared to how big the planets are.<ref>{{Cite web|title=Distances Between the Planets of the Solar System • The Planets|url=https://theplanets.org/distances-between-planets/|access-date=2025-07-09|website=The Planets|language=en-US}}</ref> For example, Earth is about 12,700 kilometers wide, but the closest it ever gets to Mars is around 96 million kilometers away.<ref>{{Cite web|date=2023-05-30|title=Facts About Earth|url=https://science.nasa.gov/earth/facts/|access-date=2025-07-09|language=en-US}}</ref><ref>{{Cite web|last=|first=|date=2025-01-12|title=Mars closest to Earth on January 12, 2025|url=https://earthsky.org/tonight/mars-closest-to-earth/|access-date=2025-07-09|website=earthsky.org|language=en-US}}</ref> Because of these huge distances, even fast [[spacecraft]] take a long [[time]], months or years, to travel between planets. The farther you go, the longer it takes.<ref>{{Cite web|date=2025-02-19|title=How Long Does it Take to Get to the Moon... Mars... Jupiter? We Asked a NASA Expert: Episode 51 - NASA|url=https://www.nasa.gov/directorates/smd/how-long-does-it-take-to-get-to-the-moon-mars-jupiter-we-asked-a-nasa-expert-episode-51/|access-date=2025-07-09|language=en-US}}</ref> For instance, [[light]] from the Sun takes only 8 minutes to reach Earth, but it takes over 4 hours to reach Neptune.<ref>{{Cite web|date=2024-04-12|title=How long does it take for light from the Sun to reach Earth?|url=https://www.skyatnightmagazine.com/space-science/how-take-light-from-sun-reach-earth|access-date=2025-07-09|website=BBC Sky at Night Magazine|language=en}}</ref><ref>{{Cite web|date=2017-11-10|title=Neptune: Facts - NASA Science|url=https://science.nasa.gov/neptune/neptune-facts/|access-date=2025-07-09|language=en-US}}</ref> All this [[Outer space|space]] makes it harder to explore and communicate with the outer planets. Sending missions to places like Neptune or beyond is a big challenge and a major achievement for [[science]] and [[engineering]].<ref>{{Cite web|title=Challenges of space travel - Space exploration - National 5 Physics Revision|url=https://www.bbc.co.uk/bitesize/guides/zqm4fcw/revision/3|access-date=2025-07-09|website=BBC Bitesize|language=en-GB}}</ref>

Some [[Scientific model|model]]s of the Solar System are made to help people understand just how big and spread out everything really is. Since the real Solar System is way too large to picture easily, scientists and educators sometimes build scale models to shrink it down while keeping the sizes and distances in the right proportion. Some of these models are small and mechanical, like [[Orrery|orreries]], which are devices that show planets moving around the Sun.<ref>{{Cite web|title=Orrery|url=https://www.hsm.ox.ac.uk/orrery|access-date=2025-07-09|website=www.hsm.ox.ac.uk|language=en}}</ref> Others are huge and can stretch across cities or even entire regions. One of the biggest models is the [[Sweden Solar System]].<ref>{{Cite web|title=Sweden Solar System|url=https://www.swedensolarsystem.se/en/|access-date=2025-07-09|website=www.swedensolarsystem.se}}</ref> In it, the [[Avicii Arena]] in [[Stockholm]] represents the Sun, and it is about 110 meters wide (361 feet). In this model, Jupiter is a 7.3-meter ball (about 24 feet wide) located at an [[airport]] 40 kilometers (25 miles) away. [[Sedna]], an object far out in the Solar System, is shown as a 10-centimeter ball (about 4 inches) placed 810 kilometers (500 miles) away.<ref>{{Cite web|title=Luleå är Sedna - Lokalt den 17 november 2005 - Arkiv - Norrbottens-Kuriren|url=http://www.kuriren.nu/arkiv/2005/11/17/Lokalt/1510647/Lule%C3%A5-%C3%A4r-Sedna.aspx|access-date=2025-07-09|website=www.kuriren.nu}}</ref><ref>{{Cite web|title=Tours of Model Solar Systems|url=http://www.psych.illinois.edu/~wbrewer/solarmodel.html|access-date=2025-07-09|website=www.psych.illinois.edu|archive-date=2010-07-24|archive-url=https://web.archive.org/web/20100724064517/http://www.psych.illinois.edu/~wbrewer/solarmodel.html|url-status=dead}}</ref> If we shrank the distance from the Sun to Neptune down to just 100 meters (about the length of a soccer field), then the Sun would only be about 3 centimeters wide (a little bigger than a [[marble]]), the [[Giant planet|giant planets]] would be all smaller than about 3 mm (0.12 in), and Earth and the other small planets would be tiny dots, smaller than a [[flea]]. These kinds of models help us understand that even though planets can be big, they are still tiny compared to the space between them.<ref>{{Cite web|title=Solar System Scale|url=https://www.nasa.gov/audience/foreducators/5-8/features/F_Solar_System_Scale.html|access-date=2025-07-09|website=www.nasa.gov|archive-date=2023-03-28|archive-url=https://web.archive.org/web/20230328021134/https://www.nasa.gov/audience/foreducators/5-8/features/F_Solar_System_Scale.html|url-status=dead}}</ref>
== Mass and Composition Distribution ==
[[File:Solar_system_mass_distribution_ppm_chart.svg|thumb|Solar system mass distribution]]
The Solar System’s mass and the way its parts are spread out tell us a lot about how it formed and changed over time. Most of the Solar System’s [[mass]], about 99.86%, is in the [[Sun]], which is a huge ball made mostly of [[hydrogen]] and [[helium]]. The Sun’s gravity controls the movement of all the planets, asteroids, comets, and space dust. It’s incredibly massive, weighing about 1.989 × 10³⁰ kilograms. This huge mass creates a strong gravitational pull that keeps everything else in the Solar System in [[orbit]], and it also helps determine how things spin and move.<ref>{{Cite web|date=2023-05-31|title=Sun - NASA Science|url=https://science.nasa.gov/sun/|access-date=2025-07-15|language=en-US}}</ref><ref>{{Cite web|title=Sun Fact Sheet|url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html|access-date=2025-07-15|website=nssdc.gsfc.nasa.gov|archive-date=1998-02-22|archive-url=https://web.archive.org/web/19980222081658/https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html|url-status=dead}}</ref> Only about 0.14% of the Solar System’s total mass is in everything else, like the eight [[planet]]s, [[dwarf planet]]s, [[Natural satellite|moon]]s, [[asteroid]]s, objects in the [[Kuiper Belt]], and the [[Oort cloud|Oort Cloud]].<ref>{{Cite web|date=2023-07-20|title=Chapter 1: The Solar System - NASA Science|url=https://science.nasa.gov/learn/basics-of-space-flight/chapter1-1/|access-date=2025-07-15|language=en-US}}</ref> Out of this small portion, more than 70% is found in [[Jupiter]], the largest planet. Jupiter weighs about 1.9 × 10²⁷ kilograms, which is 318 times heavier than [[Earth]].<ref>{{Cite web|title=Jupiter |url=https://www.dlr.de/en/research-and-transfer/projects-and-missions/juice/jupiter-gas-giant-and-ringed-planet/jupiter|access-date=2025-07-15|website=www.dlr.de|language=en}}</ref> Because of its size, Jupiter has a very strong gravitational pull. It affects the movement of asteroids, helps keep the orbits of other planets stable, and played a big role in clearing out leftover space rocks when the Solar System was still forming.<ref>{{Cite web|date=2023-05-31|title=Jupiter|url=https://science.nasa.gov/jupiter/|access-date=2025-07-15|language=en-US}}</ref>

The [[giant planet]]s, [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]], make up about 99% of the total mass of all the planets in the Solar System. This shows that the Solar System has a bimodal structure, meaning most of the mass is in these large [[Gas giant|gas]] and [[ice giant]]s, not the smaller rocky planets closer to the Sun.<ref>{{Cite book|url=https://nap.nationalacademies.org/read/13117/chapter/10|title=Read "Vision and Voyages for Planetary Science in the Decade 2013-2022" at NAP.edu|language=en}}</ref> Saturn is the second largest planet, with a mass about 95 times that of Earth. Like Jupiter, it is mostly made of hydrogen and helium, but Saturn has more ices and heavier elements deep inside.<ref>{{Cite web|date=2023-05-31|title=Saturn|url=https://science.nasa.gov/saturn/|access-date=2025-07-15|language=en-US}}</ref> Uranus and Neptune are called ice giants. They also have atmospheres made of hydrogen and helium, but inside they have a lot of [[water]], [[ammonia]], and [[Methane|methane ices]]. Each of them has a mass between 14 and 17 times the mass of Earth.<ref>{{Cite web|title=Uranus/Neptune|url=https://pages.uoregon.edu/jschombe/ast121/lectures/lec20.html#:~:text=Neptune%20is%2017%20times%20the,could%20contain%20nearly%2060%20Earths.|access-date=2025-07-15|website=pages.uoregon.edu}}</ref>

On the other hand, the [[Terrestrial planet|terrestrial (rocky) planet]]s, [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]], have only a tiny amount of the Solar System’s mass. Earth makes up more than half of the total mass of these four rocky planets because it’s the biggest and densest. These inner planets are made mostly of [[Silicate|silicate rock]] and [[metal]], with common elements like [[iron]], [[oxygen]], [[silicon]], and [[magnesium]].<ref name=":1" /> They do not have much hydrogen or helium because, when the Solar System was forming, the area close to the Sun was too hot for light gases to stick around. The Sun’s heat pushed those gases away, so only heavier materials like rock and metal were left behind to form the inner planets.<ref>{{Cite web|title=Planets and How They Formed|url=https://lco.global/spacebook/solar-system/planets-and-how-they-formed/|access-date=2025-07-15|website=lco.global|language=en}}</ref>

Besides the major planets, the Solar System has many smaller objects. Even though these objects have only a small amount of mass compared to the planets, they are very important for helping scientists understand how the planets formed. One group of these small objects is the [[asteroid belt]], which is found between Mars and Jupiter. The asteroid belt has less than 4% of the Moon’s mass, but it contains materials that were part of the early building blocks of the rocky planets. The biggest object in the belt is [[Ceres (dwarf planet)|Ceres]], which is about 940 kilometers wide. Ceres makes up about one-third of the asteroid belt’s total mass and is made mostly of [[rock]] and [[Ice|water ice]].<ref>{{Cite journal|last=Krasinsky|first=G|date=2002|title=Hidden Mass in the Asteroid Belt|url=https://linkinghub.elsevier.com/retrieve/pii/S0019103502968375|journal=Icarus|language=en|volume=158|issue=1|pages=98–105|doi=10.1006/icar.2002.6837}}</ref><ref>{{Cite journal|last1=Pitjeva|first1=E. V.|last2=Pitjev|first2=N. P.|date=2018|title=Masses of the Main Asteroid Belt and the Kuiper Belt from the Motions of Planets and Spacecraft|url=http://link.springer.com/10.1134/S1063773718090050|journal=Astronomy Letters|language=en|volume=44|issue=8–9|pages=554–566|doi=10.1134/S1063773718090050|arxiv=1811.05191 |issn=1063-7737}}</ref> Farther away from the Sun is the Kuiper Belt, which holds thousands of icy objects. Some of the best-known are [[Pluto (dwarf planet)|Pluto]], [[Eris (dwarf planet)|Eris]], [[Haumea (dwarf planet)|Haumea]], and [[Makemake (dwarf planet)|Makemake]]. These objects are much smaller than the giant planets, but their mix of rock and ices like [[methane]], [[nitrogen]], and [[carbon monoxide]] helps scientists learn about the chemistry of the early outer Solar System. The total mass of the Kuiper Belt is only a small fraction of Earth’s mass, between 0.01 and 0.1 times the mass of Earth, because over billions of years, much of its mass was lost in space due to collisions and gravitational forces.<ref>{{Cite journal|last1=Romanishin|first1=W.|last2=Tegler|first2=S. C.|last3=Rettig|first3=T. W.|last4=Consolmagno|first4=G.|last5=Botthof|first5=B.|date=2001-10-09|title=1998 SM165: A large Kuiper belt object with an irregular shape|journal=Proceedings of the National Academy of Sciences|volume=98|issue=21|pages=11863–11866|doi=10.1073/pnas.211147998 |pmid=11572937 |pmc=59815 |doi-access=free }}</ref><ref>{{Cite journal|last1=Pitjeva|first1=E. V.|last2=Pitjev|first2=N. P.|date=2018|title=Masses of the Main Asteroid Belt and the Kuiper Belt from the Motions of Planets and Spacecraft|url=http://link.springer.com/10.1134/S1063773718090050|journal=Astronomy Letters|language=en|volume=44|issue=8–9|pages=554–566|doi=10.1134/S1063773718090050|arxiv=1811.05191 |issn=1063-7737}}</ref><ref name=":17" />

Even farther out is the [[Oort cloud|Oort Cloud]], which scientists think is a huge, round shell of icy objects surrounding the Solar System. It is believed to contain trillions of comets made mostly of water ice, [[carbon dioxide]] ice, and other frozen gases. The total mass of the Oort Cloud is not known for sure, but it might be several times the mass of Earth. It stretches as far as 100,000 [[Astronomical unit|astronomical units (AU)]] from the Sun (1 AU is the distance from Earth to the Sun). The Oort Cloud is thought to be the source of [[Comet|long-period comet]]s, comets that take hundreds or thousands of years to orbit the Sun and sometimes come close enough to be seen from Earth.<ref name=":6" />

[[Natural satellite|Moon]]s and [[Planetary ring|ring]]s around planets add a little bit of mass to the Solar System, but not very much compared to the planets. For example, [[Moon|Earth’s Moon]] is the fifth largest moon in the Solar System. It is pretty big compared to Earth, with a mass that is about 1.2% of Earth’s mass. The Moon is mostly made of [[Silicate|silicate rock]], which is similar to the material found in [[Earth's crust|Earth’s crust]] and [[Earth's mantle|mantle]].<ref>{{Cite web|date=2017-11-15|title=Moon Facts - NASA Science|url=https://science.nasa.gov/moon/facts/|access-date=2025-07-16|language=en-US}}</ref> The outer planets have some very large moons too, like [[Ganymede (moon)|Ganymede]], [[Titan (moon)|Titan]], [[Callisto (moon)|Callisto]], and [[Io (moon)|Io]]. These moons are made of a mix of rock and ice. Some of them, like [[Europa (moon)|Europa]] and [[Enceladus (moon)|Enceladus]], even have [[ocean]]s beneath their icy surfaces, which shows that there are many different kinds of icy, possibly habitable places beyond Earth.<ref name=":16" />

The types of [[Chemical element|element]]s found in different parts of the Solar System also tell us a lot about how it formed. These elements were created in earlier generations of [[star]]s and then spread out into [[Outer space|space]]. In the outer Solar System, there are more volatile elements (which are lighter and can turn into gas easily), while the inner Solar System has more refractory materials (which are heavier and can stay solid at high [[temperature]]s). This difference shows how temperature, [[sunlight]], and the movement of planets affected where different materials could collect and form planets.<ref>{{Cite book|title=Encyclopedia of the solar system|url=https://archive.org/details/encyclopediaofso0000unse_m9q5|date=2014|publisher=Elsevier|isbn=978-0-12-415845-0|editor-last=Spohn|editor-first=Tilman|edition=3rd|location=Amsterdam ; Boston|editor-last2=Breuer|editor-first2=Doris|editor-last3=Johnson|editor-first3=T. V.}}</ref>
== Orbits of the planets ==
The [[Earth's orbit]] around the Sun is nearly a perfect circle, but in a very slightly oval shaped orbit, an [[orbit|elliptical orbit]]. The other [[planet]]s in the Solar System also orbit the Sun in slightly elliptical orbits. [[Mercury (planet)|Mercury]] has a more elliptical orbit than the others, and there is obviously some explanation for this. Some of the smaller objects orbit the Sun in very [[orbit|eccentric orbits]]. The planets all orbit the Sun in the same direction.<ref>{{Cite book|title=From dust to life : the origin and evolution of our solar system|url=https://archive.org/details/fromdusttolifeor0000cham|last1=Chambers|first1=John|last2=Mitton|first2=Jacqueline|publisher=Princeton University Press|year=2014|isbn=978-0-691-14522-8|location=Princeton, New Jersey|oclc=859181634}}</ref>{{rp|4-5}}

A full account of the planetary motion needs an account of the ''n''-body problem, which is not treated on this wiki. A page can be found on En wiki.
== Discovery and exploration ==
{{Main|History of astronomy}}

For thousands of years, people had no need for a name for the "Solar System". They thought the Earth stayed still at the center of everything ([[geocentrism]]). The Greek philosopher [[Aristarchus of Samos]] suggested that there was a special order in the sky.<ref>{{cite journal|title= The astronomical system of Copernicus|author=WC Rufus|journal=Popular Astronomy|year=1923|volume=31|pages=510|bibcode=1923PA.....31..510R|url=http://adsabs.harvard.edu/full/1923PA.....31..510R|accessdate=2009-05-09}}</ref> [[Nicolaus Copernicus]] was the first to develop a [[Mathematical model|mathematical system]] that described what we now call the "Solar System". This was called a "new system of the world". In the 17th century, [[Galileo Galilei]], [[Johannes Kepler]] and [[Isaac Newton]] began to understand [[physics]] more clearly. People began to accept the idea that the Earth is a planet that moves around the Sun, and that the planets are worlds, and that all worlds are governed by the same [[physical law]]s. More recently, telescopes and [[space probe]]s sometimes let us see details directly. All inner planets have surface features. The gas giants (as the name suggests) have surfaces whose make-up is gradually being discovered.
== The eight planets ==
[[File:Uklad Sloneczny 2006.png|thumb|Planetary distances, not to scale]]
In their order from the Sun:
#[[Mercury (planet)|Mercury]]
#[[Venus]]
#[[Earth]]
#[[Mars]]
#[[Jupiter]]
#[[Saturn (planet)|Saturn]]
#[[Uranus]]
#[[Neptune]]

The planets are the biggest objects that go around the Sun.  It took people many years of using [[telescope]]s to find the objects that were farthest away.  [[Planets beyond Neptune|New planets]] might still be found, and more small objects are found every year.  Most of the planets have [[Satellite (natural)|moons]] that orbit around them just as the planets orbit the Sun. There are over 400 of these moons in the Solar System.
== Dwarf planets ==
{{Main|Dwarf planet}}

Pluto was discovered by American astronomer [[Clyde Tombaugh]] in 1930. It was declared the ninth planet of the Solar System. 

Pluto's planet status changed in 2006, when the [[International Astronomical Union]] (IAU) decided on a definition of the word "planet" for the first time. By this definition, Pluto was not a planet due to its irregular orbit and size. Thus, Pluto lost its planet status. 

Eris was first announced in 2005. It was found to be 27% more massive than Pluto. Pluto did not fit in with the rest of the planets. So, the IAU defined a new category called "dwarf planet", into which Pluto did fit, along with some other small Solar System bodies. These small bodies are sometimes called [[plutino]]s.<ref>{{cite web|title=IAU 2006 General Assembly: Result of the IAU Resolution votes|url=https://www.iau.org/news/pressreleases/detail/iau0603/|url-status=dead|archive-url=https://web.archive.org/web/20140429212224/https://www.iau.org/news/pressreleases/detail/iau0603/|archive-date=2014-04-29|publisher=IAU|accessdate=2018-09-04}}</ref>

There is an unknown number of dwarf planets in the Solar System. As of 2025, these nine small Solar System bodies are often called dwarf planets:
* [[Pluto]]
* [[Ceres (dwarf planet)|Ceres]]
* [[Haumea (dwarf planet)|Haumea]]
* [[Eris (dwarf planet)|Eris]]
* [[Makemake (dwarf planet)|Makemake]]
* [[50000 Quaoar|Quaoar]]
* [[90377 Sedna|Sedna]]
* [[90482 Orcus|Orcus]]
* [[225088 Gonggong|Gonggong]]
== Structure ==
There are a few main parts of the Solar System. Here they are in order from the Sun, with the planets numbered, and [[dwarf planet]]s marked with letters.
== Inner solar system ==
[[File:Terrestrial planet size comparisons.jpg|thumb|The inner planets. From left to right: [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]]]

The first four planets closest to the Sun are called the inner planets. They are small and dense terrestrial planets, with solid surfaces. They are made up of mostly rock and metal with a distinct internal structure and a similar size. Three also have an [[atmosphere]]. The study of the four planets gives information about [[geology]] outside the Earth. 

* [[Terrestrial planet|Terrestrial planets]] region contains the four planets closest to the sun, all are rocky planets
** '''(1)''' [[Mercury (planet)|Mercury]]
** '''(2)''' [[Venus]]
** '''(3)''' [[Earth]]
** '''(4)''' [[Mars]]
* [[Asteroid belt]] region contains;
** ''(a)'' [[Ceres (dwarf planet)|Ceres]] (the only '''[[dwarf planet]]''' in this region)
** [[Asteroid]]s
== Outer solar system ==
[[File:Gas planet size comparisons.jpg|thumb|The outer planets: From left to right: [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]]]]
* [[Giant planet]]s region contains the four largest planets. Jupiter and Saturn are [[gas giant]]s, while Uranus and Neptune are [[ice giant]]s;
** '''(5)''' [[Jupiter]]
** '''(6)''' [[Saturn (planet)|Saturn]]
** '''(7)''' [[Uranus]]
** '''(8)''' [[Neptune]]
== Trans-Neptune region ==
* [[Kuiper belt]] region contains;
** ''(b)'' [[Pluto]]
** ''(c)'' [[90482 Orcus|Orcus]]
** ''(d)'' [[Haumea (dwarf planet)|Haumea]]
** ''(e)'' [[Makemake (dwarf planet)|Makemake]]
** ''(f)'' [[Quaoar]]
** [[Kuiper belt|Kuiper belt objects]] and possibly other dwarf planets
** short-period [[comet]]s
* [[Scattered disc]] region contains;
** ''(g)'' [[90377 Sedna|Sedna]]
** ''(h)'' [[225088 Gonggong|Gonggong]]
** ''(i)'' [[Eris (dwarf planet)|Eris]]
** [[Scattered disc|Scattered disk objects]] and possibly other dwarf planets
== Oort Cloud ==
The [[Oort cloud]] is separate from the '''trans-Neptune region''', and much farther out. It contains the long-period [[comet]]s.
== Ecliptic plane ==
The ''plane of the [[ecliptic]]'' is defined by the Earth's orbit around the Sun. All of the planets orbit the Sun roughly around this same [[Orbital plane (astronomy)|orbital plane]]. The farther away from this plane a planet orbits, the more ''inclined'' is its orbit to the ecliptic. If you could look at the Solar System "edge on" then all the planets would be orbiting more or less in the plane of the ecliptic.
{{clear}}

== Solar System Media ==
<gallery widths='160px' heights='100%' mode='traditional' caption=''>
File:Plane of Ecliptic.jpg|A color enhanced photograph from the [[Moon]] of a range of components of the Solar System. The three dots at the lower left are from left to right the planets [[Saturn]], [[Mars]], and [[Mercury (planet)|Mercury]], and in the middle of the picture rises the [[Sun's corona]] over the dark [[wiktionary:limb|limb]] of the Moon, which is from the right lit by [[earthshine]].

File:Solar system orrery inner planets.gif|Animations of the Solar System's [[inner planet]]s orbiting. Each frame represents 2 days of motion.

File:Solar system orrery outer planets.gif|Animations of the Solar System's [[outer planet]]s orbiting. This animation is 100 times faster than the inner planet animation.

File:The Sun in white light.jpg|The Sun in true color as viewed through a [[Astronomical filter|solar filter]]

File:Terrestrial planet sizes 3.jpg|The four terrestrial planets [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]] and [[Mars]]

File:Inner solar system objects top view for wiki.png|Overview of the inner Solar System up to Jupiter's orbit
</gallery>
== References ==
{{Reflist}}
== More reading ==
* {{cite book|first=Kenneth R.|last=Lang|title=The Cambridge Guide to the Solar System|url=https://archive.org/details/cambridgeguideto0000lang_r9p2|publisher=Cambridge University Press|edition=2nd|date=2011|isbn=978-0521198578}}
* {{cite book |last1=Iggulden |first1=Hal |last2=Iggulden |first2=Conn |title=The Dangerous Book for Boys |url=https://archive.org/details/dangerousbookfor0000iggu_u2s8 |date=2007 |publisher=HarperCollins |location=New York |isbn=978-0061243585 |pages=[https://archive.org/details/dangerousbookfor0000iggu_u2s8/page/216 217]–224 |chapter=The Solar System (a quick reference guide) }}
* Thierry Montmerle; Jean-Charles Augereau; Marc Chaussidon 2006. ''Solar System formation and early evolution: the first 100 million years. Earth, Moon, and Planets''. Springer. [[98 (number)|98]] (1–4): 39–95. Bibcode:2006EM&P...98...39M. doi:10.1007/s11038-006-9087-5. S2CID 120504344.
== Other websites ==
{{commons category}}
* [http://www.solarviews.com/eng/homepage.htm Views of the Solar System]
* [http://www.exploratorium.edu/ronh/weight/index.html Your Weight on Other Worlds]
* [https://www.gsupdate.online/2021/07/the-planets-in-the-solar-system.html The Planets in the Solar System] {{Webarchive|url=https://web.archive.org/web/20210710070248/https://www.gsupdate.online/2021/07/the-planets-in-the-solar-system.html |date=2021-07-10 }}

{{wikibooks|Wikijunior:Solar System}}
{{Solar System}}
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[[Category:Solar System| ]]