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Solar System

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Planets and dwarf planets of the Solar System. Compared with each other, the sizes are correct, but the distances are not.

The Solar System is the Sun and all the objects in orbit around it. The Sun is orbited by planets, asteroids, comets and other things.

The Sun is a star. It contains 99.9 percent of the Solar System's mass. This means that it has strong gravity. The other objects are pulled into orbit around the Sun.

There are eight planets in the Solar System. From closest to farthest from the Sun, they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The first four planets are called terrestrial planets. They are mostly made of rock and metal, and they are mostly solid. The last four planets are called gas giants. This is because they are large planets that are mostly made of gas. Even though they are made of gas, they have much more mass than the terrestrial planets.

The Solar System also contains other things. There is the asteroid belt between Mars and Jupiter. Further out than Neptune, there is the Kuiper belt and the scattered disc. In these two areas, there are dwarf planets. There are five dwarf planets in the Solar System: Ceres, Pluto, Haumea, Makemake and Eris. There are also thousands of very small objects in these areas. As well as these, there are also comets, centaurs and there is interplanetary dust.

Six of the planets and three of the dwarf planets are orbited by moons. Furthermore, planetary dust orbits the gas giants. There are many other systems like the Solar System in the universe. Each star has the potential to have a planetary system. There are more than 200 billion stars in the Milky Way galaxy.

Evolution of the Solar System

The formation and evolution of the Solar System began 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud.[1]

Most of the collapsing mass collected in the centre, forming the Sun, while the rest flattened into a protoplanetary disk of loose dust, out of which the planets, moons, asteroids, and other Solar System bodies formed.

This widely accepted model, known as the nebular hypothesis, was first developed in the 18th (1700's) century by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace. Its subsequent development has interwoven a variety of scientific disciplines including astronomy, physics, geology, and planetary science. As our knowledge of space has grown, the models have been changed to account for the new observations.

The Solar System has evolved considerably since its initial formation. Many moons have formed from circling discs of gas and dust around their parent planets, while other moons are believed to have formed independently and later been captured by their planets. Still others, as the Earth's Moon, may be the result of giant collisions.

Many collisions between bodies have occurred, and have been important to the evolution of the Solar System. The positions of the planets often shifted, and planets have switched places.[2][3] This planetary migration is thought to have been responsible for much of the Solar System's early evolution.

Earth's orbit

The Earth's orbit around the Sun is nearly a perfect circle, but when mapped it is found that the Earth moves around the Sun in a very slightly oval shaped, called an elliptical orbit. The other planets in the Solar System also circle the Sun in slightly elliptical orbits. Mercury has a more elliptical orbit than the others, and some of the smaller objects orbit the Sun in very eccentric orbits.

Discovery and exploration

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). Although the Greek philosopher Aristarchus of Samos suggested that there was a special order in the sky,[4] Nicolaus Copernicus was the first to develop a 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 helping people understand physics more clearly. This made people slowly accept the idea that the Earth is a planet and moves around the Sun, and that the planets are worlds with the same physical laws that control Earth. More recently, telescopes and spacecraft have led to discoveries of mountains and craters, and seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets.

Planetary distances, not to scale

The eight planets

In their order from the Sun:

  1. Mercury
  2. Venus
  3. Earth
  4. Mars
  5. Jupiter
  6. Saturn
  7. Uranus
  8. Neptune

The planets are the biggest objects that go around Sun. It took people many years of looking carefully through telescopes to find the objects that were farthest away. No one expects to find new planets, but more small objects are found every year. Most of the planets have moons that orbit around them just as the planet orbit the Sun. There are at least 173 of these moons in the solar system.

Dwarf planets

Pluto had been called a planet since it was discovered in 1930, but in 2006 astronomers meeting at the International Astronomical Union decided on the definition of a planet, and Pluto did not fit. Instead they defined a new category of dwarf planet, into which Pluto did fit, along with some others. These small planets are sometimes called plutinos.


There are a few main parts of the Solar System. Here they are in order from the Sun, with the planets numbered, and the dwarf planets marked with the letters a - e.

  • Terrestrial planets region contains the four planets closest to the sun, all are rocky planets

Inner solar system, Outer solar system, Trans-Neptune region

The inner planets. From left to right: Mercury, Venus, Earth, and Mars

Sometimes people use alternative names for similar regions defined above. These are less formal, and less well defined.

  • Inner solar system contains
  • Outer solar system contains
  • Trans-Neptune region contains

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.

Sometimes the Outer solar system is taken to mean the Gas Giant Planets, the Kuiper Belt, and the Scattered Disk. But with discoveries of many objects in the Kuiper Belt, and the Scattered Disk, and with more dwarf planet discoveries, the Outer solar system is now usually defined more narrowly.

The Oort cloud is separate from the Trans-Neptune region, and much farther out.

Plane of the ecliptic

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 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 in the plane of the ecliptic plane.


  1. Bouvier, Audrey and Meenakshi Wadhwa, "The age of the solar system redefined by the oldest Pb-Pb age of a meteoritic inclusion". Nature Geoscience, Nature Publishing Group, a division of Macmillan Publishers Limited. Published online 2010-08-22, retrieved 2010-08-26, doi: 10.1038/NGEO941.
    Date based on oldest inclusions found to date in meteorites, thought to be among the first solid material to form in the collapsing solar nebula.
  2. R. Gomes, H.F. Levison, K. Tsiganis, A. Morbidelli (2005). "Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets" (PDF). Nature 435 (7041): 466. doi:10.1038/nature03676 . PMID 15917802 .
  3. A. Morbidelli, J. Chambers, J.I. Lunine, J.M. Petit, F. Robert, G.B. Valsecchi, K.E. Cyr (2000). "Source regions and timescales for the delivery of water to the Earth". Meteoritics & Planetary Science 35: 1309. ISSN 1086–9379 .
  4. WC Rufus. "The astronomical system of Copernicus". Popular Astronomy 31: 510. Retrieved 2009-05--09.

More reading

  • Lang, Kenneth R. (2011). The Cambridge guide to the Solar System (2nd ed.). Cambridge University Press. ISBN 9780521198578

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