Solar System

Scale picture of the planets in the Solar System. Earth is number 3

The Solar System is the astronomical name for the Sun, the Earth, and the collection of planets and other rocky and icy objects moving around, or orbiting, the Sun.

The main component of the Solar System is the Sun, which contains 98.6 percent of the system's mass and whose gravity holds everything else in orbit. All the main objects in the Solar System formed at the same time, as part of the same process (see Age of the Earth).

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, or 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.

The Eight Planets

In their order from the Sun, and labelled with the numbers like the picture on the right:

The planets are the biggest objects that go around Sun. It took people many years of looking carefully through telescopes to find the farthest away ones. 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. 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 for the first time on the definition of a planet, and Pluto didn't fit. Instead they defined a new category of dwarf planet, into which Pluto did fit along with some other objects.

Pluto is now one of five dwarf planets, here they are in order of their distance from the Sun.:

Astronomers think they will find more dwarf planets soon.

Structure

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.

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

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

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

The Oort cloud is separate from the Tran-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 plane. The farther away from this plane a planet orbits, the more inclined is its orbit. If you could look at the solar system "edge on" then all the planets would be roughly in a horizontal plane around a centre line of the Sun, but not around the Sun's equator.

Discovery

Before the discovery of Uranus, ancients thought the solar system consisted only of the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn.

Until the 16th and 17th centuries, western scientists thought the Sun and the planets orbited the Earth. This was what ancient civilizations thought too, except for a few individuals of the Greek, Indian and Muslim civilizations.

  1. Uranus, discovered in 1781
  2. Ceres, discovered in 1801, recently (2006) defined as a dwarf planet
  3. Neptune, discovered in 1846
  4. Pluto, discovered in 1930
  5. Eris, discovered in 2005

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