What comprises the Solar System? Our Solar System consists of the Sun and the family of planets, natural satellites, asteroids, meteors, comets, and some interplanetary dust and gas. The planets of the Solar System are (in order) : Mercury, Venus, Earth , Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. What's the difference between a "Planet" and a "Moon"? The difference between a "Planet" and a "Moon" doesn't have anything to do with size. In fact, there are moons which are bigger than some planets! Planets are satellites of the Sun - they orbit around it. Moons orbit around the planets. COMMON CHARACTERISTICS OF THE PLANETS ORBIT: Every planet in our Solar System revolves around the Sun in the same direction, in a nearly circular orbit and in approximately the same plane (the plane of the ecliptic), except for Pluto (the oddball), which has a very tilted and elliptical (or oval) orbit. ROTATION: Almost all of the planets (with the exception of Venus and Uranus) and nearly all of the planetary moons rotate counterclockwise. Venus rotates clockwise, and Uranus rotates on a horizontal axis rather than a vertical one. Most of the planets have an axis that is perpendicular to the plane of the ecliptic (except for Uranus, which rotates nearly horizontal to the ecliptic (kind of like a top that has fallen over and keeps spinning). THE INNER PLANETS VS. THE OUTER (GIANT) PLANETS Inner planets have a high density (4.0 to 5.5 g/cm^3). Outer giant planets are less dense than the inner planets (0.7 to 1.7 g/cm^3). Outer planets are colder, and can therefore retain light gases such as helium and hydrogen effectively. Furthermore, since the outer planets are more massive, they have a higher gravity which also helps them to keep the lighter gases such as hydrogen and helium. Inner planets are small, and are composed of rock and metallic elements. Outer planets are a lot bigger and have more massive cores than the inner planets. What this means is that the outer giant planets are big balls of gas, compressed by their own weight, while the inner planets are made of rocks. Outer giant planets rotate faster. Rings are common features of the outer planets. Saturn has the most prominent set but rings can also be found around Jupiter, Uranus and Neptune. Inner planets do not have rings. Outer planets have significantly more moons (between 8 and 20, possibly more). The satellites (moons) of the outer planets vary greatly in size. Objects up to twice the size of Earth's moon can be found as well as objects that are range from only about ten km up to thousands of kilometers in size. THE ODDBALL: PLUTO Pluto really doesn't belong in the same category as the outer planets as it shares none of their properties. Pluto is the smallest planet, composed mostly of ice and rock. It is smaller than Earth's moon, but because it is in orbit about the Sun, we call it a planet. Material that is as far out in the Solar System as Pluto will be primarily made of ices. In composition, Pluto is basically a large comet. The orbit of Pluto is also much more eccentric and inclined to the orbital plane than any of the other planets. There is research underway to determine whether Pluto originated from the Kuiper Disk, perhaps as the result of a collision between large objects there.