Solar System Overview — Asteroids, Comets, Formation

The Solar System is dominated by the Sun, which holds ~99.8 % of its mass. Eight planets orbit the Sun, all in nearly the same plane (the ecliptic) and the same direction. Beyond them lie dwarf planets (Pluto, Ceres, Eris), 180+ moons, and vast populations of asteroids, comets, and meteoroids.

The planets split cleanly into two groups. Terrestrial planets — Mercury, Venus, Earth, Mars — are small, dense, rocky/metallic, close to the Sun, with thin or no atmospheres and few moons. Jovian planets — Jupiter, Saturn, Uranus, Neptune — are huge, low-density, gaseous (mostly hydrogen and helium), far from the Sun, with thick atmospheres, ring systems, and many moons.

Asteroids are rocky bodies, mostly in the asteroid belt between Mars and Jupiter. Comets are icy/dusty objects with a nucleus, a glowing coma, a hydrogen envelope, and two tails — a straight ion tail and a curved dust tail, both pointing away from the Sun. Short-period comets come from the Kuiper Belt (30–50 AU); long-period comets come from the much more distant Oort Cloud (2,000–200,000 AU).

The leading formation theory is the nebular hypothesis with condensation. A giant interstellar cloud of gas and dust contracted under gravity; angular momentum made it spin faster and flatten into a disk. In the hot inner disk, only rocks and metals could condense, forming the terrestrial planets. Beyond the frost line (~5 AU), water and other ices could also condense, allowing Jovian cores to grow large enough to capture H/He gas. The hot center became the Sun.

Thousands of exoplanets are now known around other stars, confirming that planet formation is a common galactic process.