Understanding the planets and small bodies that inhabit our solar system help scientists answer questions about its formation, how it reached its current diverse state, how life evolved on Earth and possibly elsewhere in the solar system, and what characteristics of the solar system lead to the origins of life.

Inner solar system bodies are rocky, unlike the gas and water giant planets of the outer solar system. Rocky planets Mercury, Venus, Earth and Mars are thought to have formed from the accretion of dust into “planetismals,” the planetismals into proto-planets and finally the proto-planets into planets. Many details of this sequence are still unknown. Equally obscure are the histories of the inner solar system worlds: although Venus, Earth, and Mars are similar to one another, they have evolved in different ways. We now know that Mars once had water on its surface, and there are tantalizing hints that Venus might have too. Yet only the Earth is currently known to be habitable. 

The outer solar system bodies consist of four “gas giants” Jupiter, Saturn, Uranus and Neptune. These four planets do not have a defined surface; Jupiter and Saturn consist mostly of hydrogen and helium and both Uranus and Neptune consist mostly of water, methane and ammonia and are sometimes referred to as the “water giants”.  At a certain point in these planets’ atmospheres, based on depth (or at a point towards the core), the gas transitions or condenses to a liquid state. Jupiter and Saturn probably have rocky cores surrounded by metallic hydrogen. Uranus and Neptune are different in that they are composed of rock, water, methane and ammonia. However, their outer atmospheres are comprised of hydrogen which is similar to Jupiter and Saturn’s. These planets combined have over a hundred moons that orbit them and Saturn’s ring system is thought to be a pulverized moonlet.

The small bodies in the solar system include comets, asteroids, the objects in the Kuiper Belt and the Oort cloud, small planetary satellites, Triton, Pluto, Charon, and interplanetary dust.  As some of these objects are believed to be minimally altered from their state in the young solar nebula from which the planets formed, they may provide insight into planet Earth and the formation and evolution of the solar system.