Spanning nine billion miles, our solar system is home to hundreds of thousands of celestial bodies including dwarf planets, moons, comets, and asteroids.  The largest and most studied however, are the eight planets. 

The four planets closest to the Sun (Mercury, Venus, Earth, and Mars) are known as terrestrial planets, because they have solid rocky surfaces.  The next two planets, Jupiter and Saturn, are known as gas giants because they do not have solid cores.  Finally, the two farthest planets from the Sun, Uranus and Neptune, are ice giants, with rocky cores coated in a thick layer of ice.

All of the planets have an atmosphere, but Earth’s is the only one that humans can survive in.  Most of the planets also have magnetic fields that extend into space.  These form magnetospheres that pull in charged particles.

Astronomers first discovered Saturn’s rings in 1659.  For over 300 years, humans believed Saturn was the only ringed planet, but in fact Jupiter, Uranus, and Neptune have rings as well.  All but two of the planets (Mercury and Venus) have moons.  There are over 140 known moons, with’s at least 27 more awaiting official validation.  Moons vary greatly – Saturn’s Titan has a thick atmosphere, while Jupiter’s Io has active volcanoes.  Another of Jupiter’s moons, Europa, is believed to have an ocean twice the size of Earth’s.

Although we have discovered a great deal about the planets, there is still much we do not know.  Better understanding of our planetary neighbors can give greater insight into our own world.  While Earth-based telescopes can provide us with some answers, space missions can help unlock the mysteries of our solar system.