Mercury, Venus, Earth, and Mars are the terrestrial planets. Mars is the only other terrestrial planet; it too has a solid surface but one that was once filled with water. This means that life could have arisen at some point in the history of Mars. The rest of the planets listed above can only be reached by spacecraft at this time. Of these planets, Jupiter is the largest and most massive body outside our solar system, Neptune is the farthest from the Sun, and Pluto is the smallest planet in terms of mass. Jupiter, Saturn, Uranus, and Neptune are called gas giants because they contain large amounts of gas such as hydrogen and helium. They formed around the same time as the Sun and share its orbit, but they are so large that they do not affect how far away the Sun is from the Earth during any given year. Pluto used to be a planet like the others but then was classified as a dwarf planet when scientists decided that it does not behave like the other objects in our Solar System. Currently, only Earth and spacecraft can land on other planets.