The Solar System formed 4. 6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system’s mass is in the Sun, with the majority of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.
Only recently have space missions begun to solve this beguiling Solar System mystery--that a small number of distant moons have been successfully hiding, from the curious eyes of astronomers, life-sustaining liquid water beneath secretive shells of ice.
Earlier theories suggested that the craggy outline of a region of the lunar surface, named Oceanus Procellarum--or the Ocean of Storms--had resulted from a large asteroid impact. If this theory had been correct, the basin it had dug out would represent the largest asteroid impact basin scarring the lunar surface. However, mission scientists, scrutinizing GRAIL data, now believe that they have discovered new evidence that the craggy outline of this rectangular region--approximately 1,600 miles across--was actually caused by the formation of ancient rift valleys.
Most of the moons of our Sun's family circle the quartet of large gaseous planets located in our Solar System's outer limits: Jupiter, Saturn, Uranus and Neptune. The four solid inner planets--Mercury, Venus, Earth and Mars--are almost entirely moonless. Earth is the only inner planet that hosts a large Moon, while Mars sports only a pathetic duo of misshapen little Moons (Phobos and Deimos), that are either captured asteroids that escaped from the Main Asteroid Belt between Mars and Jupiter, or are instead the outcome of a primordial collision between Mars and a large protoplanet.