As the sun formed, 5 billion years ago, surrounding gas and debris gradually coalesced into the planets, four inner solid planets and four outer gaseous giants; Pluto's status as a planet is a matter of debate (see The Orphaned Planet). Additional debris aligned within the Asteroid Belt, between the orbits of Mars and Jupiter, the Kuiper Belt (which includes Pluto), beyond the orbit of Neptune, and the Ort Comet Cloud, near the outer edge of the solar system (see The Nature of Comets).
The Asteroid Belt consists of millions of orbiting rock units, ranging in size from the dwarf planet Ceres down to pebble-sized debris; three proto-planets (Vesta, Palias and Hygiea) also orbit within the belt. Ceres accounts for 30% of the mass of the Asteroid Belt while this dwarf planet combined with the three proto-planets harbor 50% of the total mass. Though the Belt is relatively stable at this point in time, such was not the case during the chaotic early history of our solar system; during that period, they were slung about space, slamming into the planets and pockmarking their moons (as we observe on Mars and our cratered Moon today). Of course, asteroids within the Belt also collided with one another, a process that continues today; such collisions, combined with gravitational effects from Jupiter, still send wayward asteroids into space, threatening Earth with disastrous impacts.
Space probe Dawn, launched by NASA in 2007, explored Vesta in 2011-2012 and is now orbiting Ceres, sending back fascinating photos of the asteroid's cratered surface. Of most interest are bright white spots, up to 4 miles across, which may represent mineral deposits left behind by melted surface ice; the presence of water vapor has previously been documented on this dwarf planet.
