Ceres

Distance: 2.55 to 2.99 AU

Equatorial diameter: 606 miles (974.6 kilometers) Mass: 9.4 X 1020 kilograms, roughly 25 to 35 percent of the main asteroid belt's total mass, or 7 percent of Pluto's mass Orbital period: 4.6 years

Ceres is the odd one out among these first five dwarf planets: It's the only one located in the asteroid belt rather than the Kuiper Belt. And although a few other asteroids are biggish and roundish, Ceres may be the only asteroid ever admitted into the dwarf-planet club. Some have even suggested that it stands out from the asteroid crowd so much that it may be an interloper from the Kuiper Belt.8

Like Pluto, Ceres will be getting a visitor in 2015: the Dawn spacecraft, which is due to go into orbit around the dwarf planet after visiting Vesta, the asteroid belt's second most massive object. The findings from Dawn could well raise some eyebrows: Ceres is thought to have a rocky core and a mantle of water ice, covered over by a crust of clay and dust. This has led some researchers to call it an "embryonic planet" whose development was put on pause. "Gravitational perturbations from Jupiter billions of years ago prevented Ceres from accreting more material to become a full-fledged planet," University of Maryland astronomer Lucy McFadden says.9

Under just the right conditions—for example, if Ceres's subsurface water layer is laced with ammonia antifreeze, and if the core is giving off radioactive heat—liquid water may still lurk in the dwarf planet's interior. But even if Ceres's water is frozen solid today, it might have stayed liquid long enough when Ceres was young to support organic chemical processes, and perhaps life.10

For that reason, the Dawn team has ruled out any scenarios that call for crashing the spacecraft on the dwarf planet's surface, for fear of contaminating a potential target for future biological study. "We go into a quarantine orbit at the end of the mission," the Planetary Science Institute's Mark Sykes says.

The IAU 's planet definition may have pushed Pluto a little farther out of the spotlight, but at the same time, it has given Ceres a bigger role in the planetary play. No longer seen as just one rock amid thousands of other bits, Ceres has regained some of the prestige it had back in 1801 when Giuseppe Piazzi hailed it as what was then the eighth planet. Today, Ceres has a revised place in history as the first dwarf planet to be discovered, and the smallest of the lot.

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