Ceres: First Asteroid, Now a Dwarf Planet

Ceres was discovered on 1 January 1801 by Giuseppe Piazzi at Palermo, the first object found in the belt between Mars and Jupiter. It was counted as a planet for several decades, until the discovery of many further bodies in the same region led to the creation of the category of asteroid and to its demotion. In 2006 the same IAU resolution that reclassified Pluto made Ceres a dwarf planet, since it is round under its own gravity but shares its orbital zone; it is much the largest object in the asteroid belt and contains a substantial fraction of its mass. The Dawn spacecraft orbited it from 2015 and found bright deposits of sodium carbonate in Occator crater and evidence of subsurface brine, so that Ceres is now understood as an ice-rich body with a history of internal water.

The Dawn mission's approach to Ceres followed an earlier visit to the asteroid Vesta, making Dawn the first spacecraft to orbit two separate extraterrestrial bodies in a single mission, using ion propulsion specifically to make that fuel-efficient double rendezvous possible; the bright deposits in Occator crater were eventually identified specifically as sodium carbonate left behind by brine that reached the surface from a subsurface reservoir and evaporated, evidence that Ceres retains at least localised liquid activity rather than being wholly frozen and inert throughout its interior.