Ceres

Dwarf planet · Dwarf planet (asteroid belt) · orbits Sun

Ceres is the largest object in the asteroid belt and the only dwarf planet of the inner Solar System - alone among the asteroids, its own gravity has pulled it into a sphere. NASA’s Dawn spacecraft orbited it in 2015-2018 and found a dark, cratered world punctuated by startling bright spots of salt.

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Quick facts

It holds about a third of the asteroid belt’s entire mass, yet is still ~14 times less massive than Pluto.
The bright spots in Occator crater are sodium-carbonate salts left behind by briny water seeping up from below.
Being ~25% water ice, Ceres may contain more fresh water than all of Earth’s rivers, lakes and ice caps.

Planet, asteroid, dwarf planet

Ceres has been reclassified twice without changing at all. Piazzi announced it as a planet in 1801; once dozens of similar bodies turned up in the same zone, astronomers demoted it to asteroid; in 2006 the IAU promoted it to dwarf planet. Categories are human bookkeeping - the object was always the same round, ice-rich world.

Physical characteristics

Diameter	940 km
Mass	9.400e+20 kg
Surface gravity	0.28 m/s²
Density	2.16 g/cm³
Rotation period	9.07 h
Axial tilt	4°
Mean temperature	-105 °C
Geometric albedo	0.09
Known moons	0

Atmosphere and composition

Atmosphere	None to speak of - transient wisps of water vapour have been detected escaping from its surface.
Composition	A rocky interior under a water-rich crust of ice, clays and salts; roughly 25% of Ceres is water ice.

Interior structure

Rocky core modelled

Hydrated rock - water has altered it through and through.

Briny ice-rock mantle modelled

Pockets of salty brine survive here - the source of the bright spots.

Dusty crust modelled

A mix of clays, salts and ice, darkened by carbon-rich dust.

How we know: Dawn orbited Ceres for three years; its gravity mapping and the salty bright spots in Occator crater drive this partially differentiated model.

Discovery

Discovered by	Giuseppe Piazzi
Year	1801
How	Palermo Observatory, while charting stars - the first asteroid ever found

Missions

Dawn NASA · Orbiter · 2007–2018 · completed

Powered by gentle ion engines, Dawn became the only spacecraft ever to orbit two different deep-space worlds: the asteroid Vesta (2011-2012) and the dwarf planet Ceres (2015-2018). It ran out of fuel in 2018 and remains in a stable orbit around Ceres.