Ceres, the only dwarf planet in the asteroid belt

The asteroid belt is a zone situated between the orbits of Mars and Jupiter in which we find millions of asteroids of all sizes, which go from millimetres of diameter to hundreds of metres and even kilometres. All these asteroids contain very valued elements that make them have an enormous economical value. However, the belt isn’t only formed by asteroids, but we also find Ceres, the only dwarf planet of the asteroid belt.


Ceres, like Pluto, is considered a dwarf planet. However, Ceres hasn’t always been considered a dwarf planet. When it was discovered for the first time, it was classified as an asteroid, due to its small size. However, Pluto was considered initially a planet. All these classifications changed when, in 2006, the International Astronomical Union (IAU), published the requirements that a celestial body had to fulfil to be considered a planet. This led to Pluto being demoted to the category of dwarf planet, and Ceres to be considered a dwarf planet

The classification of Ceres as a dwarf planet was due to its difference in mass and size in comparison to the asteroids that surround it. Indeed, Ceres represents the third part of the total mass of the asteroid belt. It has a diameter of approximately 950 kilometres. In comparison, Pallas, the biggest object of the asteroid belt after Ceres, has a diameter of 512 kilometres. These factors made Ceres to be considered a dwarf planet.

Thanks to its big size, Ceres was the first object of the asteroid belt to be discovered. We owe its discovery to the astronomer Giuseppe Piazzi, in January 1801. However, after discovering it he considered Ceres a comet without nebulosity. It wasn’t until 1802, a year after, that William Herschel used the term of asteroid, after astronomers realised that there was a class of very similar bodies, of which Ceres was the first to be discovered.

As I’ve said, Ceres has a diameter of about 950 kilometres, which represent a third part of the diameter of the Moon. Even if it is the biggest body of the asteroid belt, Ceres hasn’t any natural satellite. Ceres completes an orbit around the Sun every 4 years and 222 terrestrial days, and a day on this dwarf planet lasts for only 0,38 terrestrial days, that to say, 9 hours and 7 minutes.

Its composition

According to the probes that have been sent to Ceres, the interior of this dwarf planet is formed by a rocky nucleus surrounded by an ice layer. The thickness of this ice layer, between 60 and 120 kilometres, could contain about 200 million cubic kilometres of water. This would represent more than all the freshwater that we find on Earth (although in the terrestrial oceans there is much more water).

Its surface could be made of diverse hydrated materials, such as carbonates and muds, and clays rich in iron, which are some common minerals in certain asteroids. On its surface there could also be traces of many craters formed from impacts against asteroids. Moreover, in January 2014 diverse emissions of water vapour in different regions of Ceres were detected, which could suggest that there is water in the liquid state under its surface.

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