The smallest dwarf planet in the Solar System, or its largest asteroid depending on your point of view, has a secret.
Astronomers have announced that water has been found on Ceres, a ridiculously small almost-planet orbiting far from our Sun in the asteroid belt.
Using the infrared Herschel Space Telescope, scientists from the European Space Agency were able to spot water vapour erupting up from the dwarf planet and confirm that it has both an icy surface and an atmosphere.
Above: artist's conception of Ceres
Ceres is roughly 590 miles in diameter, and when it was first found in 1801 scientists supposed it was a planet orbiting between Mars and Jupiter. But further study has shown it is in reality just another (very large) asteroid with a rocky interior.
It was eventually named a dwarf planet, along with Pluto, in 2006 by the International Astronomical Union.
NASA currently has a space craft on its way to Ceres - the Dawn mission - which is scheduled to arrive in 2015. But until it gets there we have to rely on far-off observations made closer to home.
Even so scientists are able to look in detail at its surface, and note the definite presence of water.
"This is the first time water vapour has been unequivocally detected on Ceres or any other object in the asteroid belt and provides proof that Ceres has an icy surface and an atmosphere," said Michael Küppers of ESA in Spain, lead author of a paper in the journal Nature.
Nasa said the findings blurred the lines between asteroids and comets.
"The lines are becoming more and more blurred between comets and asteroids," said Seungwon Lee of Nasa's JPL.
"We knew before about main belt asteroids that show comet-like activity, but this is the first detection of water vapour in an asteroid-like object."
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