For the first time, scientists have found evidence of a rocky planet outside our solar system that once had lots of water — a key requirement for life as we know it — suggesting there may have been habitable planets around its star.

The water-rich minor planet once orbited GD61, a star located about 150 light years away from Earth.

The star ran out of fuel about 200 million years and shrank down to a cool and dense white dwarf. In the process, the star’s gravity pulled its planets out of their usual orbit and pulverized them into a disk of dust and debris enveloping the star itself.

An analysis of the debris shows it came from a rocky minor planet or asteroid that once contained 26 per cent water by mass, reported a paper published online in the journal Science Thursday.

That would make it similar in composition to Ceres, the largest object in the asteroid belt between Mars and Jupiter, which has huge amounts of ice buried under its outer crust. Such water-rich asteroids are thought to be the source of most of the water on Earth, said a news release from the University of Warwick, where Boris Gansicke, one of the paper’s co-authors, works.

Jay Farihi of the Institute of Astronomy at Cambridge University, lead author of the paper, said in a statement that the discovery means “the building blocks of habitable planets” may exist around GD61 and other stars like it.

"These water-rich building blocks, and the terrestrial planets they build, may in fact be common," Farihi added.

"Our results demonstrate that there was definitely potential for habitable planets in this exoplanetary system."

Hubble telescope analyzed chemistry of debris

It is normally very difficult for astronomers to find out the chemical composition of planets outside our solar system. In this case, it was possible because the planet was no longer whole, but broken up into dust. That dust was analyzed using the Hubble Space Telescope and the W.M. Keck Observatory on Mauna Kea, Hawaii, which scanned the light from the star for the chemical fingerprints of different elements.

The amount of metal they detected indicates that the debris came from an asteroid or planet that was at least 90 kilometres in diameter, but probably much larger.

The researchers detected large amounts of oxygen in the debris. They assumed that all the magnesium, aluminum, silicon, calcium and iron they detected was bound to oxygen, and then calculated how much was left over. The leftover oxygen likely came from water, they suggested, providing an estimate of the amount of water in the original planet or asteroid.