The remnants of a lost water world of rocks and oceans have been discovered by astronomers – who said they had found the basic ingredients for a habitable planet beyond our Solar System. Astronomers believe the tiny blip on their telescope screens some 150 light years from Earth was once a rocky planet with huge amounts of water – the first time they have found rocks and water together in one place outside the Solar System.

Water and a rocky surface are assumed to be vital for the origin of life and the discovery of both substances in deep space indicates that the basic building blocks for habitable planets may be widely distributed throughout the cosmos, scientists said.

The rocky planetary body or asteroid is orbiting a star called GD 61, a “white dwarf” where the star’s nuclear fuel has been exhausted. The asteroid is believed to be the remnants of a small, watery planet that was knocked out of its original orbit and pulled so close to its sun that it was shredded in the process.

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Scientists said that the original planet was at least 90km in diameter, making it a minor planet in terms of size, and that it was once composed of 26 per cent water – by contrast the Earth is just 0.023 per cent water.

“At this stage in its existence, all that remains of this rocky body is simply dust and debris that has been pulled into the orbit of its dying parent star,” said Professor Boris Gänsicke of the University of Warwick.

“However, this planetary graveyard swirling around the embers of its parent star is a rich source of information about its former life. In these remnants lie chemical clues which point towards a previous existence as a water-rich terrestrial body,” Professor Gänsicke said.

“Those two ingredients – a rocky surface and water – are key in the hunt for habitable planets outside our Solar System so it’s very exciting to find them together for the first time outside our Solar System,” he said.

The study, published in the journal Science, used observations from the Hubble Space Telescope and the giant Keck telescope at Mauna Kea in Hawaii, and was based on the observation that the asteroid contains a large amount of oxygen but little or no carbon – suggesting the presence of water.

“The finding of water in a large asteroid means the building blocks of habitable planets existed – and maybe still exist – in the GD 61 system, and likely also around a substantial number of similar parent stars,” said Dr Jay Farihi of Cambridge University’s Institute of Astronomy, the study’s lead author.

“These water-rich building blocks, and the terrestrial planets that they build, may in fact be common.

“A system cannot create things as big as asteroids and avoid building planets, and GD 61 had the ingredients to deliver lots of water to their surfaces,” Dr Farihi said.

“Our results demonstrate that there was definitely potential for habitable planets in this exoplanetary system,” he said.

n A separate team of astronomers have discovered a lonely planet floating through space without a companion star, the first planet to be found without a sun.

The cold, dark planet is about six times the mass of Jupiter – a “gas giant” planet – and is estimated to have formed just 12 million years ago, making it a mere infant in astronomical terms.

Known by its code name, PSO J318.5-22, it was discovered about 80 light years away from Earth by scientists analysing data from the Pan-STARRS 1 telescope sited on top of the Haleakala volcanic mountain on the island of Maui in Hawaii.