Oceans 'may be common on planets'
Earth-like oceans may be common on planets in other solar systems, greatly increasing the chances of extra-terrestrial life, evidence suggests.
The oceans on Earth were filled long ago by water-bearing comets and asteroids, scientists believe.
Now new observations of a distant star at the end of its life have indicated that such water carriers are common in our galaxy, the Milky Way.
Astronomer Dr Roberto Raddi, from the University of Warwick, said: "Our research has found that, rather than being unique, water-rich asteroids similar to those found in our solar system appear to be frequent. Accordingly, many of planets may have contained a volume of water, comparable to that contained in the Earth.
"It is believed that the Earth was initially dry, but our research strongly supports the view that the oceans we have today were created as a result of impacts by water-rich comets or asteroids".
The scientists used the William Herschel Telescope in the Canary Islands to detect a large amount of hydrogen and oxygen - the constituents of water - in the atmosphere of a "white dwarf", the compact remnant of a sun-like star.
This was direct evidence of water being delivered to the star by a large body. The object would have been similar in size to Ceres, the largest asteroid in our solar system, measuring 900 kilometres (559 miles) across.
"The amount of water found ... is equivalent to 30 - 35% of the oceans on Earth," said Dr Raddi.
The impact of water-bearing asteroids or comets onto planets or white dwarfs would result in a mix of hydrogen and oxygen in their atmospheres.
Large quantities of both elements were found in the atmosphere of the white dwarf, known as SDSS J1242+5226.
Co-author Professor Boris Gansicke, also from the University of Warwick, said: "Oxygen, which is a relatively heavy element, will sink deep down over time, and hence a while after the disruption event is over, it will no longer be visible.
"In contrast, hydrogen is the lightest element; it will always remain floating near the surface of the white dwarf where it can easily be detected.
"There are many white dwarfs that hold large amounts of hydrogen in their atmospheres, and this new study suggests that this is evidence that water-rich asteroids or comets are common around other stars than the sun".
Water is regarded as an essential pre-requisite for Earth-like life.
Many planets have been identified that occupy "habitable" orbital paths around their parent stars where temperatures are mild enough for liquid surface water to exist. But how many of them actually possess water is unknown.
The findings appear in the journal Monthly Notices of the Royal Astronomical Society.