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Water, water everywhere
Posted: 13 April

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In today’s Water in the Solar System session at the National Astronomy Meeting, compelling evidence was presented that suggests rocky planets hosting water may be commonplace throughout the Milky Way.

White dwarf expert Jay Farihi, who has been surveying these compact, dense remains of Sun-like stars, says that they show signs of contamination by heavier elements, possibly even water. White dwarfs should have pure hydrogen or helium atmospheres, so if heavier elements – metals like calcium, magnesium and iron – are detected then these must be external pollutants.

Artist impression of a massive asteroid belt in orbit around a star. The new work with SDSS data shows that similar rubble around many white dwarfs contaminates these stars with rocky material and water. Image: NASA-JPL / Caltech / T. Pyle (SSC)

Theory suggests that the interstellar medium is the source of these pollutants, but Farihi and his team used Sloan Digital Sky Survey (SDSS) data to analyse numerous white dwarf stars to show that this is no longer a valid theory. Instead, rocky, planetary debris is almost certainly the culprit.

The new research implies that between three and 20 percent of all white dwarfs are contaminated in this way, with the debris most likely in the form of rocky minor planets amounting to a total mass of about that of a 150 kilometre wide asteroid. The implications are profound, suggesting that many other stars like our Sun and a little larger, such as Vega and Fomalhaut, might build terrestrial planetary systems. More intriguing is the signature of water, suggesting that the frequency of habitable planets might be greater than previously conceived.

“In our own Solar System with at least one watery, habitable planet, the asteroid belt – the leftover building blocks of the terrestrial planets – is several percent water by mass,” says Farihi. “From our study of white dwarfs, it appears there are basic similarities found among asteroid-like objects around other stars; hence it is likely a fraction of these white dwarfs once harbored watery planets, and possibly life.”

In the same session Dr Henry Hsieh of Queen’s University Belfast presented a new class of small bodies in our Solar System known as Main Belt Comets (MBCs). Residing in the Asteroid Belt and on the same orbits as asteroids, less than half a dozen examples have been identified that display cometary behaviour, such as sublimating ice. Fragmentation events, not ruling out impact events, might be responsible for triggering the sublimation of the ice and shows that subsurface ice can survive for millions, if not billions, of years in the inner Solar System. The presence of water-bearing objects in the inner Solar System presents strong astrobiological implications for the Earth’s primordial water source.

Combined with other presentations that revealed watery signatures in exoplanet atmospheres, today's session shows that water is really is everywhere in the Milky Way!