Comets may be creating oceans on alien planet

Comets have been caught battering an exoplanet for the first time, new observations suggest. If the existence of the planet is confirmed, the finding means that the impacts are bringing water and organic material ? the essential ingredients for life ? to a world that lies in the habitable zone around its star.

The cometary shower is taking place around a bright star about 60 light years away called Eta Corvi, which is visible to the naked eye in the northern sky.

The Spitzer Space Telescope spotted the infrared glow of a band of dust three times as far from Eta Corvi as Earth is from the sun. Carey Lisse of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, and his colleagues analysed the spectrum of light from this glow and found that it contains water, organics and rock.

The composition and amounts seen suggest that several small comets, or a single large one, crashed into a rocky world weighing up to a few times the mass of the Earth, creating a trail of debris behind the planet. For example, the dust seems to contain nanodiamonds, which form when organic materials smack into each other at ludicrous speeds, and bits of silica ? essentially glass, which forms when rock melts and then quickly re-freezes.

Astronomers already knew that Eta Corvi had a stockpile of comets: a bright ring of cold dust is seen about 150 times as far from the star as Earth is from the sun. Our solar system has an equivalent band called the Kuiper belt, an icy reservoir of leftover planet pieces where comets are born.

Barrage of impacts

The Spitzer observations suggest the planet, whose existence has not been confirmed by other methods, is suffering its own version of the solar system's "late heavy bombardment", in which a barrage of comets scarred the inner planets around 4 billion years ago. It was triggered when Jupiter and Saturn shifted positions, flinging icy bodies from the solar system's frozen fringe inwards. Eta Corvi, a relatively young star, might have a distant Neptune-like planet doing the same.

"What we've done is looked at a nearby star that's about the same age as our sun was when this happened, and we can actually see it going on, see the process of this happening," Lisse said in a press teleconference on Wednesday.

Some astrobiologists believe that comets carried water and organics ? the building blocks of life ? to Earth. Life on Earth emerged suggestively soon after the late heavy bombardment ended around 3.8 billion years ago, Lisse noted, and it wouldn't have taken much water to make the dry planet habitable.

Conveniently, the comet collision appears to be right at the distance from the star where liquid water can exist on a planet's surface. "We're showing the mechanism for [water] delivery is possible, at least in one star system," Lisse said. "The delivery of water and organics is to a place where you could actually grow life, as we know it."

Common process?

The researchers also found that the cloud around Eta Corvi also matches the composition of the Almahata Sitta meteorite, which astronomers tracked as it fell to Earth in 2008. The similarity suggests that the meteorite had its origins in the Kuiper belt.

"This could be a direct example of bringing water, organics and things that help life grow to the Earth," Lisse said.

Although they now have two examples of comet showers raining down on infant rocky worlds, Lisse and colleagues aren't sure it's a common occurrence in nascent solar systems. "It's not clear to me whether this is a typical system," he said. If late heavy bombardment-type events are very rare, it could explain why life appears to be rare as well. It is possible that "you don't form life unless they happen", he said.

Lisse presented the results on Wednesday at the Signposts of Planets meeting at NASA's Goddard Spaceflight Centre in Greenbelt, Maryland. They will also appear in the Astrophysical Journal.

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