'Exocomet' numbers nearly tripled in new study

Beta Pictoris Beta Pictoris was the first star outside our Solar System known to host a comet

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A new haul of comets around distant stars has been unveiled, more than doubling the number we know of.

The first such "exocomet" was discovered in 1987 but since then only three more had been found.

At the 221st American Astronomical Society meeting in the US, astronomer Barry Welsh gave details of seven more.

Proving that comets are common in the Universe has implications for their role in delivering water or even the building blocks of life to planets.

Comets such as Halley's Comet, which makes a long, elliptical path passing near the Sun every 75 years, make themselves known through their long "tails" of gas and debris that come off as they approach their host stars.

It is this that Dr Welsh and his collaborator Sharon Montgomery of Clarion University have measured, using the McDonald Observatory in Texas.

The exocomets' tails absorb a tiny amount of their host stars' light - and the absorption changes with time as the comets speed and slow.

With patient observation, the pair came up with seven new exocomet sightings.

In our Solar System, many comets come from the Kuiper belt, a disc of debris beyond the orbit of Neptune, and from the Oort cloud, an even larger and more distant debris disc.

Dr Welsh explained that these discs were characteristic "leftovers" of planet formation as we now understand it.

"Imagine a 'cosmic building site', where the building has already been made - the planets," he told BBC News.

"We're looking at what's left: the bricks, the mortar, the nails - the debris discs have comets, planetesimals, and asteroids."

But something must disturb the comets' orbits, putting them on a course toward their star.

While collisions between comets might do that, it is believed that the gravity of planets nearby can do the job.

In fact, in 1987 when the first exocomet was spotted around the star Beta Pictoris, it was hypothesized that a planet may have been responsible - and in 2009, a giant planet was found here.

Holds water

Recent years have seen a marked focus on exoplanets, with 461 new candidates and the prospect of billions more that are Earth-sized announced on Monday.

The new study helps illuminate the interplay between those planets and the debris discs from which they came - and in turn help to explain how our own Solar System formed.

"It looks as though the planet building process is very similar in many, many cases - and in order to prove that you need to look not only at the final product and also at the things they were made from," Dr Welsh said.

The finding of more and more comets also raises the possibility that comets could play a crucial role in delivery services.

"There are two theories: one is that comets early on in our Solar System's history brought ice to the planets, the ice melted and formed oceans," Dr Welsh explained.

"And the other one, perhaps a bit more far fetched, is that the organic [molecules in comets]… were the seeds of life on planets. And if comets are so common throughout all planetary systems, then perhaps life is as well."

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