Astronomers from the University of California in Berkeley and the Clarion University in Pennsylvania using 2.1-m telescope of the McDonald Observatory in Texas have detected six possible alien comets circling distant stars.

The newly discovered exocomets – 49 Ceti, 5 Vulpeculae, 2 Andromedae, HD 21620, HD 42111 and HD 110411 – were detected around very young type A stars, which are about 5 million years old.

According to the astronomers, the discovery suggests that exocomets are just as common in other stellar systems with planets.

“Though only one of the 10 stars now thought to harbor comets is known to harbor planets, the fact that all these stars have massive surrounding disks of gas and dust a signature of exoplanets – makes it highly likely they all do,” said Dr Barry Welsh of the University of California Berkeley’s Space Sciences Laboratory, who presented the findings at 221st meeting of the American Astronomical Society in Long Beach, California.

“This is sort of the missing link in current planetary formation studies. We see dust disks – presumably the primordial planet-forming material – around a whole load of stars, and we see planets, but we don’t see much of the stuff in between: the asteroid-like and the comets. Now, I think we have nailed it. These exocomets are more common and easier to detect than people previously thought,” said Dr Welsh, who also reported three of the newly found exocomets in the journal Publications of the Astronomical Society of the Pacific.

Dr Welsh summarized the current theory of planet formation as “interstellar dust under the influence of gravity becomes blobs, and the blobs grow into rocks, the rocks coalesce and become bigger things – planetesimals and comets – and finally, you get planets.”

Many stars are known to be surrounded by disks of gas and dust, and one of the closest, beta-Pictoris, was reported to have comets in 1987. In 2009, astronomers found a large planet around β-Pic about 10 times larger than Jupiter. Three other stars – one discovered by Welsh in 1998 – were subsequently found to have comets.

“But then, people just lost interest. They decided that exocomets were a done deal, and everybody switched to the more exciting thing, exoplanets,” Dr Welsh said. “But I came back to it last year and thought – four exocomets is not all that many compared to the couple of thousand exoplanets known – perhaps I can improve on that.”

Detecting comets may sound difficult – after all, the snowballs are typically only 5-20 kilometers (3-13 miles) in diameter.

“But once comets are knocked out of their parking orbit in the outer reaches of a stellar system and fall toward a star, they heat up and evaporate. The evaporating comet, which is what we see with comets such as Halley and next year’s highly anticipated Comet ISON, creates a brief, telltale absorption line in the spectrum of a star.”

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Bibliographic information: B. Welsh, S. L. Montgomery. Exo-comet Detection in Debris Disks Around Young A-type Stars. 221st AAS Meeting. Long Beach, CA. January 7, 2013

S.L. Montgomery, B.Y. Welsh. 2012. Detection of Variable Gaseous Absorption Features in the Debris Disks Around Young A-type Stars. Publications of the Astronomical Society of the Pacific, vol. 124, no. 920, pp. 1042-1056; doi: 10.1086/668293