To the collection of known exoplanets, worlds circling distant stars, you can now add a new class of cosmic inhabitants, astronomers report: say hello to "exocomets."

An instrument at an observatory in Chile has made the most complete census ever taken of comets surrounding another star, astronomers with the European Southern Observatory, which operates the La Silla facility, say.

A study of almost 500 comets in orbit around the star known as Beta Pictoris shows they are members of two separate "families," they say; old exocomets that've undergone multiple journeys around the star and a group of younger ones that probably originated in a recent breakup of a larger object or objects.

Similar "families" of comets exist within our own solar system, the researchers say.

In addition to the comets, Beta Pictoris -- a relatively young star at just 23 million years old -- has a freshly formed planet, Beta Pictoris b, orbiting it, they say.

That planet may be why some of the comets in the inner, older "family" appear to be ensnared in an orbital resonance, herded around the star by the planet's gravity, they say.

"This resonance is very similar to the influence of Jupiter in our own solar system, which produces most of the short-period comets around the sun," study lead author Flavien Kiefer says. "We could be seeing some of the ejected remnants from the formation of Beta Pictoris b ... It's like we are observing a much younger version of our sun, just after it formed its planets."

To study the swarms of exocomets swirling around Beta Pictoris 63 light years from Earth, the researchers analyzed more than 1,000 observations obtained between 2003 and 2011 with the ESO's 3.6-metre telescope in Chile.

Such a study of several hundred exocomets orbiting a single distant star if unique, the researchers say.

"For the first time a statistical study has determined the physics and orbits for a large number of exocomets," Kiefer says. "This work provides a remarkable look at the mechanisms that were at work in the Solar System just after its formation 4.5 billion years ago."

One mystery of the exocomets is still unexplained, the researchers say; where is their expected water?

If they are like comets in our solar system, they should be icy and water-rich, but despite years of study researchers say they've seen no signs of water vapor evaporating off of them as happens with comets that approach the Earth.

One possibility is that they consist mainly of carbon dioxide or carbon monoxide ices instead of water, they speculate, which could be disappointing for astrobiologists looking for water-rich habitable planets around distant stars.

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