Exoplanets are used to the limelight – exocomets less so. Now, a fresh crop of comets found around alien stars suggests that these icy dirt-balls stalk solar systems across the Milky Way.
Discs of debris swirling around young stars clump up to form planets. Asteroids and comets are the leftovers. But until now, astronomers had not seen many examples of these intermediate clumps.
"We have evidence of the final state, which is planets," says Barry Welsh at the University of California, Berkeley. "We have evidence of the initial state, which is discs. But what about the missing link, the stuff in between?"
Speaking on 7 January at a meeting of the American Astronomical Society (AAS) in Long Beach, California, Welsh said that excitement over comet-hunting may soon surpass that for exoplanets, of which there are now hundreds of confirmed finds and thousands of candidates.
"Exoplanets are just so last year," said Welsh. "2013, whether you know it or not, is going to be the year of the comet."
Bright tails
The first star found to host comets was Beta Pictoris in 1987. Three more stars with comets turned up before 1998. But in the 1990s the first confirmed exoplanets were spotted, so astronomers switched their focus to planet-hunting.
Welsh and colleagues have now picked up the cometary trail, discovering seven more stars with comets. The individual comets around alien suns are too small to see directly, so Welsh's team looked for the chemical signatures of their tails as they are heated by their host stars.
They used the McDonald Observatory in Texas to observe about 30 young stars between May 2010 and November 2012. They found that chemical signatures in some of the stars' light varied from night to night – a likely sign of gases being emitted by comets. Some of these comets survived the stellar encounter, while others likely disintegrated near the star
Billions of worlds
Our solar system is thought to have gone through a phase about a billion years after it formed during which incoming asteroids and comets pummelled the innermost planets. Some think that this period of heavy bombardment may have brought water and the carbon-based building blocks of life to the early Earth.
That makes the presence of comets around young stars an exciting prospect, says Russel White of Georgia State University, who was not involved in the new work. If these systems have Earth-like worlds, they could be going through their own versions of heavy bombardment, he says. "It offers some confirmation that these solar systems are rich environments, with comets and asteroids and things like that – maybe even richer than our own solar system."
For now, Beta Pictoris is the only comet-bearing star also known to have a planet, a gas giant about ten times larger than Jupiter. Longer observing times with more sensitive instruments, such as the Hubble Space Telescope, might yield comets around "adult" stars, which are known to be rich in planets.
Also at the AAS meeting, Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, reported that 17 per cent of stars host a planet about the size of the Earth, amounting to 17 billion in the Milky Way alone. These aren't true Earth twins, though, as they orbit their stars closer than the planet Mercury orbits our sun.
The research also showed that 90 per cent of sun-like stars probably have planets of any size. "If you shoot a spaceship at a sun-like star, you're 90 per cent likely to hit a planet," Fressin says. "That's pretty huge."
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