A comet blazing in the night sky can be a spectacular sight, with its bright gaseous tail liberated from the icy nucleus by the heat of the Sun. A handful of stars are now known to also harbour comets, and new research suggests that these could be as common as exoplanets.
Comets are small astronomical bodies, usually measuring between five and twenty kilometres in diameter, and they can be difficult to detect even in our own Solar System. So how can they be detected around other stars?
The answer lies not in direct detection, but in the spectra of the stars themselves. Just as in our own Solar System, a comet orbiting another star will heat up as it approaches the star, releasing a plume of gas from the nucleus. This gas can be detected as short-lived lines in the stellar spectrum, as some of the star’s light is absorbed by the gas of the comet.
These telltale spectral features vary on a night by night basis, and sometimes they even change over a few hours. “The reason they vary in velocity is that they are tracking the grazing approach of a gaseous comet towards the central star,” explains Barry Welsh of the University of California, Berkeley. “As it nears the star, the gas evaporates.”
Most of the stars known to have exocomet activity also emit excess infrared radiation that indicates the presence of debris discs surrounding the stars. This means that is also a possibility that the absorption lines are actually associated with this disc. However, because these lines are typically redshifted, and move over a short period of time, the most likely candidates for this behaviour are exocomets.
The six stars with exocomet activity discovered by Welsh and his team are not the first; exocomets were first reported around Beta Pictoris in 1987. Suspicions were raised that this star might have a large exoplanet orbiting it, as this would explain how the comets got kicked inwards towards the star.
There is also evidence of a gap in the debris disc, which was likely caused by a planet accreting material from the disc. These suspicions were confirmed in 2010, when a planet eight time the mass of Jupiter was discovered around Beta Pictoris. None of the other stars with exocomets have detected planets, but Welsh believes it is possible that all of them do.
After Beta Pictoris, three more stars revealed their cometary companions. “But then, people just lost interest. They decided that exocomets were a done deal, and everybody switched to the more exciting thing, exoplanets,” explained Welsh. “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.'”
So far, exocomets have only been detected around A-type stars of around five million years old. “The detection technique only works around young and hot stars, and you require many nights of observation per star to determine the absorption profile variability,” Welsh told Skymania News. “You can go a few nights without seeing any activity, and then on the next night you might see two or three exocomet features.”
Further observations and the use of higher resolution spectrographs might be able to push the boundaries towards older and cooler stars, and open the floodgates for exocomet discoveries.
The research was presented at the AAS meeting in California this week. Three of the exocomet-bearing stars were reported in the October 2012 issue of the Publications of the Astronomical Society of the Pacific.