Just as we have grown accustomed to stories of increasingly small planets being discovered around distant stars, along comes a new study that ups the ante.
This week, for the first time ever, scientists have announced evidence of icy comets orbiting a sun-like star about 160 light-years from Earth. Consider them the first known exocomets.
The work will be published in Monthly Notices of the Royal Astronomical Society.
The international team that worked on the observations were not able to see the comets directly. Instead, they used the European Space Observatory's ALMA radio telescope to measure low levels of carbon monoxide gas in a debris disk that had formed around the star.
A debris disk is exactly what it sounds like -- a disk of dust and debris that can be found around both new and mature stars. It is not the same as a protoplanetary disk, which is a disk of gas and dust that forms around young stars and can form planets. Debris disks are produced when large bodies collide during the protoplanetary disk phase.
"A system is usually expected to evolve from a protoplanetary disk phase to a debris disk phase," said Sebastian Marino, a doctoral student at Cambridge's Institute of Astronomy and the lead author on the paper.
The authors were drawn to look at this particular system, around a star called HD 181327, because it happens to be especially dusty, and therefore probably had a lot of collisions.
The star is 30% larger than the sun and just 23 million years old. For context, our sun is 4.6 billion years old. Using ALMA, the researchers were able to detect carbon monoxide in the ring, which was probably released as icy comets collided with one another, releasing gas and dust.
"The gas must be continually produced [for us to detect it], as well as the dust, so the natural explanation is that they are both being produced by the same phenomenon," Marino said. "As always in science, there could be other explanations, but the most simple and least biased is that it is coming out from icy bodies, i.e. exocomets."
Using computer models, the researchers were able to determine that if all the carbon monoxide they detected came from a single body, it would be 124 miles across. However, it is more likely that the gas is being generated by several comets much smaller than that.
Although the researchers were able to detect exocomets in this system, they have not yet been able to detect exoplanets. That's because planets are very compact and faint compared with their host star, Marino said. On the other hand, dust or gas can be much brighter than a planet, and therefore easier to spot.
Marino said detections of comets outside the solar system could one day help researchers determine what systems are suited for life.
"Comets can deliver large amounts of volatile elements, such as water or carbon monoxide, to terrestrial planets," he said. "This can change or create the right atmospheric conditions for the development of life on a planet."