French astronomers using the HARPS instrument on the ESO 3.6-m telescope at the La Silla Observatory in Chile have discovered nearly 500 comets around the nearby star Beta Pictoris and have found that they belong to two distinct families: old comets that have made multiple passages near the star, and younger comets that probably came from the recent breakup of one or more larger objects.
This artist’s impression shows exocomets orbiting Beta Pictoris. Image credit: ESO / L. Calçada.
Beta Pictoris is a young star located about 63 light-years from Earth. It is only about 20 million years old and is surrounded by a huge disc of material.
For almost three decades, astronomers have seen subtle changes in the light from this star that were thought to be caused by the passage of comets in front of the star itself.
Comets are small bodies of a few kilometers in size, but they are rich in ices, which evaporate when they approach their star, producing gigantic tails of gas and dust that can absorb some of the light passing through them. The dim light from the exocomets is swamped by the light of the star so they cannot be imaged directly from Earth.
To study the exocomets around Beta Pictoris, a team of astronomers headed by Dr Flavien Kiefer of the Institute of Astrophysics in Paris and Tel Aviv University analyzed more than 1,000 observations obtained between 2003 and 2011 with the HARPS instrument at ESO’s La Silla Observatory.
They selected a sample of 493 different exocomets. Careful analysis provided measurements of the speed and the size of the gas clouds. Some of the orbital properties of each of these exocomets, such as the shape and the orientation of the orbit and the distance to the star, could also be deduced.
The analysis revealed the presence of two distinct families of exocomets: one family of old exocomets whose orbits are controlled by a massive planet, and another family, probably arising from the recent breakdown of one or a few bigger objects. Different families of comets also exist in our Solar System.
The exocomets of the first family have a variety of orbits and show a rather weak activity with low production rates of gas and dust. This suggests that these exocomets have exhausted their supplies of ices during their multiple passages close to Beta Pictoris.
The exocomets of the second family are much more active and are also on nearly identical orbits. This makes them similar to the comets of the Kreutz family in the Solar System, or the fragments of Comet Shoemaker-Levy 9, which impacted Jupiter in July 1994.
According to the team, these comets all arise from the same origin: probably the breakdown of a larger object whose fragments are on an orbit grazing the star.
“For the first time a statistical study has determined the physics and orbits for a large number of exocomets. 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,” concluded Dr Kiefer, who is the first author of the paper published in the journal Nature.
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F. Kiefer et al. 2014. Two families of exocomets in the β Pictoris system. Nature 514, 462–464; doi: 10.1038/nature13849
