Astronomers Find Two Families of Comets Around Beta Pictoris

A French team of astronomers has discovered that two families of exocomets orbit the nearby star Beta Pictoris. The researchers used the HARPS instrument at ESO’s La Silla Observatory in Chile to make the most complete census of comets around another star ever created. "The expansive statistics of the comets that we observed with HARPS and the high quality of the instrument allowed us to distinguish two groups of detections," Flavien Kiefer of the Paris Institute of Astrophysics, lead author of the new study told astrowatch.net. "We found that these two groups were physically different and thus composed distinct families." The research was presented in a paper entitled "Two families of exocomets in the Beta Pictoris system" which is published in the journal Nature on Oct. 23.

Beta Pictoris is a young star located about 63 light-years from the Sun, 1.75 times as massive and 8.7 times as luminous as the Sun. It is only about 20 million years old and is surrounded by a huge disc of material — a very active young planetary system where gas and dust are produced by the evaporation of comets and the collisions of asteroids. ESO has confirmed the presence of a planet orbiting the star. Beta Pictoris b, has been discovered in orbit at about a billion kilometres from the star and studied using high resolution images obtained with adaptive optics.

For almost 30 years astronomers have seen subtle changes in the light from Beta Pictoris that were thought to be caused by the passage of comets in front of the star itself. Comets are small bodies of a few kilometres 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 brilliant star so they cannot be imaged directly from Earth.

To study the Beta Pictoris exocomets, the team analysed more than 1000 observations obtained between 2003 and 2011 with the HARPS instrument on the ESO 3.6-metre telescope at the La Silla Observatory in Chile.

The above composite shows the reflected light on the dust disc in the outer part, as observed in 1996 with the ADONIS instrument on ESO's 3.6-metre telescope. In the central part, the observations of the planet Beta Pictoris b obtained in 2003 and autumn 2009 with NACO are shown. The possible orbit of the planet is also indicated, albeit with the inclination angle exaggerated. Credit: ESO/A.-M. Lagrange

The researchers selected a sample of 493 different exocomets. Some exocomets were observed several times and for a few hours. 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.

This analysis of several hundreds of exocomets in a single exo-planetary system is unique. It 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 the Solar System.

"On the one hand, the comets of the first family are spread on a wide variety of orbits, while the comets of the second family roughly share the same orbit with the same orientation. On the other hand, the cometary tails of the second family absorbs much more starlight than the first family," Kiefer explained. "We showed that this can be explained if the first family is composed of aged comets influenced by the gravity of a massive planet (possibly Beta Pictoris b); while the other family is composed of fresh comets, which are most probably remnants of the fragmentation of one or a few bigger bodies."

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 comets have exhausted their supplies of ices during their multiple passages close to Beta Pictoris. Moreover, the orbits of these comets (eccentricity and orientation) are exactly as predicted for comets trapped in orbital resonance with a massive planet. The properties of the comets of the first family show that this planet in resonance must be at about 700 million kilometres from the star — close to where the planet Beta Pictoris b was discovered.

The exocomets of the second family are much more active and are also on nearly identical orbits. This suggests that the members of the second family all arise from the same origin: probably the breakdown of a larger object whose fragments are on an orbit grazing the star Beta Pictoris. This also 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.

This composite image represents the close environment of Beta Pictoris as seen in near infrared light. This very faint environment is revealed after a very careful subtraction of the much brighter stellar halo. The outer part of the image shows the reflected light on the dust disc, as observed in 1996 with the ADONIS instrument on ESO's 3.6 m telescope; the inner part is the innermost part of the system, as seen at 3.6 microns with NACO on the Very Large Telescope. The newly detected source is more than 1000 times fainter than Beta Pictoris, aligned with the disc, at a projected distance of 8 times the Earth-Sun distance. This corresponds to 0.44 arcsecond on the sky, or the angle sustained by a one Euro coin seen at a distance of about 10 kilometres. Because the planet is still very young, it is still very hot, with a temperature around 1200 degrees Celsius. Both parts of the image were obtained on ESO telescopes equipped with adaptive optics. Credit: ESO/A.-M. Lagrange et al.

There could also exist other families of exocomets around Beta Pictoris. "There is a possibility that another family exists. The exocomets that we saw are only the portion of the comets around Beta Pic that we were able to see because they emit Calcium elements." Kiefer said.

Asked about the future observations of this star system, Kiefer revealed that the team already has new observations with HARPS and they will continue until they are not able to extract further informations on these comets. But the Beta Pictoris system is actively observed also for other reasons like the nature of the disk, the planet.

“Beta Pictoris is a very exciting target! The detailed observations of its exocomets give us clues to help understand what processes occur in this kind of young planetary system.” Kiefer added.

The team that made this discovery is composed of F. Kiefer (Institut d’astrophysique de Paris [IAP], CNRS, Université Pierre & Marie Curie-Paris 6, Paris, France), A. Lecavelier des Etangs (IAP), J. Boissier (Institut de radioastronomie millimétrique, Saint Martin d’Hères, France), A. Vidal-Madjar (IAP), H. Beust (Institut de planétologie et d'astrophysique de Grenoble [IPAG], CNRS, Université Joseph Fourier-Grenoble 1, Grenoble, France), A.-M. Lagrange (IPAG), G. Hébrard (IAP) and R. Ferlet (IAP).