Illustration par défaut

A second cometary ring in the surprising exoplanetary system of Fomalhaut

8 January 2014

An international team of astronomers, which includes a scientist from the LERMA (Observatoire de Paris/CNRS/ENS/Université de Cergy-Pontoise/UPMC), has just discovered a second cometary ring in the Fomalhaut planetary sysstem. The star Fomalhaut A is well known for the spectacular images of its planetary system which is composed of both an exoplanet and a cometary ring. In fact, Fomalhaut A is a member of a system of three stars. Fomalhaut C, the least massive of the three, is also surrounded by a cometary ring. The scientists, working with the Herschel space telescope, have published their results in the December 17th issue of Monthly Notice of the Royal Astronomical Society.

Artist’s view of the Fomalhaut system.
The newly discovered cometary ring around Fomalhaut C is shown on the left. The other ring around Fomalhaut A is shown on the far right. The ring around Fomalhaut A is not centred, which indicates that the orbits in the ring are elliptical. According to the scenario suggested by the astronomers, the reason could be a gravitational interaction with Fomalhaut A.
Crédit : Amanda Smith

The new discovery of a second cometary ring in the triple stellar system, made using the Herschel telescope, could lead to a better understanding of exoplanetary systems. According to theory, it should be composed of both a family of planets and, on the edge of the system, and the debris left over from their formation, like the Kuiper belt in the Solar system. Of course, this theory was tailored essentially to fit our planetary system, and astronomers are now trying to show that it could have universal validity. However, numerous exo-Jupiters are being found around stars with no cometary rings and, conversely, stars with rings are often lacking in exo-Jupiters. It has also been shown that most exoplanets are on excentic orbits (i.e. their orbits are not circular), as is the case for most of the planets in the Solar System.

The discovery of the ring around Fomalhaut C could help solve this mystery. It turns out that the orbits of the comets and the exoplanet around Fomalhaut A are relatively excentic. This type of orbit can be easily induced by close encounters between stars in a triple stellar system. The danse of three stars under the influence of gravity involves set figures : the stars approach each other, draw apart to a large distance, before skimming past each other once again. Moreover, in this danse, close encounters induce, because of the omnipresent gravitational fields, collisions between comets, producing large quantities of dust which renders the ring observable.

The triple system of Fomalhaut as seen from the Earth.
The small insert zooms onto the newly discovered ring around Fomalhaut C, as seen by Herschel in the infra-red. The large insert shows the much larger ring around Fomalhaut A seen in the optical domain using the Hubble Space Telescope. Since the resolving power in the infra-red is much poorer than in the optical domain, the size of the ring around Fomalhaut C is not yet well determined.
Crédit : Grant Kennedy (Cambridge) & Paul Kalas ( UC Berkeley).

According to this scenario, a cometary ring becomes observable only after a grazing encounter with another star. Once the motion of Fomalhaut C with respect to Fomalhaut A has been determined precisely in the years to come, this idea could be tested with numerical simulation for time scales on the order of several million years, which corresponds to the periodicity of the system. Why no cometary ring has been found around Fomalhaut B is still not understood : the question will be investigated by measuring its motion with respect to the other two stars, which might well reveal, as suggested by the scenario, that a close stellar encounter has never occurred in this case.

This discovery could stimulate new work on the Fomalhaut system. It will probably have a more general impact on our understanding of the architecture of exoplanetary systems.

Know more

Fomalhaut A is one of the brightest stars in the sky. It is 25 light years away, in the constellation of the Piscis Austrinus, ; it is in the Southern hemisphere, but is visible in the Northern hemisphere in autumn, low on the horizon; it shines with an unmistakable blue colour. On the other hand, Fomalhaut C, also known as LP 876-10, is a dim red dwarf which can only be seen with a good telescope ; its membership of the Fomalhaut system was a recent discovery dating from October 2013. The bright star Fomalhaut A was a prime target for the Hubble Space Telescope, and astronomers were delighted to discover that the star is surrounded by a magnificent ring of light, scattered by the dust coming from its comets. In 2008, they were astounded by the discovery, in the astronomical image, of an exoplanet situated close the the inner edge of the ring. While on the order of a thousand exoplanets have been discovered by indirect methods, to date only three stars have been shown to have exoplanets using direct imagery.