December 18, 2013

Herschel discovers a second ring of comets in the Fomalhaut System

The Fomalhaut System is well known to the astronomers. Its proximity (25 light years), its youth - 440 millions of years, compared to the nearly 5 billions for the Solar System - and the great separation between its three stars linked by gravity make it a choice laboratory to test the theories of formation of planetary systems.

It was already known that the largest of the stars, Fomalhaut A, was surrounded by (at least) one exoplanet and a dust ring. This kind of ring is the result of collisions between small bodies such as comets. The Fomalhaut A ring features several surprises: an unusual eccentricity and a particularly high brightness (in infrared).

 Amanda Smith
Artist's impression of the Fomalhaut System. On the left: the newly discovered comet belt around Fomalhaut C. On the right: the comet belt around Fomalhaut A. Credits: Amanda Smith

But the discovery recently published by an international team, including scientists from the Paris Observatory, is about the third star of the System, Fomalhaut C, itself recently highlighted. Herschel images, taken before the end of its mission, show that Fomalhaut C, also, is surrounded by a comet ring! Besides the extreme scarcity of stellar systems having several rings (only one was known until now), the discovery of Fomalhaut C and its comets ring renew the interest of the scientists for the system: Could the eccentricity of the Fomalhaut A ring be te result of a former flyby by Fomalhaut C? Such a "graze" could cause the chaos in the orbits of the comets of the two stars, resulting in a series of collisions and the production of these dust rings so luminous today. This creation scenario of the ring was already proposed before, but with Fomalhaut A exoplanet as the disturbing element. Now new questions appear: Do the rings of the two stars could have interacted in the past? Why the last star of the triplet, Fomalhaut B, does not have a ring, as proven by Herschel images?

Fomalhaut will probably remain for long in the observation programs of telescopes. It will be, for example, the need to improve our knowledge of the orbits of the three stars that are still poorly known.

Read the complete news on the Royal Astronomical Society web site.


"Discovery of the Fomalhaut C debris disc", G. M. Kennedy, M. C. Wyatt, P. Kalas, G. Duchêne, B. Sibthorpe, J.-F. Lestrade, B. C. Matthews and J. Greaves, Monthly Notices of the Royal Astronomical Society, Oxford University Press, in press