Fomalhaut C – the least massive star in the Fomalhaut system – has been found to host comet belt, according scientists reporting in the journal Monthly Notices of the Royal Astronomical Society.

The Fomalhaut star system contains three stars: Fomalhaut A, B and C.

Located in the constellation of Piscis Austrinus about 25 light-years away from Earth, Fomalhaut A is one of the brightest stars in the sky. It hosts an exoplanet and a spectacular ring of comets.

In contrast, Fomalhaut C, also known as LP 876-10, is a dim red dwarf star invisible without a telescope, and was only found to be part of the system in October 2013.

The discovery of the debris disc around Fomalhaut C holds the key to the mysteries of the Fomalhaut system.

“It’s very rare to find two comet belts in one system, and with the two stars 2.5 light years apart this is one of the most widely separated star systems we know of. It made us wonder why both Fomalhaut A and C have comet belts, and whether the belts are related in some way,” said study lead author Dr Grant Kennedy from the University of Cambridge.

The discovery may help solve the major mystery in the Fomalhaut system: the orbits of the comet ring and planet around Fomalhaut A are elliptical – thought to be the result of close encounters with something else in the system, perhaps with another as yet undetected planet or perhaps with one of the two other stars, Fomalhaut B or C.

The comet belt around Fomalhaut C is important because such encounters can not only make the comet belts elliptical, they can also make them brighter by causing the comets to collide more often, releasing massive amounts of dust and ice. Stars are rarely seen to have such bright comet belts, so their detection around both Fomalhaut A and C suggests that they may have had their brightnesses enhanced by a previous close encounter between the two.

“We thought that the Fomalhaut A system was disturbed by a planet on the inside – but now it looks like a small star from the outside could also influence the system,” said co-author Dr Paul Kalas of the University of California.

“A good test of this hypothesis is to measure the red dwarf’s exact orbit over the next few years.”

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Kennedy GM et al. Discovery of the Fomalhaut C debris disc. MNRAS, published online December 17, 2013; doi: 10.1093/mnrasl/slt168