Fomalhaut's Stellar Sister's Comets: Exoplanet Goldmine?

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Astronomers scoping-out the vicinity of the famous star Fomalhaut have discovered that its mysterious stellar sister is also sporting a rather attractive ring of comets.

This news is brought to you by the recently-defunct Herschel space observatory, a European-led infrared mission, that was especially fond of probing the infrared signals being emitted by cool nebulae, the gas in distant galaxies and, in this case, vast icy debris fields surrounding young stars. Sadly, the orbiting telescope was lost earlier this year as it ran out cryogenic coolant, but its legacy lives on.

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Located 25 light-years away in the constellation Piscis Austrinus, Fomalhaut A is one of the brightest stars in Southern Hemisphere skies. The bright blue giant is notable in that it hosts a gigantic ring of cometary debris and dust. Embedded within this ring is the infamous Fomalhaut Ab, a massive exoplanet that has been the cause of much debate.

But this new research, published in the journal Monthly Notices of the Royal Astronomical Society, doesn’t focus on the stellar “Eye of Sauron,” it is actually centered around Fomalhaut’s less famous sibling, Fomalhaut C.

Fomalhaut C is a red dwarf star and was only confirmed to be gravitationally bound Fomalhaut A and Fomalhaut B in October. Fomalhaut is therefore a triple, or trinary, star system. The small red dwarf star may be the proverbial runt of the Fomalhaut stellar litter, but it appears to share some common ground with its larger sibling.

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“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,” said astronomer Grant Kennedy, of the University of Cambridge and lead researcher of this work. “It made us wonder why both Fomalhaut A and C have comet belts, and whether the belts are related in some way.” Interestingly, Fomalhaut B doesn't appear to have such a belt.

Astronomers have noted that both cometary belts around Fomalhaut A and C are bright and elliptical, indicating that gravitational perturbations may be destabilizing comets’ orbits, causing collisions and close encounters with the central stars. As Comet ISON dramatically demonstrated last month, stellar near-misses can result in disintegration, flinging huge quantities of ice, dust and gas around star systems. Scale that activity up in a younger star system and you have huge debris belts like the ones found around Fomalhaut A and C.

So what’s causing this activity? The researchers point out that Fomalhaut A already has a known exoplanet mixing things up from deep inside the dusty ring. Also, gravitational interactions between Fomalhaut A and C could be a huge factor.

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“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. A good test of this hypothesis is to measure (Fomalhaut C’s) exact orbit over the next few years,” said Paul Kalas, of the University of California.

The relationship between Fomalhaut A and C could go beyond their gravitational interplay, Fomalhaut C could also have hidden exoplanets in tow, jostling the comets from the inside.

This research is fascinating as it demonstrates how young, widely spaced stars interact with one another — an ideal test to see how star systems mold each other’s planetary destiny.

Publication: Discovery of the Fomalhaut C debris disc, Kennedy et al., MNRAS, 2013. doi:10.1093/mnrasl/slt168

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