The nearby star Fomalhaut A hosts the most famous planetary system outside our own Solar System, containing both an exoplanet and a spectacular ring of comets. An international team of astronomers have announced a new discovery with the Herschel Space Observatory that has made this system even more intriguing; the least massive star of the three in the Fomalhaut system, Fomalhaut C, has now been found to host its own comet belt.
To get a feeling for how far 2.5 light years is, light from the Sun takes only 8 minutes to get to the Earth, and 5.5 hours to get to Pluto, and the nearest star to the Sun, Proxima Centauri, is only 4 light years away.
This discovery may help solve the major mystery in the Fomalhaut system: the orbits of the comet ring and planet around Fomalhaut A are elliptical (which simply means that the orbits aren't circular). The elliptical orbits are 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, B or C
Fomalhaut A is one of the brightest stars in the sky. Located 25 light years away in the constellation of Piscis Austrinus, it shines with a blue-white colour and is prominent from the southern hemisphere. From northern latitudes it appears low down in the south during autumn evenings. In contrast, Fomalhaut C, also named LP 876-10, is a dim red dwarf star invisible without a telescope, and was only found to be part of the Fomalhaut system in October this year.
Fomalhaut A’s prominence made it a key target for the Hubble Space Telescope, which astronomers used to find the ring of comets, hints of and then a direct image of the planet, Fomalhaut b, in 2008 (astronomers use uppercase letters for stars, and lowercase letters are used for planets, so ‘Fomalhaut b’ is a planet, and ‘Fomalhaut B’ is the second star in the system).
The discovery of the comet belt around 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 A and C suggests that they may have had their brightnesses enhanced by a previous close encounter between the two.
Paul Kalas of the University of California discovered the orbits are elliptical and is involved in the new work. He said, "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 the red dwarf's exact orbit over the next few years."
The Hubble Space Telescope and ALMA reveal Fomalhaut's off-kilter debris disk shown below, offset by 15 a.u. from the star, which is blocked out in the middle. Click on the image to see the candidate planet Fomalhaut b.
The stellar interaction scenario isn't as unusual as it sounds. Comet ISON, which disintegrated following a close encounter with our Sun at the end of November, may have been put on a Sun-grazing orbit by a star that passed near to the Solar System in the past. Similarly, the proposed encounters between the stars in the Fomalhaut system may have sent a few comets onto star-grazing orbits. You might imagine that if there were any habitable planets around Fomalhaut A or C, their inhabitants might be luckier than us and see truly spectacular comet shows in their night sky.
The Herschel Observatory, which observed the Universe in infrared light ran out of helium coolant and stopped observing in April this year. This was seven months before Fomalhaut C was identified as part of the triple star system, but fortunately the telescope had imaged it back in 2011, so the astronomers have plenty of data on it already.
Kennedy has actually known about the comet belt for several years; "Over the last few years we used Herschel to look for comet belts around many stars within a few hundred light years of the Sun. At that stage Fomalhaut C was just called LP 876-10 and we thought it was a lone red dwarf with a comet belt. It was interesting because such discoveries are very rare, but didn’t tell us why it was there. After the discovery that this star was part of the Fomalhaut system, the existence of its comet belt made us think harder about connections between the two stars, and it may be that it helps solve the mystery of the elliptical comet belt around Fomalhaut A.”
Kennedy and his team are now trying to check the stellar encounter idea with computer simulations and more detailed observations of the Fomalhaut C belt. The apparent absence of a belt around Fomalhaut B remains a mystery. But if the simulations are in line with what the astronomers see, then this would be a ‘smoking gun’ for a stellar interaction and proof that other stars can affect how planetary systems form and evolve.
The image at the top of the page is Herschel's far-infrared observations of Fomalhaut and its disk. Scientists have been trying to understand the makeup of the disk, and new observations by the Herschel Space Observatory reveals the disk may come from cometary collisions. But in order to create the amount of dust and debris seen around Fomalhaut, there would have to be collisions destroying thousands of icy comets every day. The asymmetrical properties of the disk are thought to be due to the gravity of a possible planet in orbit around the star.
The Daily Galaxy via RAS
Image Credit ESA