Baffling Triple-Star System Reveals an Exoplanet and a Spectacular Ring of Comets
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December 18, 2013

Baffling Triple-Star System Reveals an Exoplanet and a Spectacular Ring of Comets

 

Herschel_Fomalhaut
 

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.

The new discovery might hold the key to some of the mysteries of the Fomalhaut system. The lead author Grant Kennedy, an astronomer at the Institute of Astronomy at the University of Cambridge, said, "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."

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."

Fom-b_HST_ALMA_300pxThe 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

Comments

I suspect there's a chance that the 440 Myr triple-star Fomalhaut system resulted from resonant core-collapse perturbation of a single protostar that fragmented (bifurcated) repeatedly due to excess-excess angular momentum.

And perhaps the 'A' and 'C' stars originally formed as close binaries that spiraled in and merged: thus 5 former stars evolved into the present triple-star system, since close-binary–close-binary perturbation may be a particularly-efficient core-collapse mechanism.

Then their two former close-binary mergers would explain the comet disks (more likely asteroids) around Fomalhaut A and C from planetesimals condensed by gravitational instability (GI) out of merger debris at the former magnetic corotation radius of their former (recently-merged) flare stars from merger debris. And centrifugal force of the wide-binary system around its barycenter would tend to cause the planetesimals to slowly spiral out from their original semi-major axes, again by core collapse.

This system may have a number of commonalities with our own solar system, including:

1) Both may have been (Former) Multiple-Star Systems:
Formation of a multiple star system from a single protostar. Only a companion star to our Sun could have formed the Oort cloud with its estimated mass of several Jupiters. This shoots down the Nice Model which suggests that Jupiter populated the Oort cloud from inner solar system planetesimals, since the Oort cloud has many times the angular momentum of Jupiter. However, 4.85 Myr Proxima (Centauri) has the ball park age to be our companion star
and _sufficient mass_ to have populated the Oort cloud
and the _right mass_ to have caused the late heavy bombardment (LHB) of the inner solar system when the Sun-Proxima barycenter spiraled out across Neptune's orbit at 3,900 Ma with (the starting condition of) Proxima's orbital distance from the Sun at about 75 AU at 4,567 Ma.

2) Both may Experience the Effects of Stellar Barycenter Centrifugal Force:
Then the 'recent' (22,000 years ago) loss of the Sun-Proxima barycenter (SPB) with the close approach of Alpha Centauri and the apparent extremely-close encounter of Proxima with one of the two binary components of Alpha Centauri gave Proxima its current escape-velocity kick. Along with the 22,000 ya loss of our SPB comes the loss of its centrifugal force which may have profound effects in the inner solar system including the planets:

2a) The loss of the SPB centrifugal force causes the planets to move closer to the Sun, "The maximum extent of glaciation within this last glacial period was approximately 22,000 years ago." (Last glacial period, Wikipedia)

2b) The loss of the SPB centrifugal force explains Venus' slight retrograde orbit if Venus had previously been in synchronous rotation.

2c) We may be moving from an era dominated by comet and dwarf-planet impacts from the Oort cloud to an era dominated by asteroid impacts from the inner solar system with the loss of the SPB centrifugal force which may have held the asteroids against Jupiter's inner Lindblad resonances prior to 22,000 ya.

Very interesting discovery! but the interactions between stars seems like a "no-brainer". Such a system might not be entirely rare either, just more so difficult to detect. Since there's trillions of stars how can we say anything appears rare?

I'm wondering why the term'comet belt' does not get more attention(use)....neat term!

The "comet belt" is a misnomer. It should be an asteroid belt.

"major mystery in the Fomalhaut system:"... Not!
Planetary orbits are normally elliptical.


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