Companion's comets the key to curious exoplanet system?

Dec 18, 2013
Artist’s impression of the Fomalhaut system. The newly discovered comet belt around Fomalhaut C is shown to the left. The comet belt around Fomalhaut A is in the distance to the right. The belt around Fomalhaut A is offset slightly, a signature of the elliptical orbits in the belt, which may have been caused by past interactions with the star Fomalhaut C. Credit: Amanda Smith.

( —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. Today, an international team of astronomers 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 researchers published their results today in a letter to the journal Monthly Notices of the Royal Astronomical Society.

Fomalhaut A is one of the brightest 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 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 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.

View of the Fomalhaut triple star system from Earth. The small inset shows a zoom of the newly discovered comet belt around Fomalhaut C as seen at infrared wavelengths by Herschel. The large inset shows a zoom of the much larger comet ring around Fomalhaut A as seen at optical wavelengths by Hubble. Telescope resolving power is lower at the infrared wavelengths observed by Herschel, so the size of the belt around Fomalhaut C is not well known. Image Credit: Grant Kennedy (Cambridge) & Paul Kalas (UC Berkeley).

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.

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 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 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.

Explore further: Researchers find that bright nearby double star Fomalhaut is actually a triple

More information: The new work appears in, "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, in press. A copy of the paper is available from

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SnowballSolarSystem _SSS_
1 / 5 (4) Dec 18, 2013
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.
SnowballSolarSystem _SSS_
1 / 5 (4) Dec 18, 2013
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.
SnowballSolarSystem _SSS_
1 / 5 (4) Dec 18, 2013
2) Both may Experience the Effects of Stellar Barycenter Centrifugal Force:
Then the 'recent' (22 kya) 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 kya 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 kya]." ( Wiki)

2b) The loss of the SPB centrifugal force explains Venus' slight retrograde orbit if Venus had previously been in synchronous rotation.
SnowballSolarSystem _SSS_
1 / 5 (4) Dec 18, 2013
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 kya and the loss of planetesimal showers from the inner Oort cloud (IOC) due to the SPB stroking and shaping the IOC with a former eccentric Sun-Proxima orbit.
SnowballSolarSystem _SSS_
1 / 5 (2) Dec 18, 2013
Zephir_fan, Explain how Jupiter can populate the Oort cloud which has more angular momentum than the entire inner solar system.

Poof! There goes the Nice Model.
3.7 / 5 (3) Dec 18, 2013
@ Snowball Skippy,

Is this going to be a book? Maybe you should try to sell it on Amazon. These peoples are too smart for this stuffs. Now SIT DOWN and SHUT UP. Let the smart peoples try to teach you something.

I'm firmly in your corner on this one Zephyr Fan. Ya called this idiot right. Not only is he an idiot, he is a plagiarizing idiot.

SnowballSolarSystem _SSS_
2.3 / 5 (3) Dec 18, 2013


Yea, that's me. It's not plagiarizing to repeat yourself.
for more astronomical wisdom.
SnowballSolarSystem _SSS_
2.3 / 5 (3) Dec 18, 2013
None of my stuff is plagiarized, I'll guarantee that, and I do try to include references when I borrow from others.
Captain Stumpy
not rated yet Dec 19, 2013
Q-Star - a quick question about multi star systems...

how feasible is it to have a planet capable of sustaining life in a system with multiple stars... you have obviously seen some of the models.

having multiple stars in a system... especially more than two... wouldn't that be horrific as far as stable trajectories for rocky planets with water?

5 / 5 (1) Dec 19, 2013
@ Stumpy, planetary systems is not my strong suite, so I am not sure of the nuances about this thing ya ask. GSwift and antialias seem to be more up to speed in that area. Whatever answer I might give ya would be little more than a guess.

@ Zephyr Fan, there are plenty of people commenting on this site much "smarter" than I am, but thanks anyway. That was no great trick, this is how ya can do it too: Whenever ya suspect something might be "borrowed" from another source or person, ya just copy about three or four sentences of it and paste it into a plagiarism checker and voila, it shows you where it came from. Just look for the hints that it is a copy and paste product, such as numbering for paragraphs, footnote numbers, some of the sentences seeming out of context to the topic at hand, inappropriate use of personal pronouns,, etc,,,,, There ya go, now ya can be as smart as anyone else on the site when it comes to fraud detecting.

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