Fomalhaut C, also designated LP 876-10, is the distant third

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

of the

Fomalhaut Fomalhaut (, ) is the brightest star in the southern constellation of Piscis Austrinus, the Southern Fish, and one of the brightest stars in the night sky. It has the Bayer designation Alpha Piscis Austrini, which is an alternative ...

system. It is about five degrees from Fomalhaut, roughly halfway between it and the

Helix Nebula The Helix Nebula (also known as NGC 7293 or Caldwell 63) is a planetary nebula (PN) located in the constellation Aquarius. Discovered by Karl Ludwig Harding, most likely before 1824, this object is one of the closest of all the bright planetary ...

. It is currently from Fomalhaut (A), and 3.2 light-years away from

Fomalhaut B TW Piscis Austrini (also known as Fomalhaut B) is a main sequence star in the constellation Piscis Austrinus. It lies relatively close to the Sun, at an estimated distance of . To an observer on Earth the star is visually separated from i ...

(0.987 pc). The entire system is approximately from the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

Discovery and observation

Fomalhaut C was catalogued as a high-proper-motion star by Willem Luyten in 1979, and later, in October 2013, was determined to be part of the Fomalhaut system. The star has a mass of , while Fomalhaut A is , and Fomalhaut B is . The apparent magnitude of Fomalhaut C is 12.6 requiring a six-inch aperture or larger telescope for direct visual observation. The entire system of Fomalhaut is around 440 million years old, which is roughly a tenth of the Solar System's age.

Debris disk

In December 2013, a

debris disk A debris disk (American English), or debris disc ( Commonwealth English), is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris ...

was discovered around this star. This is the second debris disk in the system, as a first one was discovered around Fomalhaut A. The debris disc was discovered with the Herschel Space Telescope, and with the telescope the debris disc's temperature has been estimated at . The distance from the star was originally thought to be around , but since it is hypothesized that it is mainly small grains, which trap more heat, it may be further. However, if it were beyond 40 AU, it would have been already cataloged, which gives it a radius between 10 and 40 AU. The disk was directly detected by

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in 2021 and by JWST in 2024.

Comets

In addition to the debris disk, there are also

comet A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...

s orbiting Fomalhaut C. The debris disk orbiting C is sometimes referred to as a comet belt, due to some very elliptical orbits. The disk around Fomalhaut A is also thought to have many comets, as it is also elliptical. The Fomalhaut A belt is thought to possibly be due to a close encounter with either an undiscovered

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

, Fomalhaut B, or Fomalhaut C. With both A and C having comet belts, the absence of one around B is a mystery. Not only does the presence of comets make the belt more elliptical, it also makes it brighter which takes a part in its discovery. It has been hypothesized that Fomalhaut C could have hidden exoplanets within its belt of comets and

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

s. It has also been hypothesized that A & C have interacted which could have formed C's comet belt if the interaction involved A giving up comets and debris. With Fomalhaut B not having any discs or belts around it, it could have been unaffected by the encounter between them.

Orbit

The orbit of Fomalhaut C around A is estimated to take 20 million years to complete. Due to the age of the three stars being 440 million years, and the distance of 2.5 light-years, this would mean that Fomalhaut C has only completed a full orbit around Fomalhaut A 22 times. The tidal radius of the Fomalhaut system is , which makes Fomalhaut C well within it, which further proves the Fomalhaut system as a triple.

Notes

References