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Proper NameDagon
DesignationsFomalhaut b, Alpha Piscis Austrini b, HD 216956 b
ClassificationUncertain; probably a debris cloud, possibly surrounding a low-mass planet
Orbital Periodc. 1,700 years
Mean Distance from Fomalhaut177 AU
MassNot known
DiameterNot known
TemperatureNot known
Parent starFomalhaut, white main sequence star in Piscis Austrinus
Other planets in this systemAn additional inner planet Fomalhaut c has been theorised, but has yet to be detected

Fomalhaut A, the main star of the ternary Fomalhaut system, is a white star about twenty-five light years from the Solar System. This star is surrounded by complex system of dust rings extending out to more than 200 AU, and of these rings one particularly dense belt of material stands out, filling a region about 133 to 158 AU from Fomalhaut A.

Imagery provided by Aladin sky atlas

Pursuing a highly eccentric orbit through this ring is a strange body named Dagon or Fomalhaut b. This body was originally tentatively identified as a planet between one and two times the mass of Jupiter, though this now appears unlikely. It is currently thought to represent a cloud of debris, possibly surrounding a low-mass planet. What is known is that its orbit is extremely elliptical, bringing it as close as 50 AU from its parent star, and carrying it out as far as 300 AU (about ten times the distance of Neptune from the Sun). This long orbit takes Dagon about 1,700 years to complete, carrying it back and forth through the rings of dust that surround Fomalhaut A.

Dagon is unusual in that it is one of the very few extrasolar planets to have been directly pictured: images taken over a period of years show it moving slowly along the inner edge of Fomalhaut's main circumstellar disc. Calculations of Dagon's mass suggest that it should not theoretically be possible to create direct images like this, which suggests that other factors are working to increase its visibility. For example, it may have an extensive ring system of its own (a system that, if it exists, would dwarf that of Saturn), or it may be shrouded in a cloud of material collected from its star's surrounding dust ring.


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