Fomalhaut b
| Exoplanet | List of exoplanets | |
|---|---|---|
 |
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| Parent star | ||
| Star | Fomalhaut | |
| Constellation | Piscis Austrinus | |
| Right ascension | (α) | 22h 57m 39.1s |
| Declination | (δ) | −29° 37′ 20″ |
| Apparent magnitude | (mV) | 1.16 |
| Distance |
25±0.1 ly (7.66±0.04 pc) |
|
| Spectral type | A3V | |
| Mass | (m) | 1.92±0.02 M☉ |
| Radius | (r) | 1.842±0.019 R☉ |
| Temperature | (T) | 8,590 K |
| Age | 0.44±0.04 Gyr | |
| Orbital elements | ||
| Semi-major axis | (a) | 177±68AU |
| Eccentricity | (e) | 0.8±0.1 |
| Orbital period | (P) | ~1700y |
| Inclination | (i) | −55±14° |
| Longitude of the node | (Ω) | 152±13° |
| Argument of periastron |
(ω) | 26±25° |
| Discovery information | ||
| Discovery date | November 13, 2008 | |
| Discoverer(s) | Kalas et al. | |
| Discovery method | Direct imaging | |
| Discovery site | Hubble Space Telescope | |
| Discovery status | Replicated (Galicher et al., Currie et al.) | |
| Other designations | ||
|
Dagon, HD 216956 b
|
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| Database references | ||
| Extrasolar Planets Encyclopaedia |
data | |
| SIMBAD | data | |
| Exoplanet Archive | data | |
| Open Exoplanet Catalogue | data | |
Fomalhaut b, also known as Dagon, is a confirmed, directly imaged extrasolar object and candidate planet orbiting the A-type main-sequence star Fomalhaut, approximately 25 light-years away in the constellation of Piscis Austrinus. The object was initially announced in 2008 and confirmed as real in 2012 from images taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope and, according to calculations reported in January 2013, has a 1,700-year, highly elliptical orbit. It has a periastron of 7.4 billion km (~50 AU) and an apastron of about 44 billion km (~300 AU). As of May 25, 2013 it is 110 AU from its parent star.
The planet was one of those selected by the International Astronomical Union as part of their public process for giving proper names to exoplanets. The process involved public nomination and voting for the new name. In December 2015, the IAU announced the winning name was Dagon. The name Dagon was proposed by Dr. Todd Vaccaro and forwarded by the St. Cloud State University Planetarium to the IAU for consideration.Dagon was a Semitic deity, often represented as half-man, half-fish.
The nature of Fomalhaut b was at first unclear. It was thought it could be a conglomeration of rubble from a recent collision between comet-to-asteroid-sized bodies, and not actually a planet. Although this scenario is possible, the likelihood of observing such a collision at the location of Fomalhaut b is extremely low. Instead, Fomalhaut b is plausibly, even probably, a planet less than twice Jupiter's mass that is either enshrouded in a spherical cloud of dust from ongoing planetesimal collisions or surrounded by a large circumplanetary ring system, either of which are responsible for scattering the primary star's light and thus making Fomalhaut b visible.
Fomalhaut b and three companions around HR 8799, whose discovery was announced simultaneously, were described as the first directly imaged extrasolar planets (among earlier claims such as e.g. 2M1207 b, GQ Lup b, DH Tau b, AB Pic b, CHXR 73 b, UScoCTIO 108 b, CT Cha b, 1RXS 1609 b) in that their emission was thought to originate at least in part from a planetary atmosphere. However, subsequent studies from the Spitzer Space Telescope and a reanalysis of the original HST data instead suggest that Fomalhaut b's light is scattered starlight, not planet thermal emission.
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