Fomalhaut b, formally named Dagon (), was a directly imaged extrasolar object and former candidate planet observed near the A-type main-sequence star Fomalhaut, approximately 25 light-years away in the constellation of Piscis Austrinus. The object's discovery was initially announced in 2008 and confirmed in 2012 via images taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. Under the working hypothesis that the object was a planet, it was reported in January 2013 that it had a highly elliptical orbit with a period of 1,700 Earth years. The planetary hypothesis has since fallen out of favor; more recently gathered data suggests a dust or debris cloud is far more likely, and more recent analysis placed the object on an escape trajectory.
The true nature of Fomalhaut b has been the subject of significant debate. Fomalhaut b was initially identified as one of the first exoplanets to be directly imaged: its detection was attributed to reflected light from circumplanetary material (e.g. a dust ring) and thermal emission from a jovian planet atmosphere. Later, Fomalhaut b was described as a low-mass planet which was only detected due to its surrounding dust cloud or, most recently, debris from a collision of asteroids instead.
The object was one of those selected by the International Astronomical Union as part of NameExoWorlds, 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.
Summary
The most likely composition of Fomalhaut b is a dust cloud. Prior to 2020, a possible alternative was that Fomalhaut b could be a planet less than twice Jupiter's mass that is enshrouded in a spherical cloud of dust from ongoing planetesimal collisions.
Fomalhaut b's discovery and preliminary identification as an exoplanet was announced at the same time as three others around HR 8799, described as the first directly imaged extra-solar 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.
History of observations
Initial discovery by Hubble
The existence of a massive planet orbiting Fomalhaut was first inferred from Hubble observations published in 2005 that resolved the structure of Fomalhaut's massive, cold debris disk (or dust belt/ring). The belt is not centered on the star, and has a sharper inner boundary than would normally be expected. The initial theory was that a massive planet on a wide orbit but located interior to this debris ring could clear out parent bodies and dust in its vicinity, leaving the ring appearing to have a sharp inner edge and making it appear offset from the star.
In May 2008, Paul Kalas, James Graham and their collaborators identified Fomalhaut b from Hubble/ACS images taken in 2004 and 2006 at visible wavelengths (i.e. 0.6 and 0.8 μm). NASA released the composite discovery photograph on November 13, 2008, coinciding with the publication of discovery by Kalas et al. in Science.
Kalas remarked, "It's a profound and overwhelming experience to lay eyes on a planet never before seen. I nearly had a heart attack at the end of May when I confirmed that Fomalhaut b orbits its parent star." In the image, the bright outer oval band is the dust ring, while the features inside of this band represent noise from scattered starlight.
Early follow-up observations and doubts
In the discovery paper, Kalas and collaborators suggested that Fomalhaut b's emission originates from two sources: from circumplanetary dust scattering starlight and from planet thermal emission. Here, the former explains most of the 0.6 μm brightness and planet thermal emission contributes to much of the 0.8 μm brightness. Their non-detections with ground-based infrared data suggested that Fomalhaut b could not be more massive than about three times Jupiter's mass if it were a planet.