|Extrasolar planet||List of extrasolar planets|
|Right ascension||(α)||22h 57m 39.1s|
|Declination||(δ)||−29° 37′ 20″|
|Semimajor axis||(a)||~175 AU
|Orbital period||(P)||~872; 2,000 y|
|Maximum stellar flux||(F⊙max)||0.0065 ⊕|
|Minimum stellar flux||(F⊙min)||0.00012 ⊕|
|Discovery date||November 13, 2008|
|Discoverer(s)||Kalas et al.|
|Discovery method||Direct imaging|
|Discovery site||Hubble Space Telescope|
|Discovery status||Confirmed (Currie et al.) |
Fomalhaut b 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 2,000-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 nature of Fomalhaut b became unsettled after its discovery. Fomalhaut b could be a conglomeration of rubble from a recent collision between comet-to-asteroid sized bodies and not actually identify 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 likely, 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 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.
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. 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 and James Graham 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 Kalas and Graham's discovery 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 about Fomalhaut b
In the discovery paper, Kalas and Graham 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 had to be less massive than about 3 Jupiter masses.
However, Fomalhaut b should be detectable in the space-based infrared data in this scenario. But sensitive infrared Spitzer Space Telescope observations failed to detect Fomalhaut b, implying that Fomalhaut b does identify emission from a planet atmosphere. Furthermore, although the planet was thought to be a plausible explanation for Fomalhaut's eccentric debris ring, the measurements in Kalas and Graham's paper implied it was moving too fast (i.e. not apsidally aligned) for this explanation to work. Finally, researchers analyzing September–October 2011 Atacama Large Millimeter Array (ALMA) data for Fomalhaut's debris ring suggested an alternate hypothesis: that the ring is shaped by much smaller, shepherding planets neither of which needed to be Fomalhaut b. These results lead to serious doubts about Fomalhaut b's claimed properties, its status as a planet and even its existence.
Recovery/independent confirmation with Hubble and further controversy
On October 24, 2012, a team lead by Thayne Currie at the University of Toronto announced the first independent recovery of Fomalhaut b and revived the claim that Fomalhaut b identifies a planet. They reanalyzed the original Hubble data using new, more powerful algorithms for separating planet light from starlight and confirmed that Fomalhaut b does exist. They also provided a new detection of Fomalhaut b at 0.4 µm.
Their analysis showed that Fomalhaut b was unlikely to have been detected in the infrared anyway and yielded a velocity for Fomalhaut b smaller than that derived in the discovery paper and consistent with that needed for Fomalhaut b to be a planet sculpting the debris ring. They modeled the optical detections and infrared upper limits for Fomalhaut b, showing that Fomalhaut b's emission can be completely explained by starlight scattered by small dust and arguing that this dust surrounds an unseen planetary-mass object. Thus, they consider Fomalhaut b to plausibly be a “planet identified from direct imaging” even if Fomalhaut b is not, strictly speaking, a directly imaged planet insofar as the light does not come from a planetary atmosphere.
A second paper made public later and lead by Raphael Galicher and Christian Marois at the Herzberg Institute for Astrophysics also independently recover Fomalhaut b and confirm the new 0.4 µm detection, claiming the spectral energy distribution (SED) of Fomalhaut b cannot be explained as due to direct or scattered radiation from a massive planet. They consider two models to explain the SED: (1) a large circumplanetary disk around a massive, but unseen, planet and (2) the aftermath of a collision during the past 100 years of two Kuiper belt objects of radii about 50 km.
The revival of the claim that Fomalhaut b is (possibly) a planet after it had been discounted led some to nickname the object a “zombie planet”, although this is a non-technical term that does not appear in any paper.
Assuming that Fomalhaut b's orbit is in the same plane as the debris disk located exterior to it, it orbits Fomalhaut at a distance of approximately 115 AU (1.72×1010 km; 1.07×1010 mi). This distance is about 18 AU (2.7×109 km; 1.7×109 mi) closer to the star than the inner edge of the debris disk. The orbital separation of Fomalhaut b is larger than that for directly imaged planets around beta Pictoris and HR 8799 (8-70 AU). Fomalhaut b appears to be moving at about 4 kilometers per second. It is unclear whether Fomalhaut b's orbit will make it cross the debris disk, cross the debris disk only in projection (i.e. it is not orbiting in the same plane as the disk), or whether its orbit is completely nested within the debris disk.
At the optical wavelengths at which Fomalhaut b is detected, it is only about 2.7×10−10 times as bright as the star and is the faintest (intrinsically) extrasolar object yet imaged. The shape of its spectrum, as determined from measurements obtained at 0.4 to 0.8 µm, appears similar to that of its host star, suggesting that the emission identifying Fomalhaut b is completely due to scattered starlight. Although the initial discovery paper for Fomalhaut b suggested that its optical brightness may be variable due to planetary accretion, later reanalyses of these data fail to find convincing evidence that Fomalhaut b is indeed variable, thus eliminating evidence for planetary accretion and also for a 'transient' dust cloud.
In order for Fomalhaut b to be detectable at optical wavelengths, it must have an emitting area much larger than the physical size of a planet, a fact further strengthening the case that what we see as Fomalhaut b is not light coming from a planet atmosphere. A circumplanetary ring system is large enough to scatter enough starlight to make Fomalhaut b visible only if it has a radius between 20 to 40 times that of Jupiter's radius. A spherical cloud of dust with a radius of 0.004 AU (600,000 km; 370,000 mi) can make Fomalhaut b visible. Fomalhaut b appears as an unresolved point source in the highest-quality data (at 0.6 µm) which would suggest that its projected emitting area cannot be larger than about 0.25 AU, about 1/4th of the Earth-Sun distance. However, it may be resolved at slightly longer wavelengths, indicating that its emitting area is larger.
The mass of Fomalhaut b, if a planet, is highly uncertain. Infrared non-detections suggest that Fomalhaut b cannot be more massive than 2 times Jupiter's mass  but a lower limit on the mass depends on uncertain details for the nature of Fomalhaut b, its circumplanetary environment, and the existence of other planet-mass bodies in the system. Models of Fomalhaut b sculpting Fomalhaut's debris disk identify 0.5 times Jupiter's mass as a plausible estimate. Models for Fomalhaut b assuming it is surrounded by a swarm of planetesimals imply that it could be much lower mass (10-100 times the mass of the Earth). If Fomalhaut b is instead one of two shepherding planets which together confine the debris disk into a narrow ring, it could be anywhere between several times the mass of Mars to slightly more massive than the Earth.
If Fomalhaut b is a gas giant planet like Jupiter or Saturn, it probably formed several million years after the host star itself was formed, making it roughly 450 million years old. Alternatively, if it is a transient dust cloud it must be extremely young, perhaps created within the last few centuries.
Other planets orbiting Fomalhaut
Fomalhaut b is orbiting its host star at a wide separation, where forming massive planets is difficult. To explain its current location, Fomalhaut b could have been dynamically scattered by a more massive, unseen body located at smaller separations. Several ground-based observations have searched for this hypothetical Fomalhaut “c” but have yet to find it. At very small, Solar-System-like scales any additional companions must have a mass less than thirteen times the mass of Jupiter. At slightly wider scales comparable to the locations of planets around HR 8799, any additional planets must have masses below about 2 to 7 Jupiter masses. Fomalhaut b could have formed in situ if it coalesced from small pebble-sized objects that rapidly formed into a protoplanetary core which in turn accreted a gaseous envelope.
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- Hubblecast 22: Hubble directly observes planet orbiting Fomalhaut
- Preprint of recovery/confirmation paper
- Preprint of discovery paper
- Preprint of prediction paper
- NASA's Hubble reveals rogue planetary orbit for Fomalhaut b