Fomalhaut b

Fomalhaut b

Extrasolar planet		List of extrasolar planets
An artist's impression of Fomalhaut b.
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
Orbital elements
Semimajor axis	(a)	~115 AU (~17300 Gm)
		~1510 mas
Periastron	(q)	~103 AU (~15400 Gm)
Apastron	(Q)	~128 AU (~19200 Gm)
Eccentricity	(e)	~0.11
Orbital period	(P)	~872 y
Inclination	(i)	~66°
Physical characteristics
Mass	(m)	0.054 – 3 MJ
Discovery information
Discovery date		November 13, 2008
Discoverer(s)		Kalas et al.
Detection method		Direct imaging
Discovery site		Hubble Space Telescope
Discovery status		Unconfirmed
Other designations
HD 216956 b
Database references
Extrasolar Planets Encyclopaedia		data
SIMBAD		data

Fomalhaut b is an unconfirmed extrasolar planet approximately 25 light-years away in the constellation of Piscis Austrinus. The planet was discovered orbiting the A-type main sequence star Fomalhaut in 2008 in photos taken by the Hubble Space Telescope. Fomalhaut b and three planets around HR 8799, whose discovery was announced simultaneously, were the first extrasolar planets whose orbital motion was confirmed via direct imaging. Fomalhaut b has yet to be independently confirmed.

Discovery

The planet orbits Fomalhaut at a distance of approximately 115 AU (1.72×10¹⁰ km; 1.07×10¹⁰ mi), which is about 18 AU (2.7×10⁹ km; 1.7×10⁹ mi) closer to the star than the inner edge of the debris disk. It was discovered after researchers spent eight years examining the system.

Fomalhaut b's position in 2004 and 2006.

The existence of the planet was inferred in 2005 from its influence on the Fomalhaut dust belt; the belt is not centered on the star, and has a sharper inner boundary than would normally be expected. However, the planet was only located in May 2008 after Paul Kalas and James Graham singled it out of Hubble photographs taken in 2004 and 2006. NASA released the composite discovery photograph, obtained by the Hubble Space Telescope's ACS, on November 13, 2008. 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. The planet was photographed again by Hubble in 2010, but it now appeared in an unexpected place. The planet would apparently need an elliptical orbit that takes it across the dust disc, yet its brightness suggests that it is too big to do so without disrupting the disc. Possible explanations include a second, hidden planet that perturbs the orbit of Fomalhaut b while holding the disc steady. It is also possible that the planet in fact is a transient dust cloud within the disc, a background star or a tiny protostar that failed to ignite. Hubble is scheduled to observe the Fomalhaut system again in 2012.

Comparison between our Solar System and the system around Fomalhaut.

Fomalhaut b is the first exoplanet observed directly in visible light, the first imaged planet since Neptune to have been predicted prior to discovery, and the first planet to have been correctly predicted based on its interaction with a debris disk. It is also believed to be the coolest, lowest-mass object ever imaged outside our own solar system. The estimated effective temperature of the planet is about 400 Kelvin and its age is about 100 million years.

Physical characteristics

The planet is estimated to be approximately the same size as Jupiter, with a maximum mass of three Jupiters and a most probable mass of 0.5 to two. It is 115 AU (17 billion km, 11 billion mi, about 20% greater than the aphelion distance of Eris and 3.8 times the semi-major axis of Neptune) from its sun, giving it an orbital period of 872 earth years. Fomalhaut has about 16 times the Sun's luminosity, so Fomalhaut is about as bright seen from Fomalhaut b as the Sun is seen from Neptune (due to the inverse square law).

Fomalhaut b is suspected, on the basis of its brightness in visible light and dimness in infrared, to be surrounded by a circumplanetary disc with a radius approximately 20–40 times the radius of Jupiter (by comparison, the outer edge of Saturn's A ring is at a distance of only about 2 Jupiter radii from the centre of the planet). This size is similar to the orbital radii of Jupiter's Galilean satellites and therefore may represent a stage in the formation of a system of moons around the planet.

Questioned Existence

Recent observations of the Fomalhaut system using Spitzer taken in 2010-2011 and published in 2012 have failed to detect any emission from the proposed planet at near-infrared wavelengths, where the bulk of the planetary emission would be expected. It has thus been suggested that the proposed planet based on the optical observations does not exist, but is instead simply scattered light from dust clouds in the system.

See also

• Direct imaging of extrasolar planets
• List of extrasolar planets

External links

• Hubblecast 22: Hubble directly observes planet orbiting Fomalhaut
• Preprint of discovery paper
• Preprint of prediction paper