Fomalhaut b
Encyclopedia
Fomalhaut b is an extrasolar planet
approximately 25 light-year
s 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.
at a distance of approximately 115 AU, which is about 18 AU closer to the star than the inner edge of the debris disk
. It was discovered after researchers spent eight years examining the system.
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.
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.
, 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.
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
approximately 25 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s away in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
of Piscis Austrinus
Piscis Austrinus
Piscis Austrinus is a constellation in the southern celestial hemisphere. The name is Latin for "the southern fish" in contrast with the larger constellation Pisces, which represents a pair of fishes. Prior to the 20th century, it was also known as Piscis Notius...
. The planet was discovered orbiting the A-type main sequence star Fomalhaut
Fomalhaut
Fomalhaut is the brightest star in the constellation Piscis Austrinus and one of the brightest stars in the sky. Fomalhaut can be seen low in the southern sky in the northern hemisphere in fall and early winter evenings. Near latitude 50˚N, it sets around the time Sirius rises, and does not...
in 2008 in photos taken by the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
. Fomalhaut b and three planets around HR 8799
HR 8799
HR 8799 is a young main sequence star located 129 light years away from Earth in the constellation of Pegasus, with roughly 1.5 times the Sun's mass and 4.9 times its luminosity. It is part of a system that also contains a debris disk and at least four massive planets...
, whose discovery was announced simultaneously, were the first extrasolar planets whose orbital motion was confirmed via direct imaging.
Discovery
The planet orbits FomalhautFomalhaut
Fomalhaut is the brightest star in the constellation Piscis Austrinus and one of the brightest stars in the sky. Fomalhaut can be seen low in the southern sky in the northern hemisphere in fall and early winter evenings. Near latitude 50˚N, it sets around the time Sirius rises, and does not...
at a distance of approximately 115 AU, which is about 18 AU closer to the star than the inner edge of the debris disk
Debris disk
A debris disk is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris disks have been found around both evolved and young stars, as well as at least one debris disk in orbit around a...
. It was discovered after researchers spent eight years examining the system.
Paul Kalas
Paul Kalas is a Greek American astronomer known for his discoveries of debris disks around stars. Kalas led a team of scientists to obtain the first visible-light images of an extrasolar planet with orbital motion around the star Fomalhaut, at a distance of 25 light years from Earth...
and James Graham
James R. Graham
James R. Graham is an Irish astrophysicist who works primarily in the fields of infrared astronomy instrumention and adaptive optics.-Biography:...
singled it out of Hubble photographs taken in 2004 and 2006.
NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
released the composite discovery photograph, obtained by the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
's ACS
Advanced Camera for Surveys
The Advanced Camera for Surveys is a third generation axial instrument aboard the Hubble Space Telescope . The initial design and scientific capabilities of ACS were defined by a team based at Johns Hopkins University. ACS was assembled and tested extensively at Ball Aerospace & Technologies Corp...
, 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.
Fomalhaut b is the first exoplanet observed directly in visible light
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
,
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
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
. The estimated effective temperature of the planet is about 400 Kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
and its age is about 100 million years.
Physical characteristics
The planet is estimated to be approximately the same size as JupiterJupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, with a maximum mass of three Jupiters and a most probable mass of 0.5 to two. It is 115 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
(17 billion km, 11 billion mi, about 20% greater than the aphelion
Apsis
An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...
distance of Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...
and 3.8 times the semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...
of Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
) from its sun, giving it an orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
of 872 earth years. Fomalhaut has about 16 times the Sun's luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
, 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
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
, 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
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.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
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
around the planet.
See also
- Direct imaging of extrasolar planets
- List of extrasolar planets