Tau Boötis Ab
Encyclopedia
Tau Boötis b, occasionally referred to as Tau Boötis Ab, is an extrasolar planet
approximately 50 light-year
s away around the primary star of the Tau Boötis system
in the constellation
of Boötes
. Announced in 1996 by Geoffrey Marcy
and R. Paul Butler
, Tau Boötis
was one of the first stars confirmed to have planets orbiting it. On 16 December 1999, the planet
was dubbed the "Millennium Planet" because the planet was then (erroneously) thought to be the first extrasolar planet
to be discovered visually.
and Geoffrey Marcy
(San Francisco Planet Search Project) using the highly successful radial velocity
method. Since the star is visually bright and the planet is massive, it produces a very strong velocity signal of 469
± 5 metres per second, which was quickly confirmed by Michel Mayor
and Didier Queloz
from data collected over 15 years. It was later confirmed also by the AFOE Planet Search Team.
over four times that of Jupiter
. It orbits the star in a so-called "torch orbit", at a distance from the star less than one seventh that of Mercury
's from the Sun
. One orbital revolution takes only 3 days 7.5 hours to complete. Because τ Boo is hotter and larger than the Sun and the planet's orbit is so low, it is assumed to be hot. Assuming the planet is perfectly grey with no greenhouse
or tidal effects, and a Bond albedo
of 0.1, the temperature would be close to 1600 K
. Although it has not been detected directly, it is certain that the planet is a gas giant
.
As Tau Boötis b is more massive than most known "hot Jupiter
s", it was speculated that it was originally a brown dwarf
, a failed star, which could have lost most of its atmosphere from the heat of its larger companion star. However, this seems very unlikely. Still, such a process has actually been detected on the famous transiting planet HD 209458 b
.
In December 1999, a group led by A. C. Cameron had announced that they had detected reflected light from the planet. They calculated that the orbit of the planet has an inclination
of 29° and thus the absolute mass of the planet would be about 8.5 times that of Jupiter. They also suggested that the planet is blue in color. Unfortunately, their observations could not be confirmed and were later proved to be spurious. A better estimate came from the assumption of tidal lock with the star, which rotates at 40 degrees; fixing the planet's mass between 6 and 7 Jupiter masses. This inclination has been confirmed by magnetic field detection.
must be less than 0.37. At 1600 K, it is (like HD 179949 b
) supposed to be hotter than HD 209458 b
(formerly predicted 1392K) and possibly even HD 149026 b
(predicted 1540 K from higher albedo 0.3, then actually measured at 2300 K). Tau Boötis b's predicted Sudarsky class is V; which is supposed to yield a highly reflective albedo of .55.
It is a candidate for "near-infrared characterisation.... with the VLTI Spectro-Imager".
There are also some indications of another, more distant planet orbiting Tau Boötis A
. However, until the possible planet has completed one orbit it remains speculation.
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 50 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 around the primary star of the Tau Boötis system
Tau Boötis
Tau Boötis is a yellow-white dwarf approximately 51 light-years away in the constellation of Boötes. The system is also a binary star system, with the secondary star being a red dwarf...
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 Boötes
Boötes
Boötes is a constellation in the northern sky, located between 0° and +60° declination, and 13 and 16 hours of right ascension on the celestial sphere. The name comes from the Greek Βοώτης, Boōtēs, meaning herdsman or plowman...
. Announced in 1996 by Geoffrey Marcy
Geoffrey Marcy
Geoffrey W. Marcy is an American astronomer, who is currently Professor of Astronomy at the University of California, Berkeley, famous for discovering more extrasolar planets than anyone else, 70 out of the first 100 to be discovered, along with R...
and R. Paul Butler
R. Paul Butler
R. Paul Butler is an astronomer who searches for extrasolar planets.He received a BA and an MS from San Francisco State University, completing a Master's thesis with Geoffrey Marcy, and then completed his doctoral studies at the University of Maryland, College Park in 1993...
, Tau Boötis
Tau Boötis
Tau Boötis is a yellow-white dwarf approximately 51 light-years away in the constellation of Boötes. The system is also a binary star system, with the secondary star being a red dwarf...
was one of the first stars confirmed to have planets orbiting it. On 16 December 1999, the planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
was dubbed the "Millennium Planet" because the planet was then (erroneously) thought to be the first extrasolar 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...
to be discovered visually.
Discovery
Discovered in 1996, the planet is one of the first extrasolar planets found. It was discovered by Paul ButlerR. Paul Butler
R. Paul Butler is an astronomer who searches for extrasolar planets.He received a BA and an MS from San Francisco State University, completing a Master's thesis with Geoffrey Marcy, and then completed his doctoral studies at the University of Maryland, College Park in 1993...
and Geoffrey Marcy
Geoffrey Marcy
Geoffrey W. Marcy is an American astronomer, who is currently Professor of Astronomy at the University of California, Berkeley, famous for discovering more extrasolar planets than anyone else, 70 out of the first 100 to be discovered, along with R...
(San Francisco Planet Search Project) using the highly successful radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
method. Since the star is visually bright and the planet is massive, it produces a very strong velocity signal of 469
Orders of magnitude (speed)
To help compare different orders of magnitude, the following list describes various speed levels between approximately 1 m/s and 3 m/s. Values in bold are exact.- List of orders of magnitude for speed :-See also:...
± 5 metres per second, which was quickly confirmed by Michel Mayor
Michel Mayor
Dr. Michel G. E. Mayor is a Swiss astrophysicist and professor emeritus at the University of Geneva's Department of Astronomy. He formally retired in 2007, but remains active as a researcher at the Observatory of Geneva...
and Didier Queloz
Didier Queloz
Didier Queloz is a Geneva-based astronomer with a prolific record in finding extrasolar planets. He is understudy to Michel Mayor.Didier Queloz was a Ph.D...
from data collected over 15 years. It was later confirmed also by the AFOE Planet Search Team.
Orbit and mass
Tau Boötis b is rather massive, with a minimum massMinimum mass
In astronomy, minimum mass is the lower-bound calculated mass of observed objects such as planets, stars and binary systems, nebulae, and black holes. Minimum mass is a widely cited statistic for extrasolar planets...
over four times that of Jupiter
Jupiter
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,...
. It orbits the star in a so-called "torch orbit", at a distance from the star less than one seventh that of Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
's from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. One orbital revolution takes only 3 days 7.5 hours to complete. Because τ Boo is hotter and larger than the Sun and the planet's orbit is so low, it is assumed to be hot. Assuming the planet is perfectly grey with no greenhouse
Greenhouse
A greenhouse is a building in which plants are grown. These structures range in size from small sheds to very large buildings...
or tidal effects, and a Bond albedo
Bond albedo
The Bond albedo, named after the American astronomer George Phillips Bond , who originally proposed it, is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space...
of 0.1, the temperature would be close to 1600 K
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...
. Although it has not been detected directly, it is certain that the planet is a gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
.
As Tau Boötis b is more massive than most known "hot Jupiter
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...
s", it was speculated that it was originally a brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
, a failed star, which could have lost most of its atmosphere from the heat of its larger companion star. However, this seems very unlikely. Still, such a process has actually been detected on the famous transiting planet HD 209458 b
HD 209458 b
HD 209458 b is an extrasolar planet that orbits the Solar analog star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapor....
.
In December 1999, a group led by A. C. Cameron had announced that they had detected reflected light from the planet. They calculated that the orbit of the planet has an inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
of 29° and thus the absolute mass of the planet would be about 8.5 times that of Jupiter. They also suggested that the planet is blue in color. Unfortunately, their observations could not be confirmed and were later proved to be spurious. A better estimate came from the assumption of tidal lock with the star, which rotates at 40 degrees; fixing the planet's mass between 6 and 7 Jupiter masses. This inclination has been confirmed by magnetic field detection.
Characteristics
The temperature of Tau Boötis b probably inflates its radius higher (1.2 times) than Jupiter's. Since no reflected light has been detected, the planet's albedoAlbedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
must be less than 0.37. At 1600 K, it is (like HD 179949 b
HD 179949 b
HD 179949 b is an extrasolar planet discovered by the Anglo-Australian Planet Search at the Anglo-Australian Observatory, which orbits the star HD 179949. The planet is a so-called "hot Jupiter", a Jupiter-mass planet orbiting very close to its parent star. In this case, orbital distance is almost...
) supposed to be hotter than HD 209458 b
HD 209458 b
HD 209458 b is an extrasolar planet that orbits the Solar analog star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapor....
(formerly predicted 1392K) and possibly even HD 149026 b
HD 149026 b
HD 149026 b is an extrasolar planet approximately 257 light-years away in the constellation of Hercules. The planet was discovered after it transited its parent star, HD 149026...
(predicted 1540 K from higher albedo 0.3, then actually measured at 2300 K). Tau Boötis b's predicted Sudarsky class is V; which is supposed to yield a highly reflective albedo of .55.
It is a candidate for "near-infrared characterisation.... with the VLTI Spectro-Imager".
There are also some indications of another, more distant planet orbiting Tau Boötis A
Tau Boötis
Tau Boötis is a yellow-white dwarf approximately 51 light-years away in the constellation of Boötes. The system is also a binary star system, with the secondary star being a red dwarf...
. However, until the possible planet has completed one orbit it remains speculation.
See also
- 51 Pegasi b51 Pegasi b51 Pegasi b , sometimes though unofficially named Bellerophon, is an extrasolar planet approximately 50 light-years away in the constellation of Pegasus...
- 70 Virginis b70 Virginis b70 Virginis b is an extrasolar planet approximately 60 light-years away in the constellation of Virgo. Announced in 1996 by Geoffrey Marcy and R. Paul Butler, 70 Virginis was one of the first stars confirmed to have planets orbiting it...
- 55 Cancri b55 Cancri b55 Cancri b is an extrasolar planet orbiting the Sun-like star 55 Cancri A every 14.65 days. It is the second planet in order of distance from its star, and is an example of a hot Jupiter. Discovered in 1996 by Geoffrey Marcy and R...
- 47 Ursae Majoris b47 Ursae Majoris b47 Ursae Majoris b is an extrasolar planet approximately 46 light-years from Earth in the constellation of Ursa Major. The planet was discovered located in a long-period orbit around the star 47 Ursae Majoris in January 1996 and as of 2011 is the innermost of three known planets in its planetary...
- Upsilon Andromedae bUpsilon Andromedae bUpsilon Andromedae b, occasionally referred to as Upsilon Andromedae Ab , is an extrasolar planet approximately 44 light-years away from Earth in the constellation of Andromeda . The planet was discovered orbiting the Solar twin star, Upsilon Andromedae, approximately every five days...