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Tau Boötis Ab

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**Tau Boötis Ab**
Parent star
Extrasolar planet		List of extrasolar planets
An artist's conception of Tau Boötis Ab looking at its sun
Star		Tau Boötis A
Constellation		Boötes
Right ascension	(α)	13^h 47^m 15.7^s
Declination	(δ)	+17° 27′ 25″
Spectral type		F6IV
Orbital elements
Semimajor axis	(a)	0.0481 AU
Eccentricity	(e)	0.023 ± 0.015
Orbital period	(P)	3.312463 ± 0.000014 d
Angular distance	(θ)	3.067 mas
Longitude of periastron	(ω)	188°
Time of periastron	(T₀)	2,446,957.81 ± 0.54 JD
Semi-amplitude	(K)	461.1 m/s
Physical characteristics
Mass	(m)	>4.13 M_J
Discovery information
Discovery date		1996
Discoverer(s)		Marcy et al.
Detection method		Doppler Spectroscopy
Discovery site		California, USA
Discovery status		Published

Tau Boötis Ab (τ Boo Ab / τ Boötis Ab) is an extrasolar planet orbiting Tau Boötis A. The designation is often written as Tau Boo b, but since Tau Boo B is a red dwarf companion of Tau Boo A, it is better to use the first mentioned designation. In 1999 the planet was dubbed the Millennium Planet.

1 Discovery
2 Orbit and mass
3 Characteristics
4 See also
5 External links
- 5.1 References
- 5.2 News

Discovery

Discovered in 1996, the planet is one of the first extrasolar planets found. It was discovered by Paul Butler 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.

Orbit and mass

Tau Boötis Ab is rather massive, with a minimum mass over four times that of Jupiter. Recent observations using Canada's MOST space telescope suggest 7-8 times Jupiter's mass. 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 one of the hottest known planets, with a temperature about 1621 K. Although it has not been detected directly, it is certain that the planet is a gas giant. As Tau Boötis Ab is more massive than most known "hot Jupiters", 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 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.

Characteristics

Tau Boötis Ab's temperature probably inflates its radius higher (1.2 times) than Jupiter's. Since no reflected light has been detected, the planet's albedo must be less than 0.39.^[1] At 1621 K, its predicted Sadursky class is V; which is supposed to yield a highly reflective albedo of .55. There are also some indications of another, more distant planet orbiting τ Boötis A. However, until the possible planet has completed one orbit it remains speculation.