Tau Boötis Ab
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| Extrasolar planet | Lists of extrasolar planets | |
|---|---|---|
 |
||
| Parent star | ||
| Star | Tau Boötis A | |
| Constellation | Boötes | |
| Right ascension | (α) | 13h 47m 15.7s |
| Declination | (δ) | +17° 27′ 25″ |
| Spectral type | F6IV | |
| Orbital elements | ||
| Semimajor axis | (a) | 0.05 AU |
| Eccentricity | (e) | 0.01 |
| Orbital period | (P) | 3.312 d |
| Inclination | (i) | ° |
| Longitude of periastron |
(ω) | 65° |
| Time of periastron | (τ) | 2,448,377.72 JD |
| Physical characteristics | ||
| Mass | (m) | >7-8 MJ |
| Radius | (r) | ? RJ |
| Density | (ρ) | ? kg/m3 |
| Temperature | (T) | ~1576 K |
| Discovery information | ||
| Discovery date | 1996 | |
| Discoverer(s) | Marcy et al. | |
| Detection method | Doppler Spectroscopy | |
| Discovery status | Confirmed | |
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.
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[edit] 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.
[edit] 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 1500 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 had lost most of its atmosphere in the heat of the 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.
[edit] Characteristics
Since no reflected light has been detected, the planet's albedo must be less than 0.39, which is rather dark for a gas giant.
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.
[edit] See also
[edit] External links
[edit] References
- Butler et al. (1997). "Three New 51 Pegasi Type Planets". The Astrophysical Journal 474: L115-L118.
- Extrasolar Planets Encyclopaedia: Tau Boo b
- Exrasolar Visions: Tau Bootis b
- The Planet Around Tau Bootes
- AFOE observations of tau Boötis
