Tau Boötis b

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Tau Boötis b
Exoplanet List of exoplanets
Artist’s impression of the exoplanet Tau Bootis b.jpg
Artist's impression of Tau Boötis b orbiting close to its parent star.
Parent star
Star Tau Boötis
Constellation Boötes
Right ascension (α) 13h 47m 15.743s[1]
Declination (δ) +17° 27′ 24.86″[1]
Apparent magnitude (mV) 4.5
Distance 50.9 ± 0.2[1] ly
(15.62 ± 0.05[1] pc)
Spectral type F6IV
Orbital elements
Semi-major axis (a) 0.0481 [2] AU
Eccentricity (e) 0.023 ± 0.015 [2]
Orbital period (P) 3.312463 ± 0.000014 [2] d
Inclination (i) 44[3]°
Argument of
(ω) 188°
Time of periastron (T0) 2,446,957.81 ± 0.54 JD
Semi-amplitude (K) 461.1 m/s
Physical characteristics
Mass (m) 5.5-6[3] MJ
Bond Albedo (Ab) <0.37
Temperature (T) 1,700 K (1,430 °C; 2,600 °F)
Discovery information
Discovery date 1996
Discoverer(s) Marcy et al.
Discovery method Doppler Spectroscopy
Other detection methods Direct detection
Discovery site United States University of California
Discovery status Published
Other designations
Tau Boötis Ab
Database references
Extrasolar Planets
Exoplanet Archive data
Open Exoplanet Catalogue data

Tau Boötis b, or more precisely Tau Boötis Ab, is an extrasolar planet approximately 51 light-years away[1] around the primary star of the Tau Boötis system in the constellation of Boötes. The planet's existence being announced in 1996 by Geoffrey Marcy and R. Paul Butler, Tau Boötis was one of the first stars confirmed to have planets orbiting it.[4] 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.[5]

The planet and its host star is one of the planetary systems selected by the International Astronomical Union as part of their public process for giving proper names to exoplanets and their host star (where no proper name already exists).[6][7] The process involved public nomination and voting for the new names, and the IAU planned to announce the new names in mid-December 2015.[8] However, the IAU annulled the vote as the winning name was judged not to conform with the IAU rules for naming exoplanets.[9]


Discovered in 1996, the planet is one of the first extrasolar planets found. It was discovered orbiting the star Tau Boo (HR 5185) by Paul Butler and Geoffrey Marcy (San Francisco Planet Search Project)[4] 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[edit]

VLT's wide-field view of the parent star of Tau Boötis b.[10]

Tau Boötis b is rather massive, with a minimum mass 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.[11] 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 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 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;[12] fixing the planet's mass between 6 and 7 Jupiter masses. In 2007, magnetic field detection confirmed this estimate.[13]

In 2012 two teams independently distinguished the radial-velocity of the planet from the radial velocity of the star by observing the shifting of the spectral lines of carbon monoxide. This enabled calculation of the inclination of the planet's orbit and hence the planet's mass. One team found an inclination of 44.5±1.5degrees and a mass of 5.95±0.28 MJ.[14] The other team found an inclination of 47−6+7 and a mass of 5.6±0.7 MJ.[15]


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 albedo must be less than 0.37.[12][16] 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 0.55.

It has been a candidate for "near-infrared characterization.... with the VLTI Spectro-Imager".[11] When its atmosphere was measured in 2011, "the new observations indicated an atmosphere with a temperature that falls higher up. This result is the exact opposite of the temperature inversion -- an increase in temperature with height -- found for other hot Jupiter exoplanets".[3] In 2014, direct detection of water vapor in atmosphere of the planet was announced.[17]

See also[edit]


  1. ^ a b c d e van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752Freely accessible. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.  Vizier catalog entry
  2. ^ a b c Butler, R. P.; et al. (2006). "Catalog of Nearby Exoplanets". The Astrophysical Journal. 646 (1): 505–522. arXiv:astro-ph/0607493Freely accessible. Bibcode:2006ApJ...646..505B. doi:10.1086/504701. 
  3. ^ a b c "New Way of Probing Exoplanet Atmospheres" in Science Daily (27 June 2012), http://www.sciencedaily.com/releases/2012/06/120627132051.htm; reporting on Nature (28 June 2012) | doi:10.1038/?
  4. ^ a b Butler, R. Paul; et al. (1997). "Three New 51 Pegasi Type Planets". The Astrophysical Journal Letters. 474 (2): L115–L118. Bibcode:1997ApJ...474L.115B. doi:10.1086/310444. 
  5. ^ Steve Connor (16 December 1999). "Scientists catch the `millennium' planet's glow". The Independent. Retrieved 26 December 2011. 
  6. ^ NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. IAU.org. 9 July 2014
  7. ^ NameExoWorlds.
  8. ^ NameExoWorlds.
  9. ^ Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.
  10. ^ "New Way of Probing Exoplanet Atmospheres". ESO Press Release. Retrieved 28 June 2012. 
  11. ^ a b Renard, S.; Absil, O.; Berger, J. -P.; Bonfils, X.; Forveille, T.; Malbet, F. (2008). "Prospects for near-infrared characterisation of hot Jupiters with the VLTI Spectro-Imager (VSI)". Proceedings of SPIE. 7013: 70132Z–70132Z–10. arXiv:0807.3014Freely accessible. doi:10.1117/12.790494. 
  12. ^ a b Leigh, Christopher; et al. (2003). "A new upper limit on the reflected starlight from Tau Bootis b". Monthly Notices of the Royal Astronomical Society. 344 (4): 1271–1282. arXiv:astro-ph/0308413Freely accessible. Bibcode:2003MNRAS.344.1271L. doi:10.1046/j.1365-8711.2003.06901.x. 
  13. ^ Catala, C.; et al. (2007). "The magnetic field of the planet-hosting star τ Bootis". Monthly Notices of the Royal Astronomical Society. 374 (1): L42–L46. arXiv:astro-ph/0610758Freely accessible. Bibcode:2007MNRAS.374L..42C. doi:10.1111/j.1745-3933.2006.00261.x. 
  14. ^ Brogi, Matteo; Snellen, Ignas A. G.; de Kok, Remco J.; Albrecht, Simon; Birkby, Jayne; de Mooij, Ernst J. W. (28 June 2012). "The signature of orbital motion from the dayside of the planet τ Boötis b". Nature. 486 (7404): 502–504. arXiv:1206.6109Freely accessible. Bibcode:2012Natur.486..502B. doi:10.1038/nature11161. PMID 22739313. 
  15. ^ Rodler, F.; et al. (2012). "Weighing the Non-transiting Hot Jupiter τ Boo b". The Astrophysical Journal Letters. 753 (1). L25. arXiv:1206.6197Freely accessible. Bibcode:2012ApJ...753L..25R. doi:10.1088/2041-8205/753/1/L25. 
  16. ^ Lucas, P. W.; et al. (2009). "Planetpol polarimetry of the exoplanet systems 55 Cnc and tau Boo". Monthly Notices of the Royal Astronomical Society. 393 (1): 229–244. arXiv:0807.2568Freely accessible. Bibcode:2009MNRAS.393..229L. doi:10.1111/j.1365-2966.2008.14182.x. 
  17. ^ Near-IR Direct Detection of Water Vapor in Tau Boo b: Alexandra C. Lockwood, John A. Johnson, Chad F. Bender, John S. Carr, Travis Barman, Alexander J.W. Richert, Geoffrey A. Blake

External links[edit]

Coordinates: Sky map 13h 47m 15.7s, +17° 27′ 25″