Tau Boötis b

Tau Boötis b, or more precisely Tau Boötis Ab, is an extrasolar planet approximately 51 light-years away. 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).[3][4] The process involved public nomination and voting for the new names, and the IAU planned to announce the new names in mid-December 2015.[5] However, the IAU annulled the vote as the winning name was judged not to conform with the IAU rules for naming exoplanets.[6]

Tau Boötis b
Artist's impression of Tau Boötis b orbiting close to its parent star.
Discovery
Discovered by[Richard Price]
Discovery siteUniversity of California
Discovery date1996
Doppler Spectroscopy
Orbital characteristics
0.0481 AU
Eccentricity0.023 ± 0.015 [1]
3.312463 ± 0.000014 [1] d
Inclination44[2]
2,446,957.81 ± 0.54
188
Semi-amplitude461.1
StarTau Boötis
Physical characteristics
Mass5.5–6[2] MJ
Temperature1,700 K (1,430 °C; 2,600 °F)

    Discovery

    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 his team (San Francisco Planet Search Project)[7] 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
    VLT's wide-field view of the parent star of Tau Boötis b[8]

    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 small, it is assumed to be hot. Assuming the planet is perfectly grey with no Greenhouse effect or tidal effects, and a Bond albedo of 0.1, the temperature would be close to 1600 K.[9] 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;[10] fixing the planet's mass between 6 and 7 Jupiter masses. In 2007, magnetic field detection confirmed this estimate.[11]

    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±1.5 degrees and a mass of 5.95±0.28 MJ.[12] The other team found an inclination of 47−6+7 and a mass of 5.6±0.7 MJ.[13]

    Characteristics
    Artist impression of the magnetic field around Tau Boötis b detected in 2020.

    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,[10][14]. The albedo constraint was tightened to less than 0.12 by 2021.[15] 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".[9] 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".[2] In 2014, direct detection of water vapor in atmosphere of the planet was announced.[16]

    In 2020, a radio emission in 14-30 MHz band was detected from Tau Boötis system, likely associated with cyclotron radiation from the poles of Tau Boötis b.[17]

    See also

    References
    1. Butler, R. P.; et al. (2006). "Catalog of Nearby Exoplanets". The Astrophysical Journal. 646 (1): 505–522. arXiv:astro-ph/0607493. Bibcode:2006ApJ...646..505B. doi:10.1086/504701.
    2. "New Way of Probing Exoplanet Atmospheres" in Science Daily (27 June 2012), https://www.sciencedaily.com/releases/2012/06/120627132051.htm; reporting on Nature (28 June 2012)
    3. NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. IAU.org. 9 July 2014
    4. NameExoWorlds.
    5. NameExoWorlds.
    6. Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.
    7. 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.
    8. "New Way of Probing Exoplanet Atmospheres". ESO Press Release. Retrieved 28 June 2012.
    9. 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)" (PDF). Proceedings of SPIE. Optical and Infrared Interferometry. 7013: 70132Z–70132Z–10. arXiv:0807.3014. Bibcode:2008SPIE.7013E..2ZR. doi:10.1117/12.790494.
    10. 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/0308413. Bibcode:2003MNRAS.344.1271L. doi:10.1046/j.1365-8711.2003.06901.x.
    11. 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/0610758. Bibcode:2007MNRAS.374L..42C. doi:10.1111/j.1745-3933.2006.00261.x.
    12. 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:1206.6109. Bibcode:2012Natur.486..502B. doi:10.1038/nature11161. PMID 22739313.
    13. Rodler, F.; et al. (2012). "Weighing the Non-transiting Hot Jupiter τ Boo b". The Astrophysical Journal Letters. 753 (1). L25. arXiv:1206.6197. Bibcode:2012ApJ...753L..25R. doi:10.1088/2041-8205/753/1/L25.
    14. 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:0807.2568. Bibcode:2009MNRAS.393..229L. doi:10.1111/j.1365-2966.2008.14182.x.
    15. Polarization of hot Jupiter systems: a likely detection of stellar activity and a possible detection of planetary polarization, 2021, arXiv:2101.07411
    16. 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
    17. The search for radio emission from the exoplanetary systems 55 Cancri, υ Andromedae, and τ Boötis using LOFAR beam-formed observations, 2020, arXiv:2012.07926

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