 Artist's impression of Tau Boötis b orbiting close to its parent star. | |
| Discovery | |
|---|---|
| Discovered by | Butleret al. |
| Discovery site | University of California |
| Discovery date | 1996 |
| Doppler Spectroscopy | |
| Orbital characteristics | |
| 0.0481AU | |
| Eccentricity | 0.023 ± 0.015[1] |
| 3.312463 ± 0.000014[1]d | |
| Inclination | 44[2] |
| 2,446,957.81 ± 0.54 | |
| 188 | |
| Semi-amplitude | 461.1 |
| Star | Tau Boötis |
| Physical characteristics | |
| 1.06[3]RJ | |
| Mass | 5.5–6[2]MJ |
| Temperature | 1,700 K (1,430 °C; 2,600 °F) |
Tau Boötis b, or more preciselyTau Boötis Ab, is anextrasolar planet approximately 51light-years away. The planet and its host star is one of the planetary systems selected by theInternational Astronomical Union as part ofNameExoWorlds, their public process for giving proper names to exoplanets and their host star (where no proper name already exists).[4][5] The process involved public nomination and voting for the new names, and the IAU planned to announce the new names in mid-December 2015.[6] However, the IAU annulled the vote as the winning name was judged not to conform with the IAU rules for naming exoplanets.[7]
Discovered in 1996, the planet is one of the first extrasolar planets found. It was discovered orbiting the star Tau Boo (HR 5185) byPaul Butler and his team (San Francisco Planet Search Project)[8] using the highly successfulradial velocity method. Since the star is visually bright and the planet is massive, it produces a very strong velocity signal of469 ± 5 metres per second, which was quickly confirmed byMichel Mayor andDidier Queloz from data collected over 15 years. It was later confirmed also by theAFOE Planet Search Team.
Tau Boötis b is rather massive, with aminimum mass over four times that ofJupiter. It orbits the star in a so-called "torch orbit", at a distance from the star less than one seventh that ofMercury's from theSun.[citation needed] 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 perfectlygrey with noGreenhouse effect ortidal effects, and aBond albedo of 0.1, the temperature would be close to 1600K.[10] Although it has not been detected directly, it is certain that the planet is agas giant.As Tau Boötis b is more massive than most known "hot Jupiters", it was speculated that it was originally abrown 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 famoustransiting planetHD 209458 b.
In December 1999, a group led byA. C. Cameron had announced that they had detected reflected light from the planet. They calculated that the orbit of the planet has aninclination 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 oftidal lock with the star, which rotates at 40 degrees;[11] fixing the planet's mass between 6 and 7 Jupiter masses. In 2007, magnetic field detection confirmed this estimate.[12]
In 2012 two teams independently distinguished theradial velocity of the planet from the radial velocity of the star by observing the shifting of thespectral lines ofcarbon 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 of5.95±0.28 MJ.[13] The other team found an inclination of 47−6+7 and a mass of5.6±0.7 MJ.[14]
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'salbedo must be less than 0.37.[11][15] The albedo constraint was tightened to less than 0.12 by 2021.[16] At 1600 K, it is (likeMastika) supposed to be hotter thanHD 209458 b (formerly predicted 1392K) and possibly evenSmertrios (predicted 1540 K from higher albedo 0.3, then actually measured at 2300 K). Tau Boötis b's predictedSudarsky 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 theVLTI Spectro-Imager".[10] 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 ofwater vapor in atmosphere of the planet was announced.[17] Later atmosphere characterization in 2021 have resulted in measured carbon abundance similar to that ofJupiter, in the form of 0.35%carbon monoxide volume admixture to hydrogen-helium atmosphere. The upper limit only of water below 2 parts per million (0.72% of that expected for solar composition) was estimated.[18]
In 2020, a radio emission in the 14-30 MHz band was detected from theTau Boötis system, likely associated withcyclotron radiation from the poles of Tau Boötis b. This could be a signature of the planet's magnetic field.[19][20]
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