In 1996, tau Bootis b was discovered via Doppler Spectroscopy. In 1999, direct detection was made by screening out the Doppler-shifted signature of starlight reflected from the planet.
[Note: These speculations are out of date. The 1999 direct detection was later found to be in error. t Boo b is still the hottest known exoplanet, so hot that silicate clouds may be present and the planet may be visibly red. No evidence of an extended or evaporating atmosphere has so far been detected, although the discovery of such an atmosphere around HD 209458 give credence to the idea.] Tau Bootes b is one of the most extreme of the so-called "Hot Jupiters", planets orbiting unusually close to their parent stars. It is both more massive and closer to its star than any of its cousins, such as 51 Pegasi b and 55 Cancri b. Its large mass of 3.87 Jupiters makes tau Bootes b almost certainly a gas giant, and one of the biggest exoplanets yet discovered. Another way that tau Bootes b stands out is its parent star. Unlike many of the other parent stars of exoplanets, Tau Bootes is an F type star, making it much hotter and brighter than more Sun-like stars. The combination of closer orbit, higher mass (meaning higher internal temperature), and hotter star makes it highly probable that tau Bootes b has the highest surface temperature of any known planet. The temperature may be so high that it is questionable whether the planet's atmosphere could survive. In such hostile environs, tau Bootes b may be slowly evaporating into space. This would make the planet appear hazy and featureless as the departing gases obscure the surface clouds. Both Saturn and Uranus have the same kind of featureless surfaces due to obscuring smog, although not for the same reason. The lost gases may also settle into a cloud around tau Bootes, or it may be blown out of the system entirely by stellar winds. The gas may even trail behind the planet in a faint comet-like tail. If tau Bootes b is indeed loosing mass due to the proximity of its star, it must have been even more massive in the past. How much more massive depends on the rate of mass loss. The percentage of mass lost may have been insignificant. Or it's possible that the planet was much more massive in the past. This brings up the interesting possibility that the planet is not really a planet at all. It may have started life as a brown dwarf (See Gliese 229) that was unfortunate enough to have formed or wandered too close to its star. Clearly tau Bootes b holds intriguing possibilities for future exploration.
View a VRML model of the system. Please be patient while the file downloads. For a VRML tour of our galaxy's exoplanets, check out Extrasolar VR.
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