Tau Bootis Planetary System

THE STAR Tau Bootis, on the border between fourth and fifth magnitude, has been classed both as an F6 subgiant and an F7 dwarf. At a distance of 51 light years, it shines with triple the solar luminosity. Warmer than the Sun, as befits its class, the surface temperature lies between 6440 and 6550 Kelvin, and probably closer to the latter. Its higher luminosity and temperature show it to have a radius 1.4 times solar and a mass 30 percent larger than the Sun. Subgiant status implies that the star is nearing the end of its hydrogen-fusing life, but age measurements suggest otherwise, that the star is only about 1.3 billion years old. About a third of the way through its normal life, the star must therefore still be a dwarf. Tau Bootis is suspected of being a slight variable with a magnitude range of under 10 percent, which would make it unusual among stars with planets. More unusual, Tau Bootis has a distant companion, a red (class M2) dwarf that orbits far outside the "planetary system" (if indeed there is an actual "system") at an average distance of 250 astronomical units with a 2000 year revolutionary period. (From Earth, the separation is now around 5 seconds of arc.) The orbit is highly elliptical, the star coming as close as 22 astronomical units and going out as far as 470. The "double sun" would present quite a sight from a hypothetical earth orbiting Tau Bootis proper. Even at the average distance it would shine with the brilliance of 16 Venuses; at its closest, it would glower redly with that of a dozen full Moons.

System Components
(Mass in Multiples of Jupiter's Mass)

Tau Bootes B

Basic Data
Parent Star: tau Bootes (F7 V)
Discovery Status: Confirmed
Planet Type: Hot Jupiter
Physical Data
Mass ( M sin i ): 3.87 Jupiters
Mean Distance: 0.046 AU
Orbital Period: 3.3128 Days
Eccentricity: 0.018
Argument of Perihelion (omega): 164°
Inferred Properties
Average Angular Size of Star: 17.923°
Planet Appearance:
MgSiO3 clouds, dim red glow
Mean Temp: 1576 Kelvin
Discovery Facts
Year Discovered: 1996
Detection Method: Doppler Spectroscopy, Reflected Starlight
Discovered By: Butler and Marcy, Reflected Starlight Detected by Cameron et al.
Speculations
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.

Planetary Orbit Around Tau Bootes

Position In Bootes