Tau Bootis

(The Planet Project)

Tau Bootis hosts both a "hot Jupiter" planet and a much more distant binary companion.


The circle shows the location of the class F star Tau Bootis (in the constellation Bootes). The planet, which has a mass at least 3.87 times that of Jupiter, has one of the shortest known periods, only 3.31 days, and orbits at a close distance of 0.046 astronomical units from the star (6.9 million kilometers, 12 percent Mercury's distance from the Sun) in a nearly circular orbit, making it one of the several "hot Jupiter's known. The planet, while not actually seen, may have been detected in light reflected from the central star.


Tau Bootis, on the border between fourth and fifth magnitude (4.50), has been classed both as an F6 subgiant and an F7 dwarf. At a distance of 51 light years, it shines with 3.1 times the solar luminosity. Warmer than the Sun, as befits its class, the surface temperature fall around 6340 Kelvin. The star's higher luminosity and temperature show it to have a radius 1.46 times solar and a mass of 1.3 times that of the Sun. The star is most likely a core-hydrogen-fusing dwarf, but one with some age to it. 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 Boo has a distant companion, a red class M dwarf of 0.4 solar masses that orbits far outside the planetary system (if indeed there is an actual "system") at an average distance of 100 or 240 astronomical units with a revolutionary period 750 or 2600 years (the highly elliptical orbit still uncertain). Like most stars that have planets, this one is also metal rich, the iron a bundance (relative to hydrogen) 1.7 times solar.
Written by Jim Kaler. Return to The Planet Project or go to STARS.