An Odd Planet

A recent paper by John Asher Johnson et al. (2012) titled “Characterizing the Cool KOIs. II. The M Dwarf KOI-254 and Its Hot Jupiter” describes the discovery of the first hot Jupiter around a red dwarf star. Red dwarf stars are the coolest and least massive type of stars, and they also make up the majority of stars. The star involved is called KOI-254 and a gas giant planet orbits it once every 2.455 days. This gas giant planet is designated KOI-254 b and it was originally discovered by the Kepler mission from its periodic transits in front of KOI-254. During each transit, KOI-254 dims by 3.909 percent and from this, the size of KOI-254 b is estimated to be 0.96 times the diameter of Jupiter.

Radial velocity observations measuring the gravitational tug exerted by KOI-254 b on KOI-254 gives KOI-254 b a mass of 0.505 times the mass of Jupiter. The estimated equilibrium temperature of KOI-254 b is 1000 degrees Kelvin. What makes KOI-254 b unique is that it is the first known Jupiter-mass planet in a short period orbit around a red dwarf star. Apart from KOI-254 b, there is no known planet above 0.2 times the mass of Jupiter orbiting a red dwarf star with an orbital period of less than 10 days. Given the fact that close-in Jupiter mass planets should be readily detected around red dwarf stars, the lack of such planets mean that KOI-254 b is likely to be the only known example of a hot Jupiter around a red dwarf star for some time.