Properties of the first five Kepler exoplanets compared to Earth and Jupiter
All of them are close to their parent stars, so they have high surface temperatures, 1500-1800K or 2240-2780 degrees F. None of this is very surprising. We knew Kepler could detect extrasolar planet transits, and the first ones were liable to be fairly large and close in. These are the easiest to detect in the shortest amount of time.
What is even more interesting to me, is the discovery of two so-called Kepler Objects of Interest (KOI). What is so interesting about KOI?
First, they were discovered because when they disappear behind the parent star from our point of view the light from the system is dimmed dramatically. I mean a lot! Usually this phase is known as the secondary eclipse, and is much less noticeable than the primary eclipse of the star as the planet transits across the face of the star. The fact that the light was dimmed more by the disappearance of the KOI means they are blazing hot and emitting a lot of light on their own. Astronomers estimate the temperature for KOI-74b to be 12250K (21590F) and KOI-81b a blazing 13500K (23840F). The hottest known exoplanet to date is a mild 2300K (3700F) in comparison.
Both KOI are actually hotter than their host stars. KOI-74b companion star is an A1V type star with a surface temperature around 9400K. KOI-81b has a companion type B9-A0V, with a temperature of approximately 10000K.
KOI-74b and KOI-81b are not massive enough to be stars, with solar masses of 0.111 and 0.212 respectively. That's just not massive enough to start nuclear burning in the core. Yet each object is far too hot to only be shining by heat absorbed from its companion star and then re-emitted into space. This may mean they have evolved from hot stars into their current state, and they are slowly cooling with time. The problem with that theory is with host stars that are relatively young type A and B stars, there doesn't appear to have been enough time for these to have evolved from massive, hot stars to the state they are in now.