# Why is the exoplanet HD 131399Ab so hot?

Recently, a fascinating triple-star system with an exoplanet has been discovered (arxiv).

I'm very surprised about the temperature of the exoplanet: it is roughly 850 kelvin, even though its orbit is ~82 AU away from the main star (~1.5 $M_☉$), and >250 AU from the other two stars. The planet itself has ~4 jupiter-masses.

Why can such a planet be so hot, for instance compared to Jupiter? I would have expected it to be very cold because it loses a lot of energy via radiation (that's actually the reason why the group discovered it via direct imaging), and it does not get much energy from its stars.

• The atmospheric composition of that planet consists of water and methane. That could be the reason why it has that temperature, considering methane can trap heat far better than CO2. – CipherBot Jul 17 '16 at 8:06

• @NicoDean The luminosity of a contracting giant planet will be $\propto t^{-1}$ (roughly), so an object (moon) in a "habitable zone" now around the giant planet would not stay habitable for very long. – ProfRob Jul 17 '16 at 17:32