Characteristics
As evidenced by its high mass and planetary radius, HAT-P-1b is a gas giant, most likely composed primarily of hydrogen and helium. The planet would thus have no well-defined surface. Current theories predict that such planets formed in the outer regions of their solar systems and migrated inwards to their present orbits.
HAT-P-1b is significantly larger than predicted by theoretical models. This may indicate the presence of an additional source of heat within the planet. One possible candidate is tidal heating from an eccentric orbit, a possibility which has not been ruled out from the available measurements. However, another planet with a significantly inflated radius, HD 209458 b, is in a circular orbit.
An alternative possibility is that the planet has a high axial tilt, like Uranus in our solar system. The problem with this explanation is that it is thought to be quite difficult to get a planet into this configuration, so having two such planets among the set of known transiting planets is problematic.
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