they saw a certain size pixel pass a larger lighter group of pixels. Sure thing. They think the planet could have a huge amount of heavy metal giving it a high gravity or if it's core is cold or if it doesn't have a magnetic field or an atmosphere. So until we get later episodes of Star Trek long range scanners this is a very rough guess at all. Hell they have a rough idea of size and a guess at it's rotational period and that is all I know for sure. Hell even the dwarf star they have an idea of photons produced by no idea of the actual heat on the planet in question.