Due to its great mass, Anubis exhibits a very hot core and shines with its own light just barely
enough to be detected. It doesn't need to be illuminated by the star to be seen. The core
temperature is expected to be about 1.8 million K and the surface temperature of 1,800 K at this point
in its existence. The planet luminosity, energy radiated, is about 1e-5 that of the Sun. It is much
more luminous than Jupiter.
The energy release due to gravitational compression is best exemplified by formation of a very massive
body, such as protostar, a star that has almost fully formed, but hasn't initiated fusion. This
protostar has a core temperature less than 10 million K. It is still collapsing in on itself and the
pressures and temperature in the core are still increasing via gravitational compression. The more
massive it gets the more gravity it exerts and so on. The protostar is emitting a great deal of heat
and light by this process as it forms. It can be very brilliant for many millions of years before it
begins nuclear fusion.
A giant planet, close to the mass of a small star, would exhibit similar characteristics. Should the
core temperatures be in the millions of K, the giant planet would also radiate great amounts of energy
in a similar manner. Though most of this energy output would be heat, there would be some visible
light radiated as well. In fact, in the case of Anubis, it was probably quite bright during its
formation. Jupiter, our closest example, radiates about 1.6 times the energy it receives from the
sun into space. Jupiter's core temperature, at 20,000 K, is far too low to emit any visible light,
but it is very clearly visible in the infra red against its cold background.
Anubis will eventually grow dark as it cools. Furthermore, the M class star in the system, being so distant, puts very little energy into the planet to slow its cooling. Jupiter currently has a core temperature of 20,000K but is approximately 4.5 billion years old. It was much hotter when it initially formed. It is not likely that Anubis will be particularly luminous for more than another few billion years.