Interterrestrials: The "aliens" are inner earthlings...

[Mister Obvious]

So the "theory" is that the inner sun (planet's core) has a buffer around it that allows for the innermost creatures to observe it in a similar way as we observe our sun here on the surface.

The vast array of different landscapes that exist underground host creatures of all kinds, just as many as are on the planet's surface, in the oceans, etc.

There could be places underground which do have some light from the core, and this could be why some of these sub-surface creatures have eyes at all.