The Heavy Planet is a dense planet with an unusually large iron core, which is one of many planet types and, one of 2 extreme planets currently not in Galactic Civilizations III.
How to Identify?
Heavy worlds can be identified by the heavy icon next to the planet and when hovering your mouse over the planet, it will indicate it's a heavy world. Unfortunately this planet type is currently not in the game, however, is expected to be added either at or shortly after release either by patch, DLC (Downloadable Content) or expansion.
Heavy worlds are a very dense planet with an unusually large iron core causing the gravity to be many times that of normal planets. Unfortunately, for most races this makes it a planet that most life forms, use to living on less dense planets, are not able to move around at all or very little. Because of this it will be one of the extreme planets that require additional technologies or racial abilities to be able to survive in this environment. This is also the only extreme planet that was in Galactic Civilizations II that is not in Galactic Civilizations III.
Unfortunately no strategy exists at this point, however, one is to expect that like the other extreme worlds it would be best to rush to the technology to unlock it as quickly as possibly in galaxies crowded or with limited options for expansion.
Which races able to immediately colonize?
Currently there are no races that are able to colonize this planet type at the start of the game, however, it is possible that a race would be added in the future in a DLC (Downloadable Content) or expansions that this will become possible.