"Goldilocks planet" is an informal term for planets in the "Goldilocks zone" or stellar habitable zone of their star. Not too hot, not too cold, just right for sustaining an earth-like amount of water and atmosphere. Such planets are believed to be the best candidates for supporting life. (They are not, however the only candidates — life might also exist on other planets, but in a subsurface, geothermally or tidally heated ocean like the one that Europa is currently thought to have.)
Currently, the Earth is the only unambiguous example, but the extrasolar planet Gliese 581d is believed to orbit within its star's habitable zone. Another planet in the same system, Gliese 581g, may also lie in the habitable zone… if it exists. Astronomers disagree over whether the discovery of planet "g" is an actual planet or a data glitch.
It's possible that Goldilocks planets are quite rare, but they may instead be rather common in our galaxy and just difficult for our current telescopes to detect. Many astronomers operate on the assumption it's the later, and several large telescopes are currently being used to search for more such worlds.
Game and Story Use
- Such planets will be valuable prizes in an interstellar sci-fi game, especially if the civilization still lives primarily on planets.
- Even a civilization which has migrated to space habitats or stellar megastructures will likely still be interested in habitable planets, especially life-bearing ones, for scientific reasons.