helium planet

A helium planet is a theoretical type of planet which has an atmosphere that is primarily helium. Solar system giant planets have atmospheres with far more hydrogen than helium. Light hydrogen molecules are the most likely to escape and seemingly under the right conditions, a planet could be left with the helium but not the hydrogen. Gliese 436 b has been suggested as a possible helium planet. A different possibility regarding how such a substellar object might come to exist is an old white dwarf losing its hydrogen to a binary-star companion.

Note that Earth atmosphere has more molecular helium than molecular hydrogen (which essentially has escaped), but both are at most a trace and Earth would not be considered a helium planet. However, Earth atmosphere does contain more hydrogen in the form of water vapor.