A helium planet is a theoretical type of planet which has an atmosphere that is primarily helium. Solar system giant planets have atmospheres of hydrogen and 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. Another theory for such a substellar object is an old white dwarf losing its hydrogen to a binary companion. Gliese 436 b has been suggested as a possible helium planet.