A helium planet is a theoretical type of planet that may form via mass loss from a low mass white dwarf star. Ordinary gas giant planets such as Jupiter and Saturn consist primarily of hydrogen, with helium as a secondary component. But a helium planet might form in an environment where all the hydrogen has been processed to helium or other heavier elements by nuclear burning.
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One scenario involves an AM CVn type of symbiotic binary star composed of two helium core white dwarf stars surrounded by a circumbinary helium accretion disk formed during mass transfer from the less massive to the more massive white dwarf. After it loses most of its mass, the less massive white dwarf may approach planetary mass.[1]
Helium planets are predicted to be of roughly similar diameter to hydrogen/helium planets of the same mass. But helium planets heavier than 13 Jupiter masses will still be planets, not brown dwarfs, because they contain no deuterium and cannot fuse anything anymore.
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