Helium Planet

A Helium Planet is a type of giant planet which, in contrast to the hydrogen dominated gas giants, is composed largely of helium. These planets would appear white or grey, and would have relatively high amounts of CO2 and CO in their atmospheres. They are predicted to have about the same mass-radius relationship as gas giants.

There are a number of theories explaining how these planets could come about, including a close-orbiting gas giant being stripped of its hydrogen and methane or even the evaporation of a low-mass white dwarf which orbits a larger star companion.

Worldbuilding in Practice
"Orbiting the white dwarf KRO2-3a is a comparatively small giant planet, KRO2-3b. Spectral data indicates that the planet, which has a mass of 3.2 MJ and a radius of 1.1 RJ, is in fact a helium planet. Computer simulations suggest that it was once a white dwarf which eventually approached planetary mass after losing much of its material to its larger companion."