Helium planet

A helium planet is a planet with a helium-dominated atmosphere. This is in contrast to ordinary gas giants such as Jupiter and Saturn, whose atmospheres consist primarily of hydrogen, with helium as a secondary component only. Helium planets might form in a variety of ways. The Gliese 436 b exoplanet is a candidate helium planet.

Theoretical ideas

There are several theoretical ideas for how a helium planet might form.

Hydrogen evaporation from giant planets

A helium planet might form via hydrogen evaporation from a gaseous planet orbiting close to a star. The star will drive off lighter gases more effectively through evaporation than heavier gasses, and over time deplete the hydrogen, leaving a greater proportion of helium behind.

Helium planets are predicted to have roughly the same diameter as hydrogen–helium planets of the same mass.

Origins

A scenario for forming helium planets from regular giant planets, involves an ice giant in an orbit so close to its host star, that the hydrogen effectively boils out of the atmosphere, evaporating from and escaping the gravitational hold of the planet. The planet's atmosphere will experience a large energy input and because light gases are more readily evaporated than heavier gases, the proportion of helium will steadily increase in the remaining atmosphere. Such a process will take some time to stabilize and completely drive out all the hydrogen, perhaps on the order of 10 billion years, depending on the precise physical conditions and the nature of the planet and the star. Hot Neptunes are candidates for such a scenario.