Helium planet

A helium planet is a special kind of planet that has an atmosphere mostly made of helium. This is different from big planets like Jupiter and Saturn, which have atmospheres that are mostly hydrogen with just a little bit of helium. Helium planets can form in many different ways, and scientists think that Gliese 436 b might be one of these special planets. These planets are interesting because they are very different from the planets we know and can help us learn more about how planets form and change.

Formation

There are several ideas about how a helium planet might form.

Hydrogen evaporation from giant planets

A helium planet might form when a gaseous planet orbits very close to its star. The star's heat can push away the lighter gases, like hydrogen, leaving behind more of the heavier helium. This process can happen to giant planets that are very close to their stars, causing the hydrogen to boil away and escape. Over a very long time, the planet's atmosphere would then have much more helium. Hot Neptunes could be examples of this.

As hydrogen disappears, the planet also loses some other gases, like methane. Normally, methane cycles through the atmosphere, but with less hydrogen, it can't form as easily, so the methane levels drop too.

Characteristics

Helium planets look different from regular planets because their air is mostly made of helium, not hydrogen. We can tell these planets apart by looking for certain gases, like carbon monoxide and carbon dioxide, in their air. Since there isn’t much hydrogen, another gas called methane can’t form. Instead, carbon links up with oxygen, making carbon monoxide and carbon dioxide. Because of what their air is made of, helium planets likely appear white or grey. One example is a planet called Gliese 436 b, which has lots of carbon monoxide and might be a helium planet.