GJ 436 b is a Neptune-sized planet with 23.2 times the mass and 4.22 times the diameter of Earth. It is in a close-in orbit around a red dwarf star and it may have a helium-dominated atmosphere. Observations of GJ 436 b indicate that the planet’s atmosphere is rich in carbon monoxide (CO) but depleted in methane (CH4). This is strange because if GJ 436 b has a hydrogen-helium atmosphere similar to Neptune’s, most of its carbon should be in the form of methane (CH4) and not carbon monoxide (CO). Nevertheless, a helium-dominated atmosphere can explain the observed atmospheric composition of GJ 436 b.
Since warm Neptunes and sub-Neptunes orbit close to their parent stars, these planets are likely tidally-locked with the same hemisphere always facing the star, leading to permanent day and night sides. The spot on the dayside that receives the most intense irradiation is known as the substellar point. Here, the star is always directly overhead. The intense irradiation creates a hot spot around the substellar point. However, the presence of winds in the atmosphere tends to shift the hot spot away from the substellar point. For a warm Neptune or sub-Neptune with a hydrogen-helium atmosphere, the hot spot can shift far from the substellar point due to the larger heat capacity of hydrogen. In contrast, for a helium-dominated atmosphere, the low heat capacity of helium means that the hot spot is likely to remain at the substellar point because it cannot shift far enough without cooling significantly.
Hu et al. (2015), “Helium Atmospheres on Warm Neptune- and Sub-Neptune-Sized Exoplanets and Applications to GJ 436 b”, arXiv:1505.02221 [astro-ph.EP]