Warm Neptunes with Helium Atmospheres

Warm Neptune-sized and sub-Neptune-sized exoplanets are a class of planets that orbit closer to their host stars than Mercury’s orbit around the Sun. Although these planets started out with hydrogen-helium atmospheres, the strong irradiation they receive due to their close proximity to their host stars can drive away the hydrogen and lead to the formation of helium-dominated atmospheres. If the planet started out with a hydrogen-helium atmosphere that is ~0.1 percent of the planet’s mass, the effect of intense stellar irradiation can drive off the hydrogen in the atmosphere in ~10 billion years. In a helium-dominated atmosphere, the scarcity of hydrogen causes the main molecular carrier of carbon to be carbon monoxide (CO) rather than methane (CH4).

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.

A helium-dominated atmosphere exhibits certain characteristics that distinguish it from a hydrogen-helium atmosphere. Helium has a much lower heat capacity compared to hydrogen because monatomic helium has three degrees of freedom while a hydrogen molecule has six degrees of freedom. As a result, the temperature gradient (i.e. the increase in temperature with depth) of a helium-dominated atmosphere is much larger compared to other planetary atmospheres because the temperature gradient of a planetary atmosphere is inversely proportional to its specific heat capacity.

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.

Reference:
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]