'Warm Neptune' Exoplanets May Have Lots of Helium

An anonymous reader writes: Phil Plait reports on new research into exoplanets that came to an unexpected and non-obvious conclusion. Throughout the galaxy, astronomers have been finding exoplanets they call "warm Neptunes" — bodies about the size of Neptune, but which orbit their parent star more closely than Mercury orbits the Sun. When astronomers looked at spectra for these planets, they found something surprising: no methane signature (PDF). Methane is made of carbon and hydrogen, and it's generally assumed that most large, gaseous planets will have a lot of hydrogen. But this class of exoplanet, being significantly smaller than, say, Jupiter, may not have the mass (and thus the gravity) to hold on to its hydrogen when it's heated by the close proximity to the star. The result is that the atmosphere may be largely made up of helium instead. If so, the planet would look oddly colorless to our eyes, very unlike the planets in our solar system.

Comments

Hydrogen can be monatomic in low pressure

[PeterM from Berkeley]

and high energy conditions.

Such as in the high, upper, tenuous atmosphere of one of these giants. The H2 molecule, which is the rule in higher pressures, can be disassociated by light, and the monatomic H can persist a while because of the low pressure and low collision rates.

So escape of monatomic hydrogen could indeed be the significant loss mechanism for these planets. H-H bond is 4.5 eV, which requires an ultraviolet C photon, or multiple photons of lower energy. Quite doable close in to a star.

--PM