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Planets with 2 $R_{oplus}$ < $R$ < 3 $R_{oplus}$ and orbital period $<$100 d are abundant; these sub-Neptune exoplanets are not well understood. For example, $Kepler$ sub-Neptunes are likely to have deep magma oceans in contact with their atmospheres, but little is known about the effect of the magma on the atmosphere. Here we study this effect using a basic model, assuming that volatiles equilibrate with magma at $T$ $sim$ 3000 K. For our Fe-Mg-Si-O-H model system, we find that chemical reactions between the magma and the atmosphere and dissolution of volatiles into the magma are both important. Thus, magma matters. For H, most moles go into the magma, so the mass target for both H$_2$ accretion and H$_2$ loss models is weightier than is usually assumed. The known span of magma oxidation states can produce sub-Neptunes that have identical radius but with total volatile masses varying by 20-fold. Thus, planet radius is a proxy for atmospheric composition but not for total volatile content. This redox diversity degeneracy can be broken by measurements of atmosphere mean molecular weight. We emphasise H$_2$ supply by nebula gas, but also consider solid-derived H$_2$O. We find that adding H$_2$O to Fe probably cannot make enough H$_2$ to explain sub-Neptune radii because $>$10$^3$-km thick outgassed atmospheres have high mean molecular weight. The hypothesis of magma-atmosphere equilibration links observables such as atmosphere H$_2$O/H$_2$ ratio to magma FeO content and planet formation processes. Our models accuracy is limited by the lack of experiments (lab and/or numerical) that are specific to sub-Neptunes; we advocate for such experiments.
Transiting planets with radii 2-3 $R_bigoplus$ are much more numerous than larger planets. We propose that this drop-off is so abrupt because at $R$ $sim$ 3 $R_bigoplus$, base-of-atmosphere pressure is high enough for the atmosphere to readily dissol
The next step on the path toward another Earth is to find atmospheres similar to those of Earth and Venus - high-molecular-weight (secondary) atmospheres - on rocky exoplanets. Many rocky exoplanets are born with thick (> 10 kbar) H$_2$-dominated atm
In the last decade, about a dozen giant exoplanets have been directly imaged in the IR as companions to young stars. With photometry and spectroscopy of these planets in hand from new extreme coronagraphic instruments such as SPHERE at VLT and GPI at
Modeling the outflow of planetary atmospheres is important for understanding the evolution of exoplanet systems and for interpreting their observations. Modern theoretical models of exoplanet atmospheres become increasingly detailed and multicomponen
In our solar system, the presence of rings is exclusive to the gas giants, but is this the case for all planetary systems? In principle, it seems that rocky exoplanets could also have rings, which could be searched for by studying their subtle imprin