The Kepler survey revealed that the radius distribution of sub-Neptunes is bimodal: there is a scarcity of planets between 1.5 and 2.0 R⊕. However, the mechanism that creates the valley is unknown. The low mean densities of sub-Neptunes imply that they formed within a few million years and accreted primordial envelopes. Because these planets receive X-ray and UV fluxes comparable to the gravitational binding energy of their envelopes, their atmospheres are susceptible to mass loss. We model the thermal and compositional evolution of sub-Neptunes undergoing escape with diffusive separation between hydrogen and helium and show that preferential loss of hydrogen can change their atmospheric compositions. Planets with radii between 1.6 and 2.5 R⊕ can obtain atmospheric helium mass fractions in excess of 40% from billions of years of photoevaporation. Such enhancement can be detected through transmission spectroscopy, providing a novel observational test to determine whether atmospheric escape creates the radius valley.
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I.M. thanks E. Rauscher, N. Calvet, N. Choksi and M. Cowherd, who provided editorial suggestions on drafts of this manuscript. I.M. acknowledges support from the Michigan Space Grant under Grant No. 80NSSC20M0124. I.M. also thanks N. Merhle, who provided code used to convolve the simulated spectra. L.R. gratefully acknowledges support from NSF FY2016 AAG Solicitation 12-589 award number 1615089 and the Research Corporation for Science Advancement through a Cottrell Scholar Award. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This work was also completed in part with resources provided by the University of Chicago’s Research Computing Center.
The authors declare no competing interests.
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Extended Data Fig. 1 The atmospheric compositions expressed as number fraction ratios planets evolved for 10 Gyr around a G type host star with homopause temperatures of 3,000 K.
The atmospheric compositions expressed as number fraction ratios planets evolved for 10 Gyr around a G type host star with homopause temperatures of 3,000 K. All model parameters are identical to those in Fig. 1. The colored dots correspond to the compositions reached at 10 Gyr. We also show the four compositions used for generating transmission spectra (Fig. 4), for both the helium enhanced compositions (the two blue Xs) and the solar H/He compositions (the two black pluses). Note, the helium to hydrogen number ratios and metallicities plotted represent the overall elemental abundances in the planet envelope, while the mean molecular weights are calculated for gas phase species after condensation is taken into account. Furthermore, we expect the metallicity of the two cases with mean molecular weights of 4.00 to be nearly identical as helium has a mean molecular weight of approximately 4.00.
Extended Data Fig. 2 The mean molecular weights as a function of pressure and the optical depths as a function of pressure for the for the solar Y/X and the maximum Y/X composition models.
The mean molecular weights as a function of pressure (top panel) and the optical depth (at a wavelength of 5.9 × 10−7 m, near the rest wavelength of the Na D-lines) as a function of pressure (bottom panel) for the solar Y/X composition atmospheres and the maximum Y/X composition atmospheres corresponding to the transmission spectra in Fig. 4.
Extended Data Fig. 3 The ratios of the solar Y/X composition abundance tables divided by the maximum Y/X composition abundance tables at 10x solar metallicity and 100x solar metallicity for H2O, CH4, CO2, and Na.
The ratios of the solar Y/X composition abundance tables divided by the maximum Y/X composition abundance tables at 10x solar metallicity and 100x solar metallicity for H2O, CH4, CO2, and Na. These tables show the abundances at different X/Y ratios but constant metallicities (defined as the number ratio between Fe and H atoms).
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Malsky, I., Rogers, L., Kempton, E.MR. et al. Helium-enhanced planets along the upper edge of the radius valley. Nat Astron (2022). https://doi.org/10.1038/s41550-022-01823-8