An Overabundance of Low-density Neptune-like Planets

Abstract

We present a uniform analysis of the atmospheric escape rate of Neptune-like planets with estimated radius and mass (restricted to $M_{\mathrm{p}}<30M_{\oplus }$). For each planet we compute the restricted Jeans escape parameter, $\Lambda$, for a hydrogen atom evaluated at the planetary mass, radius, and equilibrium temperature. Values of $\Lambda \lesssim 20$ suggest extremely high mass-loss rates. We identify 27 planets (out of 167) that are simultaneously consistent with hydrogen-dominated atmospheres and are expected to exhibit extreme mass-loss rates. We further estimate the mass-loss rates ($L_{\mathrm{hy}}$) of these planets with tailored atmospheric hydrodynamic models. We compare $L_{\mathrm{hy}}$ to the energy-limited (maximum-possible high-energy driven) mass-loss rates. We confirm that 25 planets (15\% of the sample) exhibit extremely high mass-loss rates ($L_{\mathrm{hy}}>0.1M_{\oplus }{\mathrm{Gyr}}^{-1}$), well in excess of the energy-limited mass-loss rates. This constitutes a contradiction, since the hydrogen envelopes cannot be retained given the high mass-loss rates. We hypothesize that these planets are not truly under such high mass-loss rates. Instead, either hydrodynamic models overestimate the mass-loss rates, transit-timing-variation measurements underestimate the planetary masses, optical transit observations overestimate the planetary radii (due to high-altitude clouds), or Neptunes have consistently higher albedos than Jupiter planets. We conclude that at least one of these established estimations/techniques is consistently producing biased values for Neptune planets. Such an important fraction of exoplanets with misinterpreted parameters can significantly bias our view of populations studies, like the observed mass--radius distribution of exoplanets for example.

Accepted for publication in MNRAS. 13 pages, 3 figures

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