An overabundance of low-density Neptune-like planets

作者：Cubillos Patricio; Erkaev Nikolai V; Juvan Ines; Fossati Luca; Johnstone Colin P; Lammer Helmut; Lendl Monika; Odert Petra; Kislyakova Kristina G

来源：Monthly Notices of the Royal Astronomical Society, 2017, 466(2): 1868-1879.

DOI：10.1093/mnras/stw3103

• hydrodynamics
• planets and satellites: atmospheres
• planets and satellites: fundamental parameters

We present a uniform analysis of the atmospheric escape rate of Neptune-like planets with estimated radius and mass (restricted to M-p < 30M(circle plus)). 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 less than or similar to 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-hy) of these planets with tailored atmospheric hydrodynamic models. We compare L-hy to the energy-limited (maximum-possible high-energy driven) mass-loss rates. We confirm that 25 planets (15 per cent of the sample) exhibit extremely high mass-loss rates (L-hy > 0.1M(circle plus) 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.

• 出版日期2017-4