Mon. Not. R. Astron. Soc., 466, 1868-1879 (2017/April-1)
An overabundance of low-density Neptune-like planets.
CUBILLOS P., ERKAEV N.V., JUVAN I., FOSSATI L., JOHNSTONE C.P., LAMMER H., LENDL M., ODERT P. and KISLYAKOVA K.G.
Abstract (from CDS):
We present a uniform analysis of the atmospheric escape rate of Neptune-like planets with estimated radius and mass (restricted to Mp < 30 M⊕). For each planet, we compute the restricted Jeans escape parameter, Λ, for a hydrogen atom evaluated at the planetary mass, radius, and equilibrium temperature. Values of Λ <= 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 (Lhy) of these planets with tailored atmospheric hydrodynamic models. We compare Lhy 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 (Lhy > 0.1 M[?] 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.
© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
hydrodynamics - planets and satellites: atmospheres - planets and satellites: fundamental parameters - planets and satellites: fundamental parameters
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