An overabundance of low-density Neptune-like planets

Author(s): P. E. Cubillos, N V Erkaev, I. Juvan, Luca Fossati, Colin Johnstone, Kristina Kislyakova, Helmut Lammer, Monika Lendl, Petra Odert
Abstract: 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 (L_hy) 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 (L_hy > 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.
Organisation(s): Department of Astrophysics
External organisation(s): Österreichische Akademie der Wissenschaften (ÖAW), Russian Academy of Sciences, Max-Planck-Institut für Astronomie
Journal: Monthly Notices of the Royal Astronomical Society
Volume: 466
Pages: 1868-1879
No. of pages: 12
ISSN: 0035-8711
DOI: https://doi.org/10.1093/mnras/stw3103
Publication date: 04-2017
Peer reviewed: Yes
Austrian Fields of Science 2012: 103003 Astronomy, 103004 Astrophysics
Keywords
ASJC Scopus subject areas: Astronomy and Astrophysics, Space and Planetary Science
Portal url: https://ucrisportal.univie.ac.at/en/publications/64c24c38-3082-45c1-9772-5f2403a89dcc

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