Ocean Worlds

Conclusive Evidence For A Population Of Water Worlds Around M-dwarfs Remains Elusive

By Keith Cowing
Status Report
January 11, 2023
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Conclusive Evidence For A Population Of Water Worlds Around M-dwarfs Remains Elusive
Synthetic mass-radius distributions are shown for populations of planets evolved with photoevaporation and corepowered mass-loss in left and right-hand panels, respectively, coloured by their equilibrium temperatures. Super-Earths are stripped of their H/He dominated atmospheres and fall onto a relation consistent with an Earth-like composition (brown-dashed), whilst sub-Neptunes retain a significant atmosphere. In the top panels, we assume an initial distribution of atmospheric masses appropriate for a boil-off scenario (Eq. 2), in which planets lose a significant amount of H/He mass during disc dispersal. We characterise the resulting narrow mass-radius distribution with a median line (orange dashed, Eq. 6). In the middle panels, we adopt agnostic initial conditions (Eq. 3) and parameterise this mass-radius relation with 2σ limits (orange dotted lines). In the bottom panels, we compare our theoretical mass-radius distributions (orange dashed/dotted lines, Eq. 6) with the observed sample of M-dwarf orbiting exoplanets from Luque & Pall´e (2022), together with the mass-radius relation for water-worlds (blue solid line, Eq. 1). We find that boil-off initial conditions provide mass-radius relations that are completely degenerate with that of water-worlds. Furthermore, even when adopting agnostic initial conditions, the observations are accurately reproduced since the mass-radius distribution is naturally explained due to mass-loss and cooling/contraction of H/He dominated atmospheres. We highlight planets with confirmed escaping H/He detections with blue-shaded regions (namely; K2 18 b, GJ 3470 b, GJ 436 b and, tentatively, GJ 1214 b). — astro-ph.EP

The population of small, close-in exoplanets is bifurcated into super-Earths and sub-Neptunes. We calculate physically motivated mass-radius relations for sub-Neptunes, with rocky cores and H/He dominated atmospheres, accounting for their thermal evolution, irradiation and mass-loss.

For planets ≲10 M⊕, we find that sub-Neptunes retain atmospheric mass fractions that scale with planet mass and show that the resulting mass-radius relations are degenerate with results for `water-worlds’ consisting of a 1:1 silicate-to-ice composition ratio.

We further demonstrate that our derived mass-radius relation is in excellent agreement with the observed exoplanet population orbiting M-dwarfs and that planet mass and radii alone are insufficient to determine the composition of some sub-Neptunes.

Finally, we highlight that current exoplanet demographics show an increase in the ratio of super-Earths to sub-Neptunes with both stellar mass (and therefore luminosity) and age, which are both indicative of thermally driven atmospheric escape processes. Therefore, such processes should not be ignored when making compositional inferences in the mass-radius diagram.

James G. Rogers, Hilke E. Schlichting, James E. Owen

Comments: 15 pages, 2 figures. Submitted to ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2301.04321 [astro-ph.EP] (or arXiv:2301.04321v1 [astro-ph.EP] for this version)
Submission history
From: James Rogers
[v1] Wed, 11 Jan 2023 06:02:40 UTC (39,372 KB)

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