Extrasolar Planets

Evidence For The Volatile-rich Composition Of O 1.5-R⊕ Planet

By Keith Cowing
Press Release
December 21, 2022
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Evidence For The Volatile-rich Composition Of O 1.5-R⊕ Planet
Comparison of Kepler-138 c and d to the population of super-Earth size planets. Mass-radius plot of planets with super-Earth sizes and masses below 5 M⊕. Kepler-138 c and d (bold triangles) stand out as having low densities compared to the population of small transiting exoplanets (circles colored according to instellation, planets with masses measured at better than 3σ) and solar system planets (orange letters). Error bars correspond to the 68% confidence region for the mass and radius of each planet. Modeled mass-radius curves are displayed for rocky planets and gas-enveloped planets with an Earth-like composition core (models described in the Methods and Refs.13;92;130). The transparent grey region corresponds to the “radius valley” while the solid grey region in the bottom right corner is forbidden according to models of maximum mantle strip- ping via giant impacts131. The best match to the mass and radius of Kepler-138 c and d is obtained for a volatile-rich composition with approximately 10% water. Alternatively, Kepler-138 d’s low density could be explained by a light 0.01 wt% (or about 0.0003M⊕) H2/He atmosphere, but such an envelope would be rapidly lost to space (see text). — astro-ph.EP

The population of planets smaller than approximately 1.7 R⊕ is widely interpreted as consisting of rocky worlds, generally referred to as super-Earths.

This picture is largely corroborated by radial-velocity (RV) mass measurements for close-in super-Earths but lacks constraints at lower insolations. Here we present the results of a detailed study of the Kepler-138 system using 13 Hubble and Spitzer transit observations of the warm-temperate 1.51±0.04 R⊕ planet Kepler-138 d (Teq,AB=0.3~350 K) combined with new Keck/HIRES RV measurements of its host star. We find evidence for a volatile-rich “water world” nature of Kepler-138 d, with a large fraction of its mass contained in a thick volatile layer.

This finding is independently supported by transit timing variations, RV observations (Md=2.1+0.6−0.7 M⊕), as well as the flat optical/IR transmission spectrum. Quantitatively, we infer a composition of 11+3−4\% volatiles by mass or ~51% by volume, with a 2000 km deep water mantle and atmosphere on top of a core with an Earth-like silicates/iron ratio. Any hypothetical hydrogen layer consistent with the observations (<0.003 M⊕) would have swiftly been lost on a ~10 Myr timescale.

The bulk composition of Kepler-138 d therefore resembles those of the icy moons rather than the terrestrial planets in the solar system. We conclude that not all super-Earth-sized planets are rocky worlds, but that volatile-rich water worlds exist in an overlapping size regime, especially at lower insolations.

Finally, our photodynamical analysis also reveals that Kepler-138 c (Rc=1.51±0.04 R⊕, Mc=2.3+0.6−0.5 M⊕) is a slightly warmer twin of Kepler-138 d, i.e., another water world in the same system, and we infer the presence of Kepler-138 e, a likely non-transiting planet at the inner edge of the habitable zone.

Caroline Piaulet, Björn Benneke, Jose M. Almenara, Diana Dragomir, Heather A. Knutson, Daniel Thorngren, Merrin S. Peterson, Ian J. M. Crossfield, Eliza M.-R. Kempton, Daria Kubyshkina, Andrew W. Howard, Ruth Angus, Howard Isaacson, Lauren M. Weiss, Charles A. Beichman, Jonathan J. Fortney, Luca Fossati, Helmut Lammer, P. R. McCullough, Caroline V. Morley, Ian Wong

Comments: Published in Nature Astronomy. 4 main figures, 10 extended data figures, 13 supplementary figures. 4 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2212.08477 [astro-ph.EP] (or arXiv:2212.08477v1 [astro-ph.EP] for this version)
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Submission history
From: Caroline Piaulet
[v1] Wed, 14 Dec 2022 19:06:10 UTC (31,188 KB)

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