Compositional Imprints in Density–Distance–Time: A Rocky Composition for Close-in Low-mass Exoplanets from the Location of the Valley of Evaporation

By Sheng Jin, C. Mordasini. 2017

190 citations recorded by the source corpus.

We use an end-to-end model of planet formation, thermodynamic evolution, and atmospheric escape to investigate how the statistical imprints of evaporation depend on the bulk composition of planetary cores (rocky versus icy). We find that the population-wide imprints like the location of the “evaporation valley” in the distance–radius plane and the corresponding bimodal radius distribution clearly differ depending on the bulk composition of the cores. Comparison with the observed position of the valley suggests that close-in low-mass Kepler planets have a predominantly Earth-like rocky composition. Combined with the excess of period ratios outside of MMR, this suggests that low-mass Kepler planets formed inside of the water iceline but were still undergoing orbital migration. The core radius becomes visible for planets losing all primordial H/He. For planets in this “triangle of evaporation” in the distance–radius plane, the degeneracy in composition is reduced. In the observed planetary mass–mean density diagram, we identify a trend to more volatile-rich compositions with an increasing radius (R/R⊕ ≲ 1.6 rocky; 1.6–3.0 ices, and/or H/He; ≳3: H/He). The mass–density diagram contains important information about formation and evolution. Its characteristic broken V-shape reveals the transitions from solid planets to low-mass core-dominated planets with H/He and finally to gas-dominated giants. Evaporation causes the density and orbital distance to be anticorrelated for low-mass planets in contrast to giants, where closer-in planets are less dense, likely due to inflation. The temporal evolution of the statistical properties reported here will be of interest for the PLATO 2.0 mission, which will observe the temporal dimension.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/ac12486f3c9a2c90db57b5e0bb82a3aab24f33c1

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In Astronomy and Astrophysics

1. J. Owen
2. J. V. van Saders
3. D. Huber
4. E. Gaidos
5. T. Berger
6. Yanqin Wu
7. A. Reiners
8. P. Amado

Citations in the corpus

• Planetesimal formation starts at the snow line (2017, unknown): “However, the recent models show that low mass, short period planets detected by the Kepler mission should be water-poor (see e.g. Jin & Mordasini 2017).”
• Revised Radii of Kepler Stars and Planets Using Gaia Data Release 2 (2018, background): “For example, Jin & Mordasini (2018) finds that an “evaporation valley” at 1.6R⊕ corresponds to an Earth-like composition of silicates and metals, so a valley at a larger radius implies a significant ice component.”
• The CARMENES Search for Exoplanets around M Dwarfs: A Low-mass Planet in the Temperate Zone of the Nearby K2-18 (2018, background): “However, it is well known that there is a wide range of possible compositions for a given mass and radius, all of which include low density volatiles such as water and H/He (Lopez et al. 2012; Jin & Mordasini 2018).”
• The Evaporation Valley in the Kepler Planets (2017, methodology): “While this paper was in review, Jin & Mordasini (2017) submitted a paper which used numerical modelling to reach much of the same conclusions about the composition of the Kepler planets as we do here.”
• Atmospheric Escape and the Evolution of Close-In Exoplanets (2018, background): “However, under the assumption that the observed gap is indeed the evaporation valley, several studies have used is properties to place constraints on the origin and evolution of the exoplanet population (Owen & Wu 2017; Jin & Mordasini 2018; Owen & Murray-Clay 2018).”