MOST 1.6 EARTH-RADIUS PLANETS ARE NOT ROCKY

By Leslie A. Rogers. 2014

529 citations recorded by the source corpus.

The Kepler mission, combined with ground-based radial velocity (RV) follow-up and dynamical analyses of transit timing variations, has revolutionized the observational constraints on sub-Neptune-sized planet compositions. The results of an extensive Kepler follow-up program including multiple Doppler measurements for 22 planet-hosting stars more than doubles the population of sub-Neptune-sized transiting planets that have RV mass constraints. This unprecedentedly large and homogeneous sample of planets with both mass and radius constraints opens the possibility of a statistical study of the underlying population of planet compositions. We focus on the intriguing transition between rocky exoplanets (comprised of iron and silicates) and planets with voluminous layers of volatiles (H/He and astrophysical ices). Applying a hierarchical Bayesian statistical approach to the sample of Kepler transiting sub-Neptune planets with Keck RV follow-up, we constrain the fraction of close-in planets (with orbital periods less than ∼50 days) that are sufficiently dense to be rocky, as a function of planet radius. We show that the majority of 1.6 R ⨁ ?> planets have too low density to be comprised of Fe and silicates alone. At larger radii, the constraints on the fraction of rocky planets are even more stringent. These insights into the size demographics of rocky and volatile-rich planets offer empirical constraints to planet formation theories, and guide the range of planet radii to be considered in studies of the occurrence rate of “Earth-like” planets, &eegr; ⨁ ?> .

Subfields: Astronomy and Astrophysics, Instrumentation, Statistical and Nonlinear Physics

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Citations in the corpus

STELLAR AND PLANETARY PROPERTIES OF K2 CAMPAIGN 1 CANDIDATES AND VALIDATION OF 17 PLANETS, INCLUDING A PLANET RECEIVING EARTH-LIKE INSOLATION (2015, background): “Although the planet is likely too large to be rocky (Rogers 2015), its atmosphere is likely to be the focus of many future observations, providing a cool analogue to the highly irradiated planets of a similar size found by Kepler.”
The CARMENES Search for Exoplanets around M Dwarfs: A Low-mass Planet in the Temperate Zone of the Nearby K2-18 (2018, result): “This is in agreement with Rogers (2015) and Wolfgang & Lopez (2015), who showed that most planets with radii larger than 1.6 R⊕ are not rocky.”
The California-Kepler Survey. V. Peas in a Pod: Planets in a Kepler Multi-planet System Are Similar in Size and Regularly Spaced (2017, background): “Although there is large scatter in the planet masses with respect to these mean empirical relations (Marcy et al. 2014; Weiss & Marcy 2014; Rogers 2015; Wolfgang & Lopez 2015; Chen & Kipping 2017), we have no reliable basis for deciding which planet masses should be higher or lower than the mean,…”
Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life (2017, background): “a low-molecular-weight component (i.e.,H 2,He) over geologic time. (Note this does not suggest super-Earths may have a massive H 2 envelope like that of Neptune, which is not supported by data [e.g., Rogers, 2015]; the lowmolecular-weight component would have to be small enough, relative to the plan”
Identifying Exoplanets with Deep Learning: A Five-planet Resonant Chain around Kepler-80 and an Eighth Planet around Kepler-90 (2017, background): “Kepler-90 b, c, and i all have radii small enough that they are either likely or might be rocky (Rogers 2015).”
Atmospheric characterization of Proxima b by coupling the Sphere high-contrast imager to the Espresso spectrograph (2016, background): “M⊕ objects on short-period orbits have an internal structure similar to Earth Article number, page 11 of 16 (e.g. Rogers 2015; Dressing et al. 2015).”
The Habitability of Planets Orbiting M-dwarf Stars (2016, background): “Kepler-186f is only 10% larger than the Earth, and although its mass, density, and bulk composition are unknown, recent work suggests that it is small enough to be rocky (Rogers, 2015).”
Kepler Planet Masses and Eccentricities from TTV Analysis (2016, methodology): “As a possible extension to this work, the posterior samples can be incorporated in the hierarchical modeling approach described by Hogg et al. (2010) to infer a prior (see also Rogers 2015).”
Multiple Disk Gaps and Rings Generated by a Single Super-Earth (2017, background): “Their compositions, as inferred from their bulk densities, span a wide range; some are consistent with pure rock while others have 1–10% of their mass in voluminous H/He envelopes (e.g., Lopez & Fortney 2014; Rogers 2015; Lee & Chiang 2016).”
PROBABILISTIC FORECASTING OF THE MASSES AND RADII OF OTHER WORLDS (2016, methodology): “Nevertheless, we may convert our mass posterior samples to fair radii realizations using our Forecaster public code (described later in Section 5).5 Accordingly, we find that the transition occurs at - +1.23 0.22 0.44 R⊕, which is fully compatible with Rogers (2015).”
The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets (2017, background): “Planets with larger radii and measured masses are mostly low-density and require an extended atmosphere to simultaneously explain their masses and radii (Marcy et al. 2014; Weiss & Marcy 2014; Rogers 2015; Wolfgang & Lopez 2015).”
TERRESTRIAL PLANET OCCURRENCE RATES FOR THE KEPLER GK DWARF SAMPLE (2015, background): “The integral range of 20% is within the expectations for the regime of rocky terrestrial planets (Rogers 2015; Wolfgang & Lopez 2014).”
A NEARBY M STAR WITH THREE TRANSITING SUPER-EARTHS DISCOVERED BY K2 (2015, background): “The planets range in size from 2R⊕ to 1.5R⊕, indicating that they may span the gap between rock-dominated “Earths”/“super-Earths” and low-density “sub-Neptunes” with considerable volatile content (Marcy et al. 2014; Rogers 2014; Dressing et al. 2014).”
The California-Kepler Survey. VII. Precise Planet Radii Leveraging Gaia DR2 Reveal the Stellar Mass Dependence of the Planet Radius Gap (2018, background): “These measurements revealed a transition in exoplanet bulk composition at ≈1.5RÅ, with smaller planets having bulk densities consistent with rock and larger planets having extended low-density envelopes (Weiss & Marcy 2014; Rogers 2015).”
SPITZER OBSERVATIONS CONFIRM AND RESCUE THE HABITABLE-ZONE SUPER-EARTH K2-18b FOR FUTURE CHARACTERIZATION (2016, background): “Given its radius of 2.27R⊕, the planet is likely surrounded by a thick gaseous envelope (e.g., Rogers 2015) that could be amenable to characterization through transit spectroscopy.”
PROBABILISTIC MASS–RADIUS RELATIONSHIP FOR SUB-NEPTUNE-SIZED PLANETS (2015, background): “Recently these studies have shifted to considering the available planets as a statistical ensemble (e.g. Rogers 2015; Wolfgang & Lopez 2015 sans mass constraints), which motivates detailed analyses of the observed massradius distribution.”
A Population Study of Gaseous Exoplanets (2017, methodology): “Population synthesis studies based on formation scenarios or statistics from the Kepler Space mission suggest a great diversity in the exoplanet population (e.g. Fortney et al. 2013; Lopez & Fortney 2014; Rogers 2015; Parmentier et al. 2016).”