MOST1.6 EARTH-RADIUS PLANETS ARE NOT ROCKY

Abstract

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-Neptunesize 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-Neptunesized 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 are 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, η ⊕ .

SECURE MASS MEASUREMENTS FROM TRANSIT TIMING: 10 KEPLER EXOPLANETS BETWEEN 3 AND 8 M ⊕ WITH DIVERSE DENSITIES AND INCIDENT FLUXES

“…The 10planets that we add in this study span a wide range in radii from super-Earth size to Neptune size. The wide scatter in Figure 25(a) shows a lack of rocky planets more massive than 10 M ⊕ , consistent with Rogers (2015). All the planets more massive than Neptune in Figure 25(a) are less dense than water ice and must retain deep atmospheres.…”

“…We simulate planetary orbits with an eighth-order DormandPrince Runge-Kutta integrator (Fabrycky 2011;Lissauer et al 2011aLissauer et al , 2013Jontof-Hutter et al 2014, 2015 and compare simulated transit times to the observed transit times. TTVs are expressed as the difference between the observed transit time and a calculated linear fit to the transit times (O-C).…”

“…These include parameters based on either spectroscopy or photometry depending on what observations are available. We reiterate the light-curve analysis after inferring the eccentricity vector posteriors from the TTV analysis to obtain a transit-model measurement of ρ å (Lissauer et al 2013;Jontof-Hutter et al 2014, 2015. Combining this estimate of ρ å with published stellar atmospheric values of the effective stellar temperature T eff , metallicity [Fe/H], and surface gravity g log , we matched stellar evolution models to estimate the stellar mass M å and radius R å (Demarque et al 2004).…”

“…Marcy et al (2014) conducted an RV survey of "bright" stars with short-period sub-Neptune-size planets in the Kepler field, and among planets below 10 M ⊕ they found strong detections up to around 16 days for Kepler-102 e (KOI-82.01) and , as well as useful upper limits within this mass range as far out as 69 days (Kepler-409 b, KOI-1925.01). Using the known sample of exoplanets characterized with RV, Rogers (2015) found that most planets larger than 1.6 R ⊕ with orbital periods up to ∼50 days are likely volatile rich.…”

Insights into Planet Formation from Debris Disks

“…It is becoming clear that planets with radii larger than around 1.5 R ⊕ cannot be purely rocky (e.g. Rogers 2015), however the nature and quantity of the lighter component of planets inferred to have low density is unclear. Planets with radii of around 2-3 R ⊕ can be well fit by a rocky core with a hydrogen-helium envelope that comprises ∼0.1-5 per cent of the planet mass (e.g.…”