K2-66b and K2-106b: Two Extremely Hot Sub-Neptune-size Planets with High Densities

Abstract

We report precise mass and density measurements of two extremely hot sub-Neptune-size planets from the K2 mission using radial velocities, K2 photometry, and adaptive optics imaging. K2-66 harbors a close-in subNeptune-sized ( -+ 2.49 0.24 0.34 Å R ) planet (K2-66b) with a mass of  21.3 3.6 Å M . Because the star is evolving up the subgiant branch, K2-66b receives a high level of irradiation, roughly twice the main-sequence value. K2-66b may reside within the so-called "photoevaporation desert," a domain of planet size and incident flux that is almost completely devoid of planets. Its mass and radius imply that K2-66b has, at most, a meager envelope fraction (<5%) and perhaps no envelope at all, making it one of the largest planets without a significant envelope. K2-106 hosts an ultra-short-period planet (P=13.7 hr) that is one of the hottest sub-Neptune-size planets discovered to date. Its radius ( -+ 1.82 0.14 0.20 Å R ) and mass (  9.0 1.6 Å M ) are consistent with a rocky composition, as are all other small ultra-short-period planets with well-measured masses. K2-106 also hosts a larger, longer-period planet (R p = -+ 2.77 0.23 0.37 Å R , P=13.3 days) with a mass less than 24.4 Å M at 99.7% confidence. K2-66b and K2-106b probe planetary physics in extreme radiation environments. Their high densities reflect the challenge of retaining a substantial gas envelope in such extreme environments.

Precise Masses in the WASP-47 System

“…This type of interior structure is believed to be common among super-Earths and sub-Neptunes discovered by Kepler and K2 since RV observations have shown that most of these planets larger than about 1.6 Å R have densities too low to be explained by rocky compositions Weiss & Marcy 2014;Dressing et al 2015;Rogers 2015;Sinukoff et al 2017a). Due to its short (19 hr) orbital period, WASP-47 e is so highly irradiated that any hydrogen/helium envelope would quickly be lost via photoevaporation (Penz et al 2008;Sanz-Forcada et al 2011;Lopez et al 2012), so any envelope around WASP-47 e must be made of water or some other highmetallicity volatile material (Lopez 2017).…”

Homogeneous Analysis of Hot Earths: Masses, Sizes, and Compositions

“…We felt a homogeneous analysis was warranted because the results in the literature were reported by different groups using different procedures to determine the stellar parameters and to mitigate the effects of timecorrelated noise in the radial-velocity data. For example, K2-106b was reported by Guenther et al (2017) to have an iron mass fraction of 80 +20 −30 %, while Sinukoff et al (2017) concluded that the composition was compatible with that of the Earth (35% iron). For this work, we combined all the previously published datasets, used the parallaxes from Gaia Data Release 2 (Gaia Collaboration et al 2018) to refine the stellar parameters, and employed a Gaussian Process framework to disentangle planetary radial velocity signals from cor-related noise.…”

The Bimodal Distribution in Exoplanet Radii: Considering Varying Core Compositions and H2 Envelope’s Sizes

“…Therefore the remnant cores of Neptunian or Jovian planets could be > 10M ⊕ . The possibility of very heavy cores is also supported by the discovery of rocky mega-Earths such as BD+20 594 b which has a mass and radius of 16.3 ± 6.0M ⊕ and 2.2 ± 0.1R ⊕ (Espinoza et al 2016) or K2-66 b with 21.3 ± 3.6M ⊕ and 2.50 ± 0.3R ⊕ (Sinukoff et al 2017) respectively. Therefore whilst the critical mass Υ β ∼ 10M ⊕ , the probabilistic function Γ γ could range from 10 − 25M ⊕ according to observations and theoretical predictions.…”