Tuesday, August 15, 2017

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

Sinukoff et al


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 sub-Neptune-sized (${2.49}_{-0.24}^{+0.34}$ ${R}_{\oplus }$) planet (K2-66b) with a mass of $21.3\pm 3.6$ ${M}_{\oplus }$. 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 (less than 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 (${1.82}_{-0.14}^{+0.20}$ ${R}_{\oplus }$) and mass ($9.0\pm 1.6$ ${M}_{\oplus }$) 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 (${R}_{{\rm{p}}}$ = ${2.77}_{-0.23}^{+0.37}$ ${R}_{\oplus }$, P = 13.3 days) with a mass less than $24.4$ ${M}_{\oplus }$ 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.

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