K2-291b: A Rocky Super-Earth in a 2.2 day Orbit

Abstract

K2-291 is a solar-type star with a radius of R <SUB>*</SUB> = 0.899 ± 0.034 R <SUB>☉</SUB> and mass of M<SUB>*</SUB> = 0.934 ± 0.038 M <SUB>☉</SUB>. From the K2 C13 data, we found one super-Earth planet (R <SUB> p </SUB> = {1.589}<SUB>-0.072</SUB><SUP>+0.095</SUP> R <SUB>⊕</SUB>) transiting this star on a short period orbit (P = {2.225177}<SUB>-6.8{{e</SUB>}{--}5}<SUP>+6.6{{e</SUP>}{--}5} days). We followed this system up with adaptive-optic imaging and spectroscopy to derive stellar parameters, search for stellar companions, and determine a planet mass. From our 75 radial velocity measurements using High Resolution Echelle Spectrometer on Keck I and High Accuracy Radial velocity Planet Searcher in the northern hemisphere on Telescopio Nazionale Galileo, we constrained the mass of K2-291 b to M <SUB> p </SUB> = 6.49 ± 1.16 M <SUB>⊕</SUB>. We found it necessary to model correlated stellar activity radial velocity signals with a Gaussian process (GP) in order to more accurately model the effect of stellar noise on our data; the addition of the GP also improved the precision of this mass measurement. With a bulk density of ρ = 8.84{}<SUB>-2.03</SUB><SUP>+2.50</SUP> g cm<SUP>-3</SUP>, the planet is consistent with an Earth-like rock/iron composition and no substantial gaseous envelope. Such an envelope, if it existed in the past, was likely eroded away by photoevaporation during the first billion years of the star’s lifetime.