Seeing Double with K2: Testing Re-inflation with Two Remarkably Similar Planets around Red Giant Branch Stars

Abstract

Despite more than 20 years since the discovery of the first gas giant planet with an anomalously large radius, the mechanism for planet inflation remains unknown. Here, we report the discovery of K2-132b, an inflated gas giant planet found with the NASA K2 Mission, and a revised mass for another inflated planet, K2-97b. These planets orbit on ≈9day orbits around host stars that recently evolved into red giants. We constrain the irradiation history of these planets using models constrained by asteroseismology and Keck/High Resolution Echelle Spectrometer spectroscopy and radial velocity measurements. We measure planet radii of 1.31±0.11 R J and 1.30±0.07 R J , respectively. These radii are typical for planets receiving the current irradiation, but not the former, zero age main-sequence irradiation of these planets. This suggests that the current sizes of these planets are directly correlated to their current irradiation. Our precise constraints of the masses and radii of the stars and planets in these systems allow us to constrain the planetary heating efficiency of both systems as 0.03% 0.02% 0.03% -+. These results are consistent with a planet re-inflation scenario, but suggest that the efficiency of planet re-inflation may be lower than previously theorized. Finally, we discuss the agreement within 10% of the stellar masses and radii, and the planet masses, radii, and orbital periods of both systems, and speculate that this may be due to selection bias in searching for planets around evolved stars.

Do Close-in Giant Planets Orbiting Evolved Stars Prefer Eccentric Orbits?

“…Increased irradiation due to stellar evolution is also thought to be a source of planetary heating (Lopez & Fortney 2016). Two of the close-in evolved planets with new RV measurements presented here, K2-97b and K2-132b, show signs of being significantly inflated relative to similar planets seen orbiting main sequence stars (Grunblatt et al 2017).…”

“…Two well-characterized, close-in inflated giant planets orbiting moderately evolved, or low-luminosity, red giant branch stars, K2-97b and K2-132b, were recently discovered by the K2 extension to the Kepler mission (Grunblatt et al 2016(Grunblatt et al , 2017. Here, we report new radial velocity (RV) measurements of these planets, as well as RV measurements of a previously validated planet orbiting an evolved star observed by the original Kepler mission, Kepler-643 (Huber et al 2013;Morton et al 2016).…”

“…In order to constrain orbital parameters, we fit the RV data using the publicly available software package RadVel (Fulton et al 2018). The orbital period of the planets were fixed to published values from transit measurements (Morton et al 2016;Grunblatt et al 2017), while we fit for the semiamplitude, phase, and modified eccentricity parameters of the orbit (Eastman et al 2013). We also fit for an RV jitter term for our measurements and obtained a value between 5 and 10 m s −1 for all of the systems studied here.…”

Giant Planet Occurrence within 0.2 au of Low-luminosity Red Giant Branch Stars with K2

“…The planets included in our sample are K2-97, K2-132, and K2-161 (Grunblatt et al 2016(Grunblatt et al , 2017Mayo et al 2018). All three planets are warm (> 150 F ⊕ ) gas giants larger than Jupiter 8 .…”

“…Despite the relatively long history of planet searches around evolved stars, no planets were found interior to 0.5 AU around such stars, suggestive of intrinsic differences between the main sequence and evolved system populations (Johnson et al 2010;Jones et al 2016). The recent explosion in planet discoveries around Sun-like and smaller stars fueled by transit surveys has been accompanied by only a handful of planet transit detections around evolved stars (Lillo-Box et al 2014;Barclay et al 2015;Van Eylen et al 2016;Grunblatt et al 2016Grunblatt et al , 2017. To determine whether the relatively small number of planets known around evolved stars is due to small survey size, planet detection difficulties unique to evolved stars, or an intrinsic lack of planets, a systematic transit survey of evolved stars is needed.…”