Evolution of the Radius Valley around Low-mass Stars from Kepler and K2

Abstract

We present calculations of the occurrence rate of small close-in planets around low mass dwarf stars using the known planet populations from the Kepler and K2 missions. Applying completeness corrections clearly reveals the radius valley in the maximum a-posteriori occurrence rates as a function of orbital separation and planet radius. We measure the slope of the valley to be r p,valley ∝ F −0.060±0.025 which bears the opposite sign from that measured around Sun-like stars thus suggesting that thermally driven atmospheric mass loss may not dominate the evolution of planets in the low stellar mass regime or that we are witnessing the emergence of a separate channel of planet formation. The latter notion is supported by the relative occurrence of rocky to non-rocky planets increasing from 0.5 ± 0.1 around mid-K dwarfs to 8.5 ± 4.6 around mid-M dwarfs. Furthermore, the center of the radius valley at 1.54 ± 0.16 R ⊕ is shown to shift to smaller sizes with decreasing stellar mass in agreement with physical models of photoevaporation, core-powered mass loss, and gas-poor formation. Although current measurements are insufficient to robustly identify the dominant formation pathway of the radius valley, such inferences may be obtained by TESS with O(85, 000) mid-to-late M dwarfs observed with 2-minute cadence. The measurements presented herein also precisely designate the subset of planetary orbital periods and radii that should be targeted in radial velocity surveys to resolve the rocky to non-rocky transition around low mass stars.Note. -Only the first five rows are shown here to illustrate the table's content and format. The complete table in csv format is available in the arXiv source.

Constraints on Metastable Helium in the Atmospheres of WASP-69b and WASP-52b with Ultranarrowband Photometry

“…Such extreme environments can radically alter planetary evolution, potentially driving atmospheric mass loss via thermal escape (e.g., Tian 2015; Owen 2019). Mass loss can in turn leave substantial imprints on observed planetary statistics, such as the dearth of planets between 1.5 and 2 Earth radii (the "radius gap" or "evaporation valley") and the so-called "Neptune desert" in the radius-period plane (Lopez & Fortney 2013;Owen & Wu 2013Fulton et al 2017;Fulton & Petigura 2018;van Eylen et al 2018;Cloutier & Menou 2020;Hardegree-Ullman et al 2020). Over the past two decades, most measurements of mass-loss rates for close-in planets have been conducted at ultraviolet wavelengths, with Lyα detections for HD 209458b (Vidal-Madjar et al 2003), HD 189733b (Lecavelier Des Etangs et al 2010;Lecavelier des Etangs et al 2012), GJ 436b (Kulow et al 2014;Ehrenreich et al 2015;Lavie et al 2017), and GJ 3470b (Bourrier et al 2018); tentative/marginal signals for TRAPPIST-1b and c (Bourrier et al 2017a), Kepler-444e and f (Bourrier et al 2017b), and K2-18b (dos Santos et al 2020); and nondetections for 55 Cnc e (Ehrenreich et al 2012), HD 97658b (Bourrier et al 2017c), GJ 1132 b (Waalkes et al 2019), and π Men c (García Muñoz et al 2020).…”

A Pair of TESS Planets Spanning the Radius Valley around the Nearby Mid-M Dwarf LTT 3780

“…The resulting Keplerian RV signals are clearly discernible in their phase-folded RV time series. The rms values of the RV residuals are found to be 1.55 and 1.74 m s −1 for HARPS and HARPS-N, respectively.The M dwarfs are known to commonly host two to three planets per star out to 200 days (e.g.,Dressing & Charbonneau 2015;Ballard & Johnson 2016;Hardegree-Ullman et al 2019;Cloutier & Menou 2020…”

A super-Earth and a sub-Neptune orbiting the bright, quiet M3 dwarf TOI-1266

“…This affects the prediction for the mass ratios of the two planets; in particular, a shallow slope means that the two masses can be more similar than estimated above. If we assume that the slope of the valley is instead the same as the slope observed for FGK stars (Van Eylen et al 2018;Cloutier & Menou 2020), and not the one predicted in the model, then we can estimate that the inner planet must have a mass that is at least 25% higher than the mass of the outer planet (Mordasini 2020). Then, the inner planet can keep some H/He.…”

“…Demory et al: A super-Earth and a sub-Neptune orbiting the M3V TOI-1266 (Ginzburg et al 2018;Gupta & Schlichting 2020). The recent discovery of a change in the fraction of planets above and below the valley over ∼ Gyr timescales has indeed been interpreted in favour of the core-powered mass loss scenario for some solartype stars (Berger et al 2020), although there is evidence that the formation pathway may be different in low-mass stars (Cloutier & Menou 2020).…”