Absence of extended atmospheres in low-mass star radius-gap planets
Vigneshwaran Krishnamurthy, Teruyuki Hirano, Eric Gaidos, Bun’ei Sato, Ravi Kopparapu, Thomas Barclay, Katherine Garcia‐Sage, Hiroki Harakawa, K. W. Hodapp, Shane Jacobson, Mihoko Konishi, Takayuki Kotani, Tomoyuki Kudo, Takashi Kurokawa, Masayuki Kuzuhara, Eric Lopez, Jun Nishikawa, Masashi Omiya, Joshua E. Schlieder, Takuma Serizawa, Motohide Tamura, Akitoshi Ueda, Sébastien Vievard
Abstract
ABSTRACT Kepler showed a paucity of planets with radii of 1.5–2 R⊕ around solar mass stars but this radius-gap has not been well studied for low-mass star planets. Energy-driven escape models like photoevaporation and core-powered mass-loss predict opposing transition regimes between rocky and non-rocky planets when compared to models depicting planets forming in gas-poor environments. Here, we present transit observations of three super-Earth sized planets in the radius-gap around low-mass stars using high-dispersion InfraRed Doppler spectrograph on the Subaru 8.2 m telescope. The planets GJ 9827 b and d orbit around a K6V star and TOI-1235 b orbits a M0.5 star. We limit any planet-related absorption in the 1083.3 nm lines of triplet He i by placing an upper-limit on the equivalent width of 14.71, 18.39, and 1.44 mÅ for GJ 9827 b (99 per cent confidence), GJ 9827 d (99 per cent confidence), and TOI-1235 b (95 per cent confidence), respectively. Using a Parker wind model, we cap the mass-loss at >0.25 M⊕ Gyr−1 and >0.2 M⊕ Gyr−1 for GJ 9827 b and d, respectively (99 per cent confidence), and >0.05 M⊕ Gyr−1 for TOI-1235 b (95 per cent confidence) for a representative wind temperature of 5000 K. Our observed results for the three planets are more consistent with the predictions from photoevaporation and/or core-powered mass-loss models than the gas-poor formation models. However, more planets in the radius-gap regime around the low-mass stars are needed to robustly predict the atmospheric evolution in planets around low-mass stars.