A Super-Earth Orbiting Near the Inner Edge of the Habitable Zone around the M4.5-dwarf Ross 508

Abstract

We report the near-infrared radial-velocity (RV) discovery of a super-Earth planet on a 10.77-day orbit around the M4.5 dwarf Ross 508 ($J_{\mathrm{mag}}=9.1$). Using precision RVs from the Subaru Telescope IRD (InfraRed Doppler) instrument, we derive a semi-amplitude of $3.92_{-0.58}^{+0.60}$ ${\mathrm{m}\mathrm{s}}^{-1}$, corresponding to a planet with a minimum mass $m\sin i=4.00_{-0.55}^{+0.53}{M}_{\oplus }$. We find no evidence of significant signals at the detected period in spectroscopic stellar activity indicators or MEarth photometry. The planet, Ross 508 b, has a semimajor-axis of $0.05366_{-0.00049}^{+0.00056}$ au. This gives an orbit-averaged insolation of $\approx$1.4 times the Earth's value, placing Ross 508 b near the inner edge of its star's habitable zone. We have explored the possibility that the planet has a high eccentricity and its host is accompanied by an additional unconfirmed companion on a wide orbit. Our discovery demonstrates that the near-infrared RV search can play a crucial role to find a low-mass planet around cool M dwarfs like Ross 508.

Accepted for publication in PASJ (May 23, 2022)

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