A Sub-Neptune-sized Planet Transiting the M2.5 Dwarf G 9-40: Validation with the Habitable-zone Planet Finder
Abstract
We validate the discovery of a 2-Earth-radii sub-Neptune-sized planet around the nearby high-proper-motion M2.5 dwarf G 9-40 (EPIC 212048748), using high-precision, near-infrared (NIR) radial velocity (RV) observations with the Habitable-zone Planet Finder (HPF), precision diffuser-assisted ground-based photometry with a custom narrowband photometric filter, and adaptive optics imaging. At a distance of d = 27.9 $\,\mathrm{pc}$ , G 9-40b is the second-closest transiting planet discovered by K2 to date. The planet's large transit depth (∼3500 ppm), combined with the proximity and brightness of the host star at NIR wavelengths (J = 10, K = 9.2), makes G 9-40b one of the most favorable sub-Neptune-sized planets orbiting an M dwarf for transmission spectroscopy with James Webb Space Telescope, ARIEL, and the upcoming Extremely Large Telescopes. The star is relatively inactive with a rotation period of ∼29 days determined from the K2 photometry. To estimate spectroscopic stellar parameters, we describe our implementation of an empirical spectral-matching algorithm using the high-resolution NIR HPF spectra. Using this algorithm, we obtain an effective temperature of ${T}_{\mathrm{eff}}=3404\pm 73\,{\rm{K}}$ and metallicity of $[\mathrm{Fe}/{\rm{H}}]=-0.08\pm 0.13$ . Our RVs, when coupled with the orbital parameters derived from the transit photometry, exclude planet masses above 11.7M⊕ with 99.7% confidence assuming a circular orbit. From its radius, we predict a mass of $M={5.0}_{-1.9}^{+3.8}{M}_{\oplus }$ and an RV semiamplitude of $K={4.1}_{-1.6}^{+3.1}\,{\rm{m}}\ {{\rm{s}}}^{-1}$ , making its mass measurable with current RV facilities. We urge further RV follow-up observations to precisely measure its mass, to enable precise transmission spectroscopic measurements in the future.
- Publication:
-
The Astronomical Journal
- Pub Date:
- March 2020
- DOI:
- 10.3847/1538-3881/ab5f15
- arXiv:
- arXiv:1912.00291
- Bibcode:
- 2020AJ....159..100S
- Keywords:
-
- Exoplanet astronomy;
- Exoplanet systems;
- Radial velocity;
- Exoplanet detection methods;
- Transit photometry;
- Low mass stars;
- Mini Neptunes;
- Exoplanets;
- 486;
- 484;
- 1332;
- 489;
- 1709;
- 2050;
- 1063;
- 498;
- Astrophysics - Earth and Planetary Astrophysics;
- Astrophysics - Instrumentation and Methods for Astrophysics;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- Accepted for publication in AJ, 22 pages, 15 figures