The NASA-UC-UH Eta-Earth Program: IV. A Low-mass Planet Orbiting an M Dwarf 3.6 PC from Earth

TL;DR

This paper reports the discovery of a low-mass exoplanet orbiting an M dwarf star at 3.6 parsecs from Earth, using radial velocity measurements, photometry, and spectroscopy, and characterizes both the planet and its host star.

Contribution

It presents the detection and detailed characterization of a new low-mass planet orbiting an M dwarf, combining multiple observational techniques for the first time in this context.

Findings

Discovery of a 5.35 Earth-mass planet with an 11.44-day orbit.

Detection of stellar activity signals at 44 days confirming stellar rotation.

Precise measurements of the host star's properties, including spectral type and metallicity.

Abstract

We report the discovery of a low-mass planet orbiting Gl 15 A based on radial velocities from the Eta-Earth Survey using HIRES at Keck Observatory. Gl 15 Ab is a planet with minimum mass Msini = 5.35 $\pm$ 0.75 M$_\oplus$, orbital period P = 11.4433 $\pm$ 0.0016 days, and an orbit that is consistent with circular. We characterize the host star using a variety of techniques. Photometric observations at Fairborn Observatory show no evidence for rotational modulation of spots at the orbital period to a limit of ~0.1 mmag, thus supporting the existence of the planet. We detect a second RV signal with a period of 44 days that we attribute to rotational modulation of stellar surface features, as confirmed by optical photometry and the Ca II H & K activity indicator. Using infrared spectroscopy from Palomar-TripleSpec, we measure an M2 V spectral type and a sub-solar metallicity ([M/H] = -0.22, [Fe/H] = -0.32). We measure a stellar radius of 0.3863 $\pm$ 0.0021 R$_\odot$ based on interferometry from CHARA.