A temperate exo-Earth around a quiet M dwarf at 3.4 parsec

Abstract

The combination of high-contrast imaging and high-dispersion spectroscopy, which has successfully been use to detect the atmosphere of a giant planet, is one of the most promising potential probes of the atmosphere of Earth-size worlds. The forthcoming generation of extremely large telescopes (ELTs) may obtain sufficient contrast with this technique to detect O2 in the atmosphere of those worlds that orbit low-mass M dwarfs. This is strong motivation to carry out a census of planets around cool stars for which habitable zones can be resolved by ELTs, i.e. for M dwarfs within ∼5 parsecs. Our HARPS survey has been a major contributor to that sample of nearby planets. Here we report on our radial velocity observations of Ross 128 (Proxima Virginis, GJ447, HIP 57548), an M4 dwarf just 3.4 parsec away from our Sun. This source hosts an exo-Earth with a projected mass m sin i = 1.35M⊕ and an orbital period of 9.9 days. Ross 128 b receives ∼1.38 times as much flux as Earth from the Sun and its equilibrium ranges in temperature between 269 K for an Earth-like albedo and 213 K for a Venus-like albedo. Recent studies place it close to the inner edge of the conventional habitable zone. An 80-day long light curve from K2 campaign C01 demonstrates that Ross 128 b does not transit. Together with the All Sky Automated Survey (ASAS) photometry and spectroscopic activity indices, the K2 photometry shows that Ross 128 rotates slowly and has weak magnetic activity. In a habitability context, this makes survival of its atmosphere against erosion more likely. Ross 128 b is the second closest known exo-Earth, after Proxima Centauri b (1.3 parsec), and the closest temperate planet known around a quiet star. The 15 mas planet-star angular separation at maximum elongation will be resolved by ELTs (> 3λ/D) in the optical bands of O2.

Photometric rotation periods for 107 M dwarfs from the APACHE survey

“…In such cases, the ability to discriminate the true nature of the signals crucially depends on the availability of spectroscopic proxies for stellar activity and/or photometric monitoring. When it comes to M dwarf type planet hosts, the stellar temperate zone, within which the signals of potentially habitable planets can today be readily identified in principle (e.g., Anglada-Escudé et al 2016a;Dittmann et al 2017;Suárez Mascareño et al 2017a;Astudillo-Defru et al 2017b;Bonfils et al 2018;Zechmeister et al 2019), corresponds to orbital periods typically in the range of tens of days (e.g., Kopparapu et al 2013Kopparapu et al , 2014Kopparapu 2018). However, these timescales can coincide with those of stellar rotation periods of low-mass stars in the middle of their main-sequence lifetime (e.g., Barnes 2007;McQuillan et al 2013;Vanderburg et al 2016;Newton et al 2016;Suárez Mascareño et al 2018).…”

The Impact of Planetary Rotation Rate on the Reflectance and Thermal Emission Spectrum of Terrestrial Exoplanets around Sunlike Stars

“…SMART was developed for Solar System terrestrial planet atmospheres and has been rigorously validated on Earth [Robinson et al, 2011] and Venus [Meadows and Crisp, 1996;Arney et al, 2014], as well as applied to study a diversity of exoplanet environments, including the spectra of M-dwarf habitable planets [Segura et al, 2003[Segura et al, , 2005Meadows et al, 2018;Lincowski et al, 2018], habitable haze-enshrouded exoplanets [Arney et al, 2016;2018] and H2-dominated mini-Neptunes [Charnay et al, 2015]. Water cloud wavelength-dependent and scattering optical properties used by SMART are specified by phase as either cirrus clouds (liquid) [Baum et al, 2005] or stratocumulus clouds (ice) [Hale and Querry, 1973], as described in Meadows et al [2018].…”

“…Recently, terrestrial-size or mass planets have been detected in the habitable zones of a variety of host stars, some of which include Proxima Centauri b [Anglada-Escudé et al, 2016], Kepler 62f [Borucki et al, 2013], Kepler 442b [Torres et al, 2015], Ross 128b [Bonfils et al, 2018], and the TRAPPIST-1 system [Gillon et al, 2017]. Detailed transmission spectroscopy across the near-infrared is within reach of the James Webb Space Telescope for numerous known terrestrial-sized exoplanets [Barstow and Irwin, 2016;Morley et al, 2017;Batalha et al, 2018;Lustig-Yaeger et al, 2019].…”

Exoplanet Terra Incognita

“…The only exoplanet discovered so far in this system is Proxima b (Anglada-Escudé et al 2016), possibly a rocky planet in the WHZ of the M5 red dwarf. Other nearby possibly rocky planets in a WHZ are Ross 128 b (11 ly) orbiting an M4 red dwarf (Bonfils et al 2017) and Tau Ceti planets e and f orbiting an orange dwarf G8 V (12 ly) (Feng et al 2017). These nearby exoplanets are excellent candidates for first-time exoplanet surface imaging.…”