A terrestrial planet candidate in a temperate orbit around Proxima Centauri

Abstract

At a distance of 1.295 parsecs, 1 the red-dwarf Proxima Centauri (α Centauri C, GL 551, HIP 70890, or simply Proxima) is the Sun's closest stellar neighbour and one of the best studied low-mass stars. It has an effective temperature of only ∼ 3050 K, a luminosity of ∼0.1 per cent solar, a measured radius of 0.14 R ⊙ 2 and a mass of about 12 per cent the mass of the Sun. Although Proxima is considered a moderately active star, its rotation period is ∼ 83 days, 3 and its quiescent activity levels and X-ray luminosity 4 are comparable to the Sun's. New observations reveal the presence of a small planet orbiting Proxima with a minimum mass of 1.3 Earth masses and an orbital period of ∼11.2 days. Its orbital semi-major axis is ∼ 0.05 AU, with an equilibrium temperature in the range where water could be liquid on its surface. 5 The results presented here consist of the analysis of previously obtained Doppler measurements (pre-2016 data), and the confirmation of a signal in a specifically designed follow-up campaign in 2016. The Doppler data comes from two precision radial velocity instruments, both at the European Southern Observatory (ESO): the High Accuracy Radial velocity Planet Searcher (HARPS) and the Ultraviolet and Visual Echelle Spectrograph (UVES). HARPS is a high-resolution stabilized echelle spectrometer installed at the ESO 3.6m telescope (La Silla observatory, Chile), and is calibrated in wavelength using hollow cathode lamps. HARPS has demonstrated radial velocity measurements at ∼1 ms −1 precision over time-scales of years, 6 including on low-mass stars. 7 All HARPS spectra were extracted and calibrated with the standard ESO Data Reduction Software, and radial velocities were measured using a least-squares template matching technique. 7 HARPS data is separated into two datasets. The first set includes all data obtained before 2016 by several programmes (HARPS pre-2016 work, and its value is then used to assess the false-alarm probability (or FAP) of the detection. 14 A FAP below 1% is considered suggestive of periodic variability, and anything below 0.1% is considered to be a significant detection. In the Bayesian framework, signals are first searched using a specialized sampling method 16 that enables exploration of multiple local maxima of the posterior density (the result of this process are the gray lines in Figure 1), and significances are then assessed by obtaining the ratios of evidences of models. If the evidence ratio exceeds some threshold (e.g. B 1 /B 0 > 10 3 ), then the model in the numerator (with one planet) is favoured against the model in the denominator (no planet).A well isolated peak at ∼11.2 days was recovered when analyzing all the night averages in the pre-2016 datasets (Figure 1, panel a). Despite the significance of the signal, the analysis of pre-2016 subsets produced slightly different periods depending on the noise assumptions and which subsets were considered. Confirmation or refutation of this signal at 11.2 days was the main driver for proposing the HARPS PRD campaign. T...

Stable habitable zones of single Jovian planet systems

“…CODEX, Pasquini et al 2010). As the radial velocity resolution decreases, the resultant noise from stellar activity in Sun-like stars becomes significant (Dumusque et al 2011a;Anglada-Escudé et al 2016). We do not consider stellar noise in our assessment herein.…”

Calculations of periodicity from Hαprofiles of Proxima Centauri

“…The rotation period of Proxima is nevertheless uncertain. The rotation period is of interest for various studies, including flare cycles (Davenport et al 2016) and for the correct identification of radial velocity signals from orbiting planets (Anglada-Escudé et al 2016) and subsequent work (Ribas et al 2016). Previous studies have reported periods ranging from 31.5 ± 1.5 days (Guinan & Morgan 1996), through 41.3 days (Benedict et al 1993) to between 82 and 84 days (Benedict et al 1992(Benedict et al , 1998.…”

No Snowball on Habitable Tidally Locked Planets

“…This topic is particularly timely given the recent discoveries of likely terrestrial planets in the habitable zones of three M-dwarfs within 40 light years of Earth (TRAPPIST-1e, Proxima Centauri b, and LHS 1140b, Anglada-Escudé et al 2016;Gillon et al 2017;Dittmann et al 2017). One issue that influences planetary habitability is the occurrence of global glaciations, which are called snowball Earth events in Earth history (Kirschvink 1992;Hoffman et al 1998).…”