A super-Earth orbiting around an old M star. The study: “GJ 357 b. A super-Earth orbiting an extremely inactive host star” of Modirrousta-Galian (UNIPA/INAF-OAPA) recently appeared on A&A

Despite theoretical models predict a large abundance of Earth-like planets in our Galaxy, the family of the 4284 planets discovered so far (updated at 2020/9/25, credits: NASA) counts relatively a few of such a planets. This is clearly an observational bias, due to the fact that the methods used to detect planets are not sensitive to small planets. For this reason, however, the few Earth-like planets known are important to study properties, formation and evolution of this class of planets.

 

A super-Earth has been recently identified around the M star GJ 357, which is very close to the Sun (30.8 light years away), and it has a mass of 0.34 Solar Masses. The transiting planet GJ 357 b has been discovered by the NASA satellite TESS (Transiting Exoplanets Survey Satellite). The planet has a mass of 1.8 Earth Masses and a radius of 1.22 Earth Radii, being thus compatible with a Earth-like planet. The planetary system of GJ 357 also hosts other two non-transiting super-Earths more massive than GJ 357 b.

 

With the aim of understanding the properties, formation and evolution of GJ 357 b, the team led by the astronomer D. Modirrousta-Galian (University of Palermo and INAF – Astronomical Observatory of Palermo) have analysed an X-ray observation of GJ 357 obtained with the ESA telescope XMM-Newton in the framework of a survey aimed at studying nearby M stars. Authors have calculated a very low X-ray luminosity for GJ 357 (log(L_x)[erg/s]=25.73), compatible with an age of 5 billion years. Given its low mass, it is unlikely that GJ 357 b hosted a primordial atmosphere. Nevertheless, a secondary atmosphere could been produced by outgassing and geological activity. Thanks to the estimate of the age of the system, the evolution of the X-ray luminosity of the star, and of the stellar and planetary parameters, authors have produced three possible models for the atmosphere of GJ 357 b: a 100% CO₂ composition, 100% SO₂, or 75% N₂, 24% CO₂, and 1% H₂O. The research is described in the paper: “GJ 357 b. A super-Earth orbiting an extremely inactive host star“, recently appeared on Astronomy & Astrophysics. Among the coauthors, the astronomers B. Stelzer, J. Maldonado e G. Micela of INAF – Astronomical Observatory of Palermo.

 

The figure (click here to visualize the entire image) shows the synthetic spectra of the atmosphere of GJ 357 b. The three colors mark the three different compositions: 100% CO₂  (red), 100% SO₂  (yellow), e 75% N₂, 24% CO₂ e 1% H₂O (blue). The spectra are calculated both for a cloud-less atmosphere (top panel) and considering clouds at 0.01 bar (bottom panel)