Strong indications for the existence of a second planet in orbit around Proxima Centauri, the nearest star to the Sun, have been recently unveiled by an international team of scientists.

Proxima Centauri is a small red dwarf star about 4.3 light-years away from us. It is colder and smaller than our Sun, and since 2016 it has been known to host at least one planet, Proxima b. However, new data indicate the existence of a second planet, named Proxima c, orbiting in the same system. The discovery was announced by an international team of researchers, led by Fabio Del Sordo, postdoctoral researcher at the Institute of Astrophysics of FORTH, and Mario Damasso, National Institute of Astrophysics (INAF) in Torino, and it was published yesterday in Science Advances.

An artist ‘s impression of Proxima Centauri ‘s planetary system which was selected as cover for the current Science Advances issue (Credit: Lorenzo Santinelli)

They used data obtained in 17 years of observations performed with the spectrographs HARPS, at La Silla Observatory, and UVES, at the Very Large Telescope on Cerro Paranal, in Chile. From these spectra, the team extracted 279 radial velocities aimed at monitoring the possible wobbling of Proxima Centauri as a consequence of planets orbiting around it. In this dataset, Damasso and Del Sordo unveiled a signal consistent with the existence of Proxima c, an exoplanet at least six times more massive than our Earth. The detection of Proxima c is very challenging due to its long orbital period of about 5.2 years and its small mass. Its estimated equilibrium temperature would be of about 40 K, or -230 degrees Celcius, as it orbits at around 1.5 Astronomical Units away from its host star: an icy planet, unlikely to host life as we know it.

What makes this exoplanet fascinating, beyond its proximity to our Solar System, is the possibility to observe it with various techniques, such as astrometry and direct imaging. In particular, the team illustrated the possibility of confirming the existence of Proxima c with astrometric observations underway with the Gaia satellite. When the Gaia end-of-mission dataset is released, astrometry will probably prove its existence, and tell us both its real mass and exact orbit.

With this discovery, Proxima becomes the nearest multi-planetary system to us, and new compelling questions immediately arise:

What is the composition of Proxima c's atmosphere? Does it host a system of Saturn-like rings? How did it form?

New observations, analysis and modelling will hopefully soon shed light on this brand new celestial object.

Article: M. Damasso, F. Del Sordo, et al., A low-mass planet candidate orbiting Proxima Centauri at a distance of 1.5 AU. Sci. Adv. 6, eaax7467 (2020) https://advances.sciencemag.org/content/6/3/eaax7467