Mass determination of the three planets of K2-3. Published on A&A the study: “Eyes on K2-3: A system of three likely sub-Neptunes characterized with HARPS-N and HARPS” of M. Damasso (INAF-OA Torino)
In the field of exoplanetary science, it is particularly important the study of exoplanets orbiting in the habitable zone, e. g. at a distance from their stars which allows water to be in liquid state at the surface of rocky planets. In particular, several projects such as MEarth (Irwin et al. 2015), APACHE (Sozzetti et al. 2013), and TRAPPIST (Gillon et al. 2017) focused on the search of exoplanets around M stars (with effective temperature between 2400K and 3700K and with a mass ranging from 0.08 and 0.6 solar masses). In M stars, in fact, the two techniques mainly used to detect exoplanets, the method of transits (which consists in detecting the fading of emission from the stars during the transit of exoplanets across stellar disc), and the method of radial velocity (which consists in detecting the periodic oscillations of stars around the common center of mass of the star-planets system), are more effective in detecting planets in the habitable zone.
The star K2-3 is a M0 star at a distance of 147 light years from the Sun. The satellite Kepler (which has recently finished its fuel completing its long and successful mission) has detected three planets around this star: K2-3 b (with a radius of 2 R⊕ and an orbital period of 10 days), K2-3 c (1.7 R⊕ and 24.6 days), and K2-3 d (1.6 R⊕ and 45.5 days). In particular, K2-3 d orbits close to the inner edge of the habitable zone of K2-3.
Given these properties, it was important to obtain an accurate determination of the masses and thus the bulk densities of these three planets. This was the purpose of the study “Eyes on K2-3: A system of three likely sub-Neptunes characterized with HARPS-N and HARPS” of M. Damasso (INAF-Astronomical Observatory of Turin), recently published by Astronomy & Astrophysics. By analysing a total of 349 high resolution spectra, 138 acquired with HARPS and 211 with HARPS-N, some of which as part of the program GAPS (Global Architecture of Planetary Systems) the authors have measured the mass of two of these three planets after removing the principal component in the radial velocity signal due to stellar activity. The resulting mass fo K2-3 b is 6.6 ± 1.1 M⊕, while for K2-3 c is 3.1−1.2+1.3 M⊕. The mass of planet d has not been measured directly, since its signal was very low, and it was estimated by simulations as equal to 2.7−0.8+1.2 M⊕. According to these results, these three planets are of the sub-Neptune class. The study has been published with the collaboration of the astronomers J. Maldonado, L. After, E. Gonzalez-Alvarez, A. Maggio, S. Masiero, and G. Micela of INAF – Astronomical Observatory of Palermo.
The figure (link) shows the mass-radius diagram for exoplanets for which the mass and radius have been both measured with a relative error better than 30$\%$. The location of the K2-3 planets is emphasized. The curve for bulk density 3 g cm-3 is shown in grey passing through the three positions occupied by the K2-3 planets. The planetary data are taken from the NASA exoplanet archive and updated to 30th August 2017.
