A system of super-Earths orbiting the star GJ 3998 described in the article: ‘HADES RV Programme with HARPS-N at TNG XVI. A super-Earth in the habitable zone of the GJ 3998 multi-planet system’ by A. K. Stefanov (IAC)

Just 60 light years away from us, the star GJ 3998, of spectral class M, hosts a system of three super-Earths, discovered and studied thanks to observations made by the Galileo National Telescope.

M-type stars are among the most important targets for exoplanet research, mainly for two reasons: they are the most numerous stars in our Galaxy, and the techniques most commonly used to detect exoplanets, namely transits and radial velocity, are more effective than for stars of greater mass. As a result of several decades of exoplanetary research, we now know that, for M-class stars in the Milky Way, hosting planets is more the rule than the exception.

The radial velocity technique consists of measuring the small oscillations of stars caused by gravitational interaction with their planets. These observations exploit the Doppler effect, which influences stellar radiation precisely because of these oscillations. Since the speeds at which stars oscillate are typically on the order of metres or centimetres per second, very high spectral resolution instruments are needed to detect them. One of the most advanced instruments for this technique is HARPS-N, mounted on the Galileo National Telescope, a key instrument in numerous projects for the study and characterisation of exoplanets, including HADES (HARPS-N Red Dwarf Exoplanet Survey Radial Velocity), which began in 2012 and is led by astronomers from our observatory.

As part of the HADES project, a team of researchers led by astrophysicist A. K. Stefanov (Instituto de Astrofísica de Canarias, Departamento de Astrofísica) analysed data from a long campaign of observations of the star GJ 3998, conducted with HARPS-N. This star, located only 60 light-years away, was already known to host two super-Earth-type planets, discovered in a 2016 study led by astronomers from our observatory.

The new observations not only confirm the presence of the two planets, with orbital periods of 2.6 and 13.7 days and masses equal to 2.5 and 6.8 Earth masses, but have also led to the discovery of a third planet of the same class, with a mass of 6.1 Earth masses and an orbital period of 41.8 days.
A peculiarity of this new planet is the intensity of the radiation flux it receives from its star, very similar to that which Earth receives from the Sun. This characteristic is shared by very few exoplanets known to date.
The research is described in the article: 'HADES RV Programme with HARPS-N at TNG. XVI. A super-Earth in the habitable zone of the GJ 3998 multi-planet system', recently published in Astronomy & Astrophysics. Co-authors include astrophysicists L. Affer and G. Micela and astrophysicist J. Maldonado of INAF - Palermo Astronomical Observatory.

The figure (click here to view it in full) shows two graphs comparing the mass, period, density and intensity of the incident radiation flux for planets known in the literature with planets b, c and d of GJ 3998. In particular, in the panel on the right, it can be seen that GJ 3998 d is one of the few known planets whose incident flux (mean insolation flux) is comparable to that of Earth.