An international team, led by the Instituto de Astrofísica de Canarias (IAC) in which the Institute of Space Sciences (ICE-CSIC) and the Institute of Space Studies of Catalonia (IEEC) participate, has detected a super-Earth orbiting in the habitable zone of GJ 3998, a nearby red dwarf located 59 light-years away. The new planet, named GJ 3998 d, is the third planet found in the system. 

“GJ 3998 d is a welcome addition to the planetary census of our cosmic neighbourhood”, states Atanas Stefanov, La Caixa funded PhD student at IAC and the University of La Laguna (ULL) and main author of the study published in Astronomy & Astrophysics. “This super-Earth appears to be in the habitable zone of one of the nearer stars to the Sun. This gives us one more reason to keep searching for habitable planets at our doorstep”, he adds. 

The newly discovered planet, GJ 3998 d, is a super-Earth with a mass 6 times larger than the mass of the Earth. It resides in the optimistic habitable zone of its star and completes an orbit once every 41.8 days. At this distance, GJ 3998 d gets just 20% more stellar insolation compared to what Earth receives by the Sun. 

“Despite we already know more than 5800 of them, only a very small number of exoplanets show such characteristics, which are interesting for further studies on the presence of so-called biomarkers in their atmospheres”, says Manuel Perger, researcher at ICE-CSIC and IEEC, and co-author of the study. 

Researchers at the ICE-CSIC and IEEC contributed with the analysis of the complex data both of the planetary companions and the magnetic activity of the host star, manifested in dark spots on the stellar surface.   

Credits: Gabriel Pérez Díaz (SMM, IAC)

‘GJ 3998 is significantly smaller and cooler than the Sun, and this moves the habitable zone closer to the star’, explains Alejandro Suárez Mascareño, a researcher in the IAC and co-author of the study. ‘While it is certainly different from the Earth, if the planet is rocky, it might be able to host liquid water on its surface, one of the main requirements for life’, noted Jonay I. González Hernández, a researcher at the IAC and a co-author. 

The proximity of this system to the Sun makes GJ 3998 d an attractive candidate for atmosphere characterisation. “It should be possible to check for the presence of an atmosphere and probe for oxygen using the future ANDES spectrograph at ESO’s Extremely Large Telescope (ELT). GJ 3998 d would also be a good target for the future 50m Exo Life Finder (ELF) telescope, led by the IAC, which will be looking for biosignatures in exoplanet atmospheres”, adds Rafael Rebolo, researcher at the IAC and co-author of the study.

The discovery is part of the HADES programme, an international effort to explore planetary systems around red dwarfs through the Telescopio Nazionale Galileo (TNG) telescope at Roque de los Muchachos Observatory, La Palma. Red dwarfs are smaller, cooler stars than our own, and make up nearly three quarters of the stellar population in our galaxy. Their low masses and their abundance make them prime targets in the search of low-mass planets. GJ 3998, one such red dwarf, has been attractive to the community for its proximity (59 ly) and for its rather tame stellar activity. Using the HARPS-N spectrograph at the TNG telescope, the team was able to detect minute wobbles in the star’s motion, caused by the gravitational tug of orbiting planets.

A previous study conducted in 2016 by the same team had already detected two planets. The presence of an additional signal in the data prompted the continuation of the observations and the reanalysis of the dataset. With three known planets now detected in the system, GJ 3998 highlights once again how common multi-planetary systems are. “Planets, in particular low-mass ones, are rarely on their own — they prefer to have company. Often, when we revisit a system with new measurements and new methods, we find new planets that had been overlooked before”, concludes Atanas Stefanov. 

Press release in collaboration with Instituto de Astrofísica de Canarias (IAC)