Two super-Earths and a possible third exoplanet have been discovered orbiting a neighboring star, forming the closest known compact planetary system to Earth, located approximately 10.7 light-years away.
Named GJ887, the alien Sun is considered the twelfth nearest star to Earth. The star system is of great interest to astronomers, which say that it might be the “best star by far” to study for exoplanets with atmospheres similar to our own.
The planets have been found in the framework of the international collaboration RedDots thanks to the Doppler technique, which allows detecting the small movement that the planets produce in their star when revolving around it.
GJ887 is a red dwarf star, the most common type of star in the Milky Way, and has a mass equivalent to half that of the Sun and an approximate temperature of about 3,400 degrees, which is around 2,100 degrees cooler than the Sun.
The recently discovered planets, named GJ887b and GJ887c respectively have a minimum mass of about four and seven times that of the Earth, and both orbit their star at a distance that falls just short of the habitable zone; a region in space, surrounding a star that allows the existence of liquid water on the surface of a planet.
This means that the two worlds probably are too hot to maintain liquid water on their surface.
However, the second planet orbits close to the inner edge of the habitable zone, which increases its chances of being habitable and supporting liquid water on its surface.
“We have found two super-Earths, planets that are more massive than Earth, orbiting a star called GJ887, said Eloy Rodríguez, a researcher at the Institute of Astrophysics of Andalusia (IAACSIC) who participated in the finding, published in Science. “Furthermore, we have found indications of the existence of a third super-earth that, if confirmed, would fall just within the habitable zone.”
GJ887, therefore, becomes one of the closest known multi-planetary systems to Earth, only behind Proxima Centauri and Wolf359, located respectively 4.2 and 7.9 light-years away.
Of the three, GJ887 constitutes the most compact; its two planets orbit around their star every 9.3 and 21.8 days, and the third exoplanet candidate orbits it’s star every 51 days, according to a statement from the University of Göttingen.
Unlike Proxima Centauri and Wolf359, which have great magnetic activity on their surfaces, GJ887 seems to be a very calm star.
GJ887 has been observed for three months with the HARPS spectrograph, one of the most accurate planet-hunting instruments, and archival data from several spectrographs spanning more than twenty years has been used, in addition to both ground-based and space photometric observations.
Given their minimum mass, in principle, the planets orbiting GJ887 might be rocky super-Earths, but astronomers can’t know for sure in the absence of measurement of the radius of the planets.
Furthermore, depending on the inclination of the orbit with respect to us, the masses could be much larger and move to the range of mini-Neptunes, with higher water content in their structure.
Whether the super-Earths are habitable is something experts can’t know for sure, at least not now.
It is also possible that the planets are tidally locked to their Sun.
“Close orbiting planets like these have a high chance of being tidally locked to their host star. This means that as the planet orbits its star, the same hemisphere always faces the star,” explained Dr. John Barnes, an astrophysicist at the Open University and one of the authors on the study.
“For the planets orbiting GJ 887, half of the planets would be in perpetual daytime and the other half in a perpetual night time. So, it could be much cooler on some parts of the planets than others.”