"In the last year, the temperate exoplanet Proxima b has been discovered orbiting Proxima Centauri, the nearest star, but it probably does not transit ahead and its true mass is unknown. On the other hand, seven planets the size of the Earth transiting the very low mass star TRAPPIST-1 located 12 parsecs (about 40 light years) away, but its masses and, in particular, its densities are not well defined ”.
With this introduction an international team of astronomers presents this week in the magazineNature his new discovery: the exoplanet LHS 1140b, which also orbits a cold red dwarf star located 40 light years from Earth, and which, due to its characteristics, could be the best place to look for signs of life beyond the system solar.
The newly discovered super-earth has a diameter 1.4 times that of Earth (almost 18,000 kilometers) and a mass about seven times that of our planet. Therefore, its density is also much higher, and this implies that LHS 1140b is probably rocky with a dense core of iron inside.
The authors also note that the new exoplanet orbits in the habitable zone (where there may be liquid water) around the faint red dwarf star LHS 1140, visible from the southern hemisphere in the constellation Cetus (the whale or the sea monster). .
Red dwarfs are, in addition to the most common stars in our galaxy, much smaller and cooler than the Sun. Although LHS 1140b is ten times closer to its star than Earth is to the Sun, it only receives about half as much light as our planet and is situated in the middle of the habitable zone. From Earth, its circular orbit is seen almost edge-on, and as the exoplanet passes in front of its star, it blocks a little of its light every 25 days.
The most interesting exoplanet of the decade
"It is the most interesting exoplanet I have seen in the last decade," says lead author Jason Dittmann of the Harvard-Smithsonian Center for Astrophysics (USA), who stresses: "It is the perfect goal to carry out one of the Science's Greatest Missions: Search for Evidence of Life Beyond Earth ".
At present, conditions for this red dwarf are particularly favorable, as it spins more slowly and emits less high-energy radiation than other similar low-mass stars. For life, as we know it, to develop, a planet must have liquid water on its surface and retain an atmosphere.
In this case, the large size of the planet implies that, millions of years ago, an ocean of magma could have existed on its surface. This seething ocean of lava could have provided steam to the atmosphere long after the star had calmed down, reaching its current constant brightness, thus replenishing water that might have been lost by the action of the star in its most active phase.
Best candidate to study the possible atmosphere
The researchers emphasize that this super-earth may be the best candidate so far for future observations whose objective is to study and characterize, if it has one, the exoplanet's atmosphere. Two of the European team members, Xavier Delfosse and Xavier Bonfils, both from the CNRS and IPAG centers in Grenoble (France) emphasize: "For the future characterization of planets in the habitable zone, the LHS 1140 system could be an even more objective. important than Proxima bo TRAPPIST-1. This has been an extraordinary year for the discovery of exoplanets! "
In the case of LHS 1140b, the initial finding was made with the MEarth-South telescope array installation, located at the Cerro Tololo observatory (Chile), from where the first signs were detected: characteristic changes in light that occur when the exoplanet passes in front of the star.
A crucial follow-up was then carried out with the HARPS instrument (High Accuracy Radial velocity Planet Searcher) that has the European Southern Observatory (ESO) in La Silla (Chile). It is a high precision planet finder using the radial velocity method, thus confirming the presence of the super-earth.
HARPS also helped establish the orbital period and made it possible to deduce the mass and density of the exoplanet, which researchers think formed at its current location at least 5 billion years ago, similar to how Earth did.
Observations of the new super-earth will soon be made with the Hubble space telescope (NASA / ESA) to determine exactly how much high-energy radiation acts on it, which will be able to define its capacity to host life.
Furthermore, when new telescopes, such as the James Webb spacecraft and ESO's terrestrial Extremely Large Telescope, come into operation in the future, it is likely that scientists will be able to observe in detail the atmospheres of exoplanets like LHS 1140b, an exceptional candidate for this type of investigations. Will it be the first in which an atmosphere with possible signs of life is detected?
Jason Dittmann et al. "A temperate rocky super-Earth transiting a nearby cool star."Nature, April 20, 2017.