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Only six light years away in the constellation Serpent Bearer, and thus practically in our interstellar front garden, is Barnard's star, a red dwarf of rather low luminosity. Although he was named after the three members of the Alpha Centauri Systems is the fourth star, it cannot be seen in the night sky with the naked eye. His apparent brightness of 9.54 mag (i.e. in the range of Neptune) already requires the use of a slightly better telescope.

In November 2018, astronomers around Ignasi Ribas from the Institute of Space Studies of Catalonia published the Discovery of an exoplanet in orbit around Barnard's star in front. Their results were based on data from 20 years of measuring time from a total of seven different instruments. The scientists were not only able to work out its existence, but also some key data from this nearby world.

Giant rocky planet or mini Neptune?

Barnard's star b has at least 3.2 times the mass of our earth, which makes the exoplanet a so-called super-earth - possibly a rocky planet, but perhaps already beyond the threshold of the ice giant, such as Uranus or Neptune. It orbits its parent star at a distance of 0.4 astronomical units (AU), that is a good 60 million kilometers; it takes the planet 233 days to complete a full lap. Our Mercury is practically at the same distance from the Sun, but Barnard's star is only 0.00044 times as bright as our home star.

This means that Barnard's star b, unlike Mercury, orbits in an extremely cold zone of its system, close to the so-called snow line. During planet formation, this crucial boundary separates the inner region, in which rocky planets are formed, from the outer region, in which gas planets can condense. The data therefore also suggest that temperatures of at most -170 degrees Celsius prevail on Barnard's star b - at least that doesn't seem to be life-friendly.

Life not excluded

Nevertheless, it cannot be completely ruled out that there is life in this exotic world, as is now the case a team led by Edward Guinan and Scott Engle from Villanova University in Pennsylvania explained. The astronomers put on the American Astronomical Society (AAS) annual meeting in Seattle presented a thesis according to which Barnard's star b could have parallels to Jupiter's moon Europa: a kilometer-thick ice crust under which, melted by geothermal processes, lakes or entire oceans of liquid water could exist - provided, of course, it is actually one Super earth made of rock and not a small gas planet.

Guinan and Engle argue that similar phenomena have already been demonstrated on Earth. Scientists now have hundreds under the ice of Antarctica subglacial lakes discoveredthat could house a completely independent ecosystem in isolation for millions of years. In addition, the system is around 9 billion years old, twice as old as the solar system - a lot of time to bring about life.

"The most significant aspect of the discovery of Barnard's star b, however, is its proximity to Earth," says Engle. "The fact that the two closest star systems are home to exoplanets supports the assumption that planets are extremely common celestial bodies in our galaxy." Even if most of them orbit unpredictable red dwarfs, it would increase the likelihood that some of these worlds would have good conditions for the evolution of life.

Soon in sight

How the potential living conditions on Barnard's star b actually stand could be found out in the next few years. Both the Hubble successor James Webb Space Telescope (JWST) as well as the terrestrial Extremly Large Telescope the Eso have the ability to make the nearby exoplanet directly visible. "If the JWST shows a bright planet, it should probably be a mini-Neptune," says Guinan. In that case one can assume that Barnard's star b is a sterile planet. "If, on the other hand, the celestial body is rather faint, then we are probably dealing with the assumed frozen super-earth." In other words, a world that, in the best-case scenario, hides life under its ice crust, which may even be detected from a distance using biomarkers. (tberg, January 21, 2019)