ESO/M. Kornmesser

ESO/M. Kornmesser

Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani

ESO/M. Kornmesser/G. Coleman

Digitized Sky Survey 2 Acknowledgement: Davide De Martin/Mahdi Zamani

After weeks of rumors and unconfirmed reports, European scientists have officially announced the discovery of an Earth-sized world around the closest star to the Sun, Proxima Centauri. Although the planet theoretically orbits its star in a region where water could exist as a liquid on its surface, no direct information can be gleaned about whether Proxima b has an atmosphere, water, or other characteristics that would increase its habitability. Nonetheless, it is tantalizing to imagine a habitable world so close to home.

The existence of a likely terrestrial world orbiting Proxima Centauri, which is 4.2 light years from the Sun and part of a three-star system that includes two distant members, Alpha Centauri A and B, has been speculated about for some time. That's because Guillem Anglada-Escudé, an astronomer at the Queen Mary University of London, and colleagues used spectrograph data from the European Southern Observatory in Chile during the 2000s to discern a wobble in the star’s movement.

Then, earlier this year, with a concerted observing program over 60 nights at the observatory, they gathered additional data to rule out other explanations for Proxima Centauri's wobbling, such as flares or other stellar activity. It had to be the gravitational effect of a nearby planet. “Statistically, there can be no doubt now,” Anglada-Escudé said during a telephone news briefing about the findings, published Wednesday in Nature.

What we know about Proxima b

Of Proxima b, the scientists know this much for sure: the planet orbits its star every 11.2 days at a distance of approximately 7.5 million km, about 5 percent of the distance that Earth lies from the much larger and hotter Sun. At such a distance from a red dwarf star like Proxima Centauri, the newly confirmed planet receives about 65 percent of the radiation that Earth does from the Sun. Therefore, without an atmosphere, the surface would have an equilibrium temperature of -40 degrees Celsius (by comparison, if Earth had no atmosphere, it would have a surface temperature of -20 degrees Celsius). Finally, the astronomers say the planet has a minimum mass of 1.3 Earth masses.

With this information, said Ansgar Reiners, one of the scientists involved in the discovery, it is possible to make some educated guesses about what kind of a world Proxima b might be. The planet is probably rocky, rather than a gas giant, and tidally locked to its star, meaning one side is always sunny, while the other is perpetually gloomy. Therefore, if the planet does have an atmosphere, the surface temperature may vary from +30 degrees Celsius on the light side to -30 degrees Celsius on the dark side. This would allow water, if the planet has any, to exist on the surface of the world as a liquid at some locations.

The biggest question is whether the planet has an atmosphere, and at present there is just no way for scientists to know. Absent hard data, they have run model simulations of how such a planet might have formed close to its parent star or have moved into the inner system later. Confounding their speculations is the fact that astronomers know little about the early evolution of long-lived red dwarf stars. Was Proxima Centauri once much more active, in terms of X-ray fluxes and other high-energy radiation, than it is today?

If that is the case, had the planet formed long ago near its present orbit around the star, an active Proxima Centauri likely would have blown away any atmosphere and any attendant water billions of years earlier. However it is also plausible that the planet formed farther out, beyond the system’s ice line, and migrated farther inward at a later date. Among the multitudes of possibilities, Reiners said, “We have scenarios that lead to the formation of an atmosphere."

It’s full of... life?

Well before the discovery of Proxima b, exoplanet hunters were increasingly interested in red dwarf stars like Proxima Centauri. This is not only because they are the most common stars in the galaxy, but also because Earth-sized worlds can be found in the habitable zone around these stars using the “Doppler technique.” With this method scientists can infer the presence of a nearby planet by observing its gravitational pull on its parent star.

Because such stars are much cooler than the Sun, planets in the so-called habitable zone must necessarily orbit close to their stars. This makes for a happy coincidence in that the very types of planet that astronomers most want to find around red dwarfs—close-in planets where it is warm enough for water to exist as a liquid—are the easiest types for them to observe with existing telescopes. In the years ahead, if astronomers can confirm that Earth-sized worlds like Proxima b are common around red dwarfs, it will provide a powerful argument indeed that life may well be very common in the galaxy.

Nature, 2016. DOI: doi:10.1038/nature19106 (About DOIs).

Listing image by ESO/M. Kornmesser