A terrestrial exoplanet orbiting in the habitable zone of the nearest star Proxima Centauri has just been discovered. It has a mass about 30% larger than our planet and an orbital period (year) of 11.2 Earth days. Could it have an atmosphere? Could it have liquid water on its surface? How long is its day? What is its climate like today? Can we see it directly with a telescope now? And in the future? This website provides answers to these questions as a result of the studies that we have carried out and that have just been submitted for publication to the specialized Astronomy & Astrophysics journal.

Is Proxima Cen b habitable?

In two publications, a team of astronomers from Spain, Belgium, France, Germany, UK, and USA has studied the prospects for the recently discovered terrestrial planet Proxima b being a habitable world. Proxima b is a 1.3 Earth mass planet orbiting its star at about 1/20th of the Sun-Earth distance, which places it well within the so-called Habitable (Goldilocks) Zone. It receives an amount of energy from its central star that is about 2/3 of that received by the Earth from the Sun.

The key element to habitability is whether liquid water could exist on the planetary surface. While visible and infrared light just warm the planet, high-energy radiation (X-rays and extreme-UV) and particles are a threat to the gases and liquids (volatiles as a whole) as they can be eroded away and escape to space. Proxima b receives a significantly higher amount of such radiation compared to Earth, and therefore its is crucial to assess whether the planet has been able to maintain its water, and gases in general, over the course of the 5 billion years of history since it was born.

The studies, which have just been submitted for publication to the Astronomy & Astrophysics journal, make use of the most advanced tools and observations available today to investigate the past history of Proxima b and the amount of gases and liquids that could still be on its surface. Researchers find that, currently, Proxima b receives 60 times more high-energy radiation than the Earth, and that the total from the time of formation is about 7 to 16 times higher (because the Earth was also irradiated more strongly in the past).

Although the amount of initial water on the planet is unknown, calculations reveal that Proxima b could have lost about 1 ocean's worth of water due to the early irradiation in the first 100-200 million years after formation. What happened after that time is very uncertain. The planet could have continued to lose significant amounts of atmosphere gases and end up as a dry, atmosphereless world. Or perhaps it could have been able to keep most of its atmosphere and preserve liquid water on the surface and therefore be hospitable to life as we know it. Both possibilities are still open.

Much research is still due, but the studies conclude that the presence of liquid water on the surface today cannot be ruled out and thus Proxima b can be considered a viable candidate habitable planet.

To study the scenario in which Proxima b had surface water and an atmosphere, the authors examined the possible climates using a sophisticated 3D model and exploring a broad variety of atmospheric compositions and water inventories. They find that that liquid water may be present over the surface of the planet only in the sunniest regions, either an area in the hemisphere of the planet facing the star or in a tropical belt.

Future observations, for example using the 39-m ESO E-ELT telescope under construction in Chile, will allow further investigation of Proxima b and of the hypothetical presence of a thick atmosphere and a liquid water reservoir. If this turned out to be the case, it would be very exciting that the nearest star to the Sun also hosts the nearest habitable (perhaps inhabited?) planet.