A new terrestrial planet
The star Proxima is part of a triple star system, Alpha Centauri, which consists of two solar-type stars separated by about 20 AU (1 AU = distance Earth-Sun) and a distant red dwarf, Proxima, 15,000 AU from the other two, which is currently the closest star to the Sun (4.24 light years). Proxima is a red dwarf. It has 12% of the mass of the Sun and its brightness is only 0.15% of that of our star. The estimated age of this star system is close to that of the Sun, between 4.5 and 5 billion years. A team led by G. Anglada-Escudé observed periodic variations in the speed of the star caused by the presence of a planet. This detection, published on Aug 25 in the journal Nature was essentially performed using data collected by an ESO telescope in Chile, equipped with the most precise spectrograph today, HARPS. This planet, called Proxima b, is at least 1.27 times the mass of Earth and completes one orbit in 11.2 days, corresponding to a star-planet distance of 0.05 AU. This may seem very close but because of the low brightness of the star the planet receives only 65-70% of the radiation flux that the Earth receives from the Sun. The real mass of the planet depends on the geometry of the orbit, which is not known. The most likely mass is 1.4 Earth masses, which makes it probably a rocky planet.
Could it be habitable?
This major discovery is accompanied by a series of works about this planetary system. A team of astronomers from Spain, Belgium, France, Germany, UK, and USA has studied in detail the habitability of this planet, and, in particular, whether the planet could have liquid water on its surface. This work has resulted in two articles submitted to the journal Astronomy & Astrophysics.
The evolution of the star and its planet
While Proxima b is today in the so-called "habitable zone" of its star, where surface oceans may exist, it has not always been the case. Its star has evolved differently from solar-type stars, and its brightness has decreased over time. Early in its history, the planet received a much greater flux of energy. During the warm phase, water was vaporized into a thick atmosphere exposed to high-energy radiation from its star. Proxima, like most red dwarfs, is very active and the planet is exposed to more X-ray and extreme-UV radiation than Earth. The combination of these two factors, vaporization of the water and strong exposure to high-energy radiation and particles, generates evaporation from the atmosphere to space and erosion of the water content. The purpose of this article was to characterize the radiation spectrum of the star in the range from X-rays to the UV in order to estimate the atmospheric losses and determine whether the water reservoir and the atmosphere could survive this early hot phase. The fate of Proxima b depends on the amount of water and gas the planet inherited during its formation, which was very different from that of the Earth. We do not know if b Proxima began its history with more or less water than Earth and the planet could still possess a thick atmosphere and oceans despite early atmospheric losses.
Two scenarios for the early evolution of Proxima b. It could have lost all gases and liquids before it entered the habitable zone of its star. Or it could have kept water and an atmosphere until today and thus be habitable.
The climate of a habitable Proxima b
Building up on the hypothesis that the planet may harbor water and atmosphere, albeit in quantities that we cannot estimate today, the second article examines the possible climates of Proxima b by exploring a broad variety of atmospheric compositions and water inventories. To achieve this theoretical exploration, the scientists used a 3D climate model similar to those used to study the Earth's climate but especially developed for exoplanets and including all the relevant characteristics of the Proxima system. At the short orbital distance of Proxima b, strong tidal forces exerted by the star allow only two possible rotations for the planet. In the first case the planet is synchronous, its rotation period is equal to its orbital period (11.2 days) and it always presents the same face to its star. In the second case the planet rotates 3 times every 2 orbits (3:2 spin-orbit resonance, like Mercury), a situation that can arise if the orbit is slightly eccentric (which is possible but not yet determined). In all cases, Proxima b should not have seasons because tidal forces cancel the obliquity, bringing the equator on the planet's orbital plane. Numerical simulations show that liquid water is possible for a wide range of atmospheric compositions. Depending on the rotation period and the amount of greenhouse gases, water may be present over the surface of the planet only in the sunniest regions: that is to say in the area facing permanently the star in the synchronous case and in a tropical belt in the asynchronous case.
A numerical simulation of possible surface temperatures on Proxima b performed with the Laboratoire de Météorologie Dynamique's Planetary Global Climate Model. Here it is hypothesized that the planet possesses an Earth-like atmosphere and that it is covered by an ocean (the dashed line is the frontier between the liquid and icy oceanic surface). The planet is in synchronous rotation (like the moon around the Earth), and is seen as a distant observer would do during one full orbit.
Same as above but for the case of the planet trapped in the 3:2 resonance (3 rotations of the planet for every revolution around the star).
Surface liquid water?
Note that subsurface (underground) liquid water can also provide habitable conditions (similar to Jupiter's moon Europa in the Solar System). However, such biosphere would not allow for remote detection from Earth. If liquid water is present at the surface, biological photosynthesis is possible and its affects the entire planetary environment so that it can potentially be observable from interstellar distances.
The next step: seeing the planet
The authors also used the models to simulate and prepare for future observations. They determine what telescopes could see and in particular the 39-m ESO E-ELT whose construction just began in Chile. This large telescope will actually "see" the world by separating it from its star, something that is feasible today only for some newly formed gas giant planets. These observations will tell us whether Proxima b has water, an atmosphere and a habitable climate.
Not the Earth 2.0
In any case, and although it is an excellent candidate planet that could harbor life, Proxima b cannot be considered an Earth twin. One thing is certain, in fact: the history of Proxima b and its star was radically different from the history of the Earth and the Sun. The formation of Proxima b, the irradiation by its star, the tidal forces it suffers affecting its internal structure and rotation, its possible climates have no equivalent in the history of our planet.