An international team of astrophysicists from France and the United States has determined that the surface properties of Proxima Centauri b — an Earth-mass exoplanet orbiting Proxima Centauri, the closest star to our Solar System — favor its habitability.

Proxima Centauri is a red dwarf star only 4.23 light-years away in the constellation Centaurus.

This small, cool star is not visible to the naked eye and lies near to the much brighter pair, Alpha Centauri AB.

Earlier this year, a terrestrial exoplanet was discovered orbiting around Proxima Centauri. Named Proxima Centauri b, or simply Proxima b, the planet is in an orbit that would allow it to have liquid water on its surface, thus raising the question of its habitability.

According to a new study led by Marseille Astrophysics Laboratory researcher Bastien Brugger, Proxima b could be a ‘water world,’ with an ocean covering its entire surface.

The study shows that the planet has a mass close to that of Earth and orbits its star with a period of 11.2 days at an average distance of 0.05 AU (one tenth of the Sun-Mercury distance).

Despite this close orbit, the fact that Proxima Centauri’s luminosity is only 0.15% of that emitted by the Sun keeps Proxima b in the crucial habitable zone.

However, very little is known about the planet itself, particularly its radius. It is therefore impossible to know what the planet looks like, or what it is made of.

The radius measurement of an exoplanet is normally done during transit, when it eclipses its star. But Proxima b is not known to transit.

There is another way to estimate the radius of a planet. If we know its mass, we can simulate the behavior of the constituent materials. This is the method used by Dr. Brugger and his colleagues from Cornell University and the Marseille Astrophysics Laboratory.

With the help of a model of internal structure, the team explored the different compositions that could be associated with Proxima b and deduced the corresponding values for the radius of the planet.

The authors restricted their study to the case of potentially habitable planets, simulating dense and solid planets, formed with the metallic core and rocky mantle found in terrestrial planets in the Solar System. They also allowed the incorporation of a large mass of water in their composition.

These assumptions allow a wide variety of compositions for Proxima b.

The radius of the planet may vary between 0.94 and 1.4 times the radius of the Earth (3,959 miles, or 6,371 km).

The study shows that Proxima b has a minimum radius of 3,722 miles (5,990 kilometers), and the only way to get this value is to have a very dense planet, consisting of a metal core with a mass equal to 65% of the planet, the rest being rocky mantle. The boundary between these two materials is then located about 932 miles (1,500 km) depth.

With such a composition, Proxima b is very close to the planet Mercury, which also has a very solid metal core. This first case does not exclude the presence of water on the planet’s surface, as on Earth where the water body does not exceed 0.05% of the mass of the planet.

In contrast, Proxima b can also have a radius of 5,543 miles (8,920 km), provided that it is composed of 50% rock surrounded by 50% water.

In this case, Proxima b would be covered by a single liquid ocean 124 miles (200 km) deep. Below, the pressure would be so strong that liquid water would turn to high-pressure ice before reaching the boundary with the mantle to 1,926 miles (3,100 km) depth.

In these extreme cases, a thin gas atmosphere could cover Proxima b, as on Earth, making the planet potentially habitable.

Such findings provide important additional information to different composition scenarios that have been proposed for Proxima b. Some involve a completely dry planet, while others permit the presence of a significant amount of water in its composition.

Details of the research will be published in an upcoming edition of the Astrophysical Journal Letters. The article is also publicly available at arXiv.org.

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B. Brugger et al. 2016. Possible Internal Structures and Compositions of Proxima Centauri b. ApJL, accepted fore publication; arXiv: 1609.09757

This article is based on a press-release issued by NASA.