When the discovery of the planet Proxima b was announced back in August, it caused quite a flutter among astronomers looking for alien life.

The planet, orbiting a star just 4.2 light-years from Earth, seemed perfect — it was at the right distance from its parent star, it was the right size, and it was rocky. What else could one want from an exoplanet?

Water.

In a new study accepted for publication in the Astrophysical Journal Letters, a team of researchers state that Proxima b is most likely an “ocean planet” similar to Earth.

“[The planet] could have ocean covering its entire surface, similar to some icy moons of Jupiter or Saturn,” researchers from the French National Center for Scientific Research (CNRS) said in a statement.

Proxima b orbits Proxima Centauri — a red dwarf that is the nearest known star to the sun. Its 11-day orbit around its parent star is much smaller than Mercury’s orbit around the sun, but, since Proxima Centauri is only about 12 percent as massive as the Sun, and way dimmer, the planet lies well within the “Goldilocks zone” — a region where the temperature is just right for liquid water to exist.

“Contrary to what one might expect, such proximity does not necessarily mean that Proxima b's surface is too hot,” the researchers said.

Another key factor in determining an exoplanet’s habitability is its size. This is usually determined by observing the planet during its transit — an event wherein it passes in front of its host star, as seen from Earth. However, the relative positions of Proxima b and Earth make the likelihood of observing such a transit extremely low — just 1.5 percent, by some estimates.

So, the researchers had to resort to the next best thing — computer simulations.

“This is the method used by the team of researchers from the Marseille Astrophysics Laboratory (CNRS/Aix-Marseille University) and the Department of Astronomy at Cornell University, " the researchers said. "With the help of a model of the internal structure, they explored different compositions that Proxima b could present and deduced the corresponding values of the radius of the planet. They restricted their study to the case of potentially habitable, dense and solid planets, formed of a metallic core and a rocky mantle.”

Using this technique, they calculated that the planet’s radius is between 0.94 and 1.4 times that of Earth. If it’s near the lower end of the estimate, the planet would be very dense and would contain a metallic core accounting for two-thirds of its mass. In this case, surface water, if present, would account for roughly 0.05 percent of the planet’s total mass — comparable to Earth, where it accounts for 0.02 percent of the mass. 

On the other hand, if the upper end of the estimate is correct, Proxima b's mass would be split evenly between a rocky centre and surrounding water, and it would be covered by a single, gigantic ocean over 120 miles deep. 

“In both cases, a thin, gassy atmosphere could surround the planet, like on Earth, rendering Proxima b potentially habitable,” the researchers concluded.