Proxima Centauri b, the Earth-like planet orbiting the red dwarf Proxima Centauri may have oceans, scientists say. The planet was discovered in August 2016 and caused excitement because it’s in the habitable zone of its star, and it’s rocky. And it’s in the Alpha Centauri, the closest star system to us!

Researches from France’s National Center for Scientific Research (CNRS) have calculated the size and surface properties of the planet and concluded it may be an “ocean planet” similar to Earth. There are possibilities, though, Proxima Centauri b may have continents and oceans like Earth, or its entire surface may be covered by a massive ocean.

The researchers used mathematical models to make these predictions. In fact, we could learn much more if we were able to observe the planet passing between us and its star, but it seems we won’t see it, despite it orbits its host star every 11.186 days. Because of the planet’s orbital inclination (see notes 1), the likelihood of transit had been calculated at only 1.5 percent. There are other problems like Proxima Centauri’s relative faintness and constant flares which made it difficult to see if the planet passed in front of the star.

The researchers calculated the radius of Proxima Centauri b was between 0.94 and 1.4 times that of Earth (see notes 2). If we take the minimum radius of 5,990 km (3,722 mi), the planet would be very dense, with a metallic core making up two-thirds of the entire planet’s mass, surrounded by a rocky mantle. In this scenario, if there is surface water, it would not contribute more than 0.05 percent to the planet’s total mass (similar to Earth, where it is about 0.02%).

In the larger planet scenario, with a radius of 8,920 km (5,542.6 mi), Proxima b’s mass would be split 50-50 between a rocky center and surrounding water. The researchers say “In this case, Proxima b would be covered by a single, liquid ocean 200 km (124.3 mi) deep”.

## Proxima Centauri b: the “New Earth”?

The second scenario is interesting. If you read Isaac Asimov‘s 1986 science fiction novel “Foundation and Earth”, it must be familiar. In the novel, the characters’ spaceship Far Star lands on a marine world Alpha, orbiting Alpha Centauri A: the final home of the last human refugees from some dying, radioactive Earth.

The survivors, encountered here by Foundation councilor Golan Trevize, dwell in seeming Polynesian simplicity on Alpha’s only landmass, a Jamaica-sized island called by them “New Earth”. It is, in fact, an artificial island, built by the material gathered from the ocean’s base, to make the planet habitable to humans. There were no islands before, the “Alpha” was covered by a massive ocean.

The main character, Golan Trevize says “a habitable planet cannot be orbiting a red dwarf”, though. So, Asimov puts this marine world into Alpha Centauri A’s orbit, not the Proxima Centauri. But the similarities still very interesting.

(Now we know that a red dwarf can have a habitable zone, so a habitable planet can orbit a red dwarf, at least theoretically)

## Notes

1. The orbital inclination is the acute (smaller) angle between a reference plane and the orbital plane or axis of direction of an object in orbit around another object. In the Solar System, the orbital inclination is usually stated with respect to Earth’s orbit.
2. Earth’s radius is 6,371 km (3,958.8 mi)

## Sources

• “Failed hunt for Proxima b’s star transit leaves us in the dark” on New Scientist