What’s Inside Proxima Centauri B, Our Nearest Exoplanet Neighbor

The closest potentially habitable planet has an intriguing interior
On the left, what Proxima b might look like if its composition is like Mercury's; on the right, what it might look like if it was more like Titan. Earth is provided in the middle for comparison. NASA/B. Brugger, et al.

What might the closest exoplanet to Earth look like? Anything from a cannonball-like world to an ocean-covered planet made up half of water, researchers say.

In August, astronomers revealed that the nearest star system to the sun, Alpha Centauri, possesses a world roughly 1.3 times Earth’s mass. Alpha Centauri consists of three stars — Alpha Centauri A, Alpha Centauri B, and a small red dwarf named Proxima Centauri — and this newfound planet appears to call Proxima Centauri home.

The exoplanet, named Proxima b, orbits Proxima Centauri at a distance one-tenth that between Mercury and the sun. However, because the red dwarf is more than 600 times dimmer than the sun, Proxima b may actually lie within its star’s habitable zone, the area around the star warm enough for the planet to possess liquid water on its surface — and thus, perhaps, life as we know it.

However, one of the many details that remains unknown about Proxima b is its diameter. This means that astronomers cannot calculate its density, which in turn makes it impossible to say for sure what the exoplanet is made of.

Still, scientists can dream. Based on Proxima b’s mass, astrophysicist Bastien Brugger at the Marseille Astrophysics Laboratory in France and his colleagues modeled what its size and structure might be like given a range of potential ingredients. “Even if we don’t know the planet’s radius, we can still try and get an idea of what the planet could look like,” Brugger says.

If Proxima b is a cannonball-like world like Mercury, it would be about 6 percent smaller than Earth. This dense, completely dry version of the planet would consist of a metal core making up about two-thirds of the planet’s mass sheathed by a rocky shell.

On the other hand, if Proxima b is made up half of water ice like Saturn’s largest moon Titan might be, it would be roughly 40 percent bigger than Earth. In this scenario, the planet would be covered completely in water about 120 miles deep. Under this global ocean, the pressure would be so strong that water would turn into a layer of high-pressure ice, which would sit on top of a rocky mantle and a metal core.

These findings suppose that the abundances of elements in Alpha Centauri are similar to those in the solar system, and there is no guarantee that is the case, says planetary astronomer Franck Marchis, chair of the exoplanet group at the SETI Institute in Mountain View, California, who did not take part in this research. Brugger agrees, noting that future observations of Proxima Centauri’s elemental abundances could improve their models and help pin down which version of Proxima b is more likely.

And who knows, “with the Breakthrough Starshot project, maybe in 20 to 40 years, we’ll have pictures of Proxima b, telling us exactly what it looks like,” Brugger says.

The scientists detailed their findings Oct. 17 at the meeting of the American Astronomical Society Division for Planetary Sciences and the European Planetary Science Congress in Pasadena, California.