We’ve never gotten a clear look at an exoplanet, in part because they’re all so far away. That’s not the case with Proxima b, a small exoplanet that orbits the nearest star to our own, Proxima Centauri. It’s a mere 24 trillion miles away! Hey, four light years is right next door in astronomical terms. Scientists haven’t been able to spot Proxima b transiting its star yet, but a new study of the Proxima Centauri system shows how Proxima b could be a habitable ocean planet.
The new study will be published in The Astrophysical Journal Letters, and was headed by researchers at the Marseille Astrophysics Laboratory. We already know that Proxima b is in the habitable zone of its parent star, so water on the surface would at least have a chance at being liquid. The team was able to narrow down Proxima b’s dimensions and simulate its composition — these figures lay out a vision for Proxima b that include a planet-wide ocean and Earth-like atmosphere.
Scientists don’t know how large Proxima b is, but we have a good handle on its mass. It’s about 1.3 times the mass of Earth. Using models of planetary structure, the Marseille team worked out what sort of compositions we could expect from Proxima b at both ends of that scale. However, this model is based on what we know of planets in our own solar system. We have no choice but to assume there are some commonalities among planets across the universe until we can get a better look at exoplanets.
Based on its mass, Proxima b likely has a radius between 3,722 and 5,543 miles. At the smaller end of the scale, Proxima b would have to be extremely dense (it would be slightly smaller than Earth, but 30% more massive). That would mean its internal structure is mostly metallic like Mercury. This doesn’t rule out water on the surface, but there would not be much of it.
The far more interesting option is that Proxima b is toward the larger end of the radius scale. If it were significantly larger than Earth, its mass would be a 50/50 split between rock and water. That would mean a single planet-wide ocean about 124 miles deep. Toward the bottom of this sea, the incredible pressure would compress water into super-dense ice. This model also supports a thin, Earth-like atmosphere.
There’s a lot of speculation about Proxima b right now, but that’s all we have. It’s possible that Proxima b has a structure entirely unlike the planets in our solar system. The only way to know for sure it to observe it directly. Our best hope of that is the James Webb Telescope, which is scheduled to launch in about a year.