Our Sun’s nearest neighbor has another planet

We've now cataloged thousands of planets that orbit distant stars. For most of them, our knowledge is limited to basic statistics: their size, mass, and orbital distance from their host star. And, due to the difficult-to-comprehend distances within our galaxy, it's likely that this will remain the sum of our knowledge about them for generations.

For the small number of planets closer to Earth, however, there's the chance to learn much more. Plans are already underway to study the atmospheres of planets within about 30 light years of Earth over the next few decades, and improvements in existing technologies have the potential to reveal even more. So the discovery of an Earth-sized planet orbiting Proxima Centauri, the closest star to the Sun, was exciting news. We now have the potential to learn much more about this rare planet.

And now, scientists have confirmed that this planet isn't alone; at least one more planet orbits Proxima Centauri. And it turns out to be an unusually light one, with only about double the mass of Mars.

Previous hints

The original discovery of Proxima Centauri b, an Earth-sized planet within its star's habitable zone, was made using of what's known as the radial velocity method. This method relies on the fact that, if a planet's orbit is oriented in the right way, traversing the orbit can take the planet nearer or closer to Earth. And, because gravitational attraction is mutual, it will tug the star it orbits closer to or farther from Earth, as well.

The motion of the star can be read from the light we receive from it. As it moves toward Earth, its light will be shifted toward the bluer end of the spectrum; when it's pulled away from Earth, the light becomes a bit redder. If these shift show a regular pattern, that can be an indication of a planet's orbit, with the magnitude of the shift telling us the planet's relative mass compared to the mass of the star. Since many planets are in multi-planet systems, however, interpreting the changing red and blue shifts can be challenging.

In the case of Proxima Centauri, there was a clear signal of planet b, which takes about 11 days to complete an orbit. That short orbit means it's very close to the star. Since Proxima Centauri is a dim red dwarf star, however, its habitable zone is quite close as well, with Proxima Centauri b being solidly within it.

Beyond that, there's strong indication of a planet (Proxima Centauri c) much farther out, with an orbit of roughly five years. We haven't, however, been doing the right sort of observations of Proxima Centauri for long enough to have data on more than a single orbit. As a result, this is still considered a candidate.

Finally, follow-up observations have hinted at the possibility of a Proxima Centauri d, orbiting close to its host star. But the signal had never reached the point of statistical significance. Until now, that is.

More is better

Since that follow-up observation, an international team has been using the Very Large Telescope at the European Southern Observatory to continue characterizing Proxima Centauri, prioritizing following up the potential signal of a five-day orbit. The signal remained, and the team used a variety of statistical methods to show that it was likely real.

For example, models of the system using two planets, rather than one (and ignoring the slow-changing signal of the distant Proxima c) provided a better fit to the data than a model with only the confirmed Proxima Centauri b. In another test, they showed that the evidence for the signal grew as additional data was gathered, exactly what you'd expect if it were real.

Given the current data, it's impossible to tell how eccentric the orbit is; it may be largely circular, but more ellipsoidal options are also possible. The planet takes just 5.12 days to complete an orbit, which means it's closer to the host star and hot enough to be outside the habitable zone.

Mass estimates from radial velocity only provide minimal possible masses, since the same signal could be generated by a small planet orbiting with a plane that's close to "flat" from Earth's perspective, or a large planet orbiting in a steeply tilted plane. If we had evidence of transits, where the planets pass between the star and Earth, we could know the orbital tilt, and therefore the mass. But there's no sign of any transits here.

Within that limitation, it appears that Proxima Centauri d may be a very light planet, with only about a quarter of the mass of Earth. For context, that would make it roughly twice Mars' mass. It's also potentially the lightest planet ever detected using radial velocity.

So, we now know that our nearest stellar neighbor is well populated with planets. It's not clear how long it will take us to take advantage of that knowledge. Without a transit, we can't look for atmospheres, and the two closest planets are too close to the star to image directly without a major technological breakthrough. We may need to wait until something like Breakthrough Starshot manages to get hardware there.

The better news is what this finding tells us about the state of the new generation of planet-hunting hardware that has come online. Most stars have a degree of variability in their light output that can interfere with radial velocity measurements. It's estimated that Proxima Centauri's variability can often look like a radial velocity signal equivalent to a meter per second. Despite that, the team here was able to extract the signal of a planet that was so light that it only caused changes on the order of a few tens of centimeters a second.

Astronomy & Astrophysics, 2022. DOI: 10.1051/0004-6361/202142337  (About DOIs).