Discoveries Reveal Another Planet Orbiting Our Sun's Nearest Neighbour

Thousands of planets orbiting faraway stars have now been cataloged. For the most part, we only have basic information about them, such as their size, mass, and orbital distance from their host star. And, because of the difficult-to-comprehend distances within our galaxy, this is likely to be the total of our knowledge about them for generations.

However, there is a potential to learn a lot more about the few planets that are closest to Earth. Over the next few decades, plans are already in the works to investigate the atmospheres of planets within 30 light years of Earth, and advances in existing technologies could disclose considerably more. So it was thrilling news when an Earth-sized planet was discovered circling Proxima Centauri, the nearest star to the Sun. 

Scientists have now proven that this planet isn't the only one around Proxima Centauri; at least one other planet orbits the star. It also turns out to be very light, weighing only roughly twice as much as Mars.

Previous Discoveries

The radial velocity approach was used to discover Proxima Centauri b, an Earth-sized planet within its star's habitable zone, in the first place. This strategy is based on the notion that if a planet's orbit is aligned correctly, traversing it can bring the planet closer or farther away from Earth. Because gravitational attraction is reciprocal, it will also pull the star it orbits closer or farther away from Earth.

The light we receive from the star can be used to deduce the star's motion. Its light will shift toward the bluer end of the spectrum as it approaches Earth; as it goes away from Earth, the light will shift toward the redder end of the spectrum. If there is a regular pattern to these shifts, it might be a sign of a planet's orbit, with the size of the shift indicating the planet's relative mass to the star's mass. However, because many planets are part of multi-planet systems, understanding the shifting red and blue shifts can be difficult.

There was a clear indication of planet b in the instance of Proxima Centauri, which takes around 11 days to complete an orbit. Because of its brief orbit, it is relatively close to the star. However, because Proxima Centauri is a faint red dwarf star, its habitable zone is also relatively near, with Proxima Centauri b firmly within it.

Beyond that, there's a strong suggestion of a planet considerably further out (Proxima Centauri c) with a five-year orbit. However, we haven't been observing Proxima Centauri in the appropriate way for long enough to gather data on more than one orbit. As a result, this is still a viable option.

Finally, additional observations have shown that a Proxima Centauri d, circling close to its host star, may exist. The signal, however, had never attained statistical significance. That is, until now.

The More The Merrier

An international team has been utilizing the Very Large Telescope at the European Southern Observatory to continue characterizing Proxima Centauri since that follow-up observation, with a focus on following up on the potential signal of a five-day orbit. The signal persisted, and the researchers employed a number of statistical techniques to prove that it was most likely real.

Those of the system with two planets instead of one (and neglecting the slow-changing signal of the distant Proxima c) fit the data better than models with only the confirmed Proxima Centauri b. In another experiment, they demonstrated that as more data was acquired, the evidence for the signal expanded, which is exactly what you'd anticipate if it were real.

It's impossible to say how eccentric the orbit is based on the available data; it could be substantially circular, but more ellipsoidal choices are also plausible. The planet completes an orbit in just 5.12 days, indicating that it is closer to the host star and hot enough to be outside the habitable zone.

Because the same signal may be created by a small planet circling in a plane that is near to "flat" from Earth's perspective, or a massive planet orbiting in a steeply tilted plane, mass estimates based on radial velocity only yield the smallest potential masses. We could determine the orbital tilt and thus the mass if we had evidence of transits, which occur when planets pass between the star and Earth. However, there is no evidence of any transits here.

Within that constraint, Proxima Centauri d looks to be an extremely light planet, with only approximately a quarter of Earth's mass. To put that in perspective, it's nearly double the mass of Mars. It could also be the lightest planet ever discovered by radial velocity.

As a result, we now know that our nearest stellar neighbor has a thriving planet population. It's unclear how long it will take us to put that information to use. We can't look for atmospheres without a transit, and the two closest planets are too close to the star for direct imaging without a big technological leap. It's possible that we'll have to wait till something like Breakthrough Starshot succeeds in getting hardware there.

The better news is what this discovery reveals about the state of the latest generation of planet-hunting technology. The light output of most stars varies to some degree, which might skew radial velocity readings. The variability of Proxima Centauri is thought to resemble a radial velocity signal comparable to a meter per second. Despite this, the researchers were able to retrieve the signal of a planet that was so small that it barely moved a few tens of millimeters every second.