Episode for September 6th, 2023 Download

Astronomers have recently concluded an observation study of the closest exoplanet to Earth, Proxima Centauri b and found that the planet does not transit its star. These observations settle a question that astronomers have been asking since the exoplanet’s discovery in 2016 using the radial velocity method.

Proxima Centauri b was discovered in August 2016 at the European Southern Observatory using a technique that watches the wobble of a star as the gravity from a planet in orbit around it, pulls the star to and fro.

By watching the star in this way, astronomers can tell several things: how many planets are there (in this case there’s just one), how massive they are (proxima b has roughly the same mass as the Earth), and how fast they go around the star (proxima b takes 11 days to go around the star once).

The thing is, astronomers also want a transit; they have never seen the planet pass between us and the star, leading astronomers to take a longer look at our nearest exoplanet neighbor. Why is observing a transit such a big deal? When a planet passes in front of the star the light from that star can give us clues to the physical size of the planet, but more importantly, the James Webb Space Telescope can measure that light and tell us if there is an atmosphere present on the planet.

This would be great news because not only is Proxima b our closest neighbor at a little over four light years, but it lies in the habitable zone of Proxima Centauri. If proxima b had an atmosphere, and if it had water, then that water could be liquid and may sustain life as we understand it.

So measuring a transit of Proxima Centauri b would give us very important data about the habitability of the planet.

The results of this recent study were disappointing however. Looking at the system with data from the Spitzer Space telescope and instruments in operation on the ground at the European Southern Observatory, astronomers confirmed that the exoplanet, proxima centauri b does not transit the star, not even partially.

Surprisingly little has been learned about proxima b since its discovery. We don’t know how old it is, whether it has an atmosphere, whether it is tidally locked, nor it’s precise size. But because it is so close, it provides one of the best opportunities for learning about Earth-sized worlds around M-class dwarf stars. This is important because there are a lot of systems like this and having one in our celestial backyard is an ideal laboratory.

Astronomers estimate that planets less than ten times the mass of Earth occur around M-class stars at a rate of about one per star. There is at least one rocky planet around every M dwarf, and there are more M-class stars than any other star out there. The more we understand the Proxima Centauri system, the more we understand most of the exoplanets in our galaxy.

Although a transit event would have been ideal, we can still learn a lot about Proxima b from other sources of information. For example, the star Proxima Centauri is a flaring M-class dwarf, and it flares, a lot, over 60 times a day. This, coupled with the assumption that it is tidally locked to the star, and that it orbits very close, the chance of keeping an atmosphere is low and that would make habitability seem rather a rather remote possibility.

We need to keep looking, observing, measuring and analyzing the closest exoplanet system to the Earth. We have in our celestial backyard a pristine laboratory for understanding the question of habitability on planets around other stars. Most of the planets in our galaxy orbit M-class stars like this, and we are sitting right next door to one.