Artist impression of Proxima Centauri c. Press “HD” on the player for the best image quality (E. Tasker).

The discovery of Proxima Centauri b in 2016 caused a flood excitement. We had found an extrasolar planet around our nearest star, making this the closest possible world outside of our solar system!

But despite its proximity, discovering more about this planet is difficult. Proxima Centauri b was found via the radial velocity technique, which measures the star’s wobble due to the gravity of the orbiting planet. This technique gives a minimum mass, the average distance between the star and planet and the time for one orbit, but no details about conditions on the planet surface.

If the planet had transited its star, we might have tried detecting starlight that passed through the planet’s atmosphere. This technique is known as transit spectroscopy, and reveals the composition of a planet’s atmosphere by detecting what wavelengths of light are absorbed by the molecules in the planet’s air. But searches for a transit proved fruitless, suggesting the planet’s orbit did not pass in front of the star from our viewpoint.

The radial velocity technique measures the motion of the star due to the gravity of the planet. As the star moves away from the Earth, its light becomes stretched and redder. As it moves back towards Earth, the light shifts to bluer wavelengths. The technique gives the planet’s period, distance from the star and its minimum mass. (E. Tasker)

Another option for planet characterization is to capture a direct image of the planet. This is one of the most exciting observational techniques, as it reveals the planet itself, not its influence on the star. Temporal changes in the planet’s light could reveal surface features as the planet rotates, and if enough light is detected to analyze different wavelengths, then the atmospheric composition could be deduced.

But direct imaging requires that the planet’s light can be differentiated from the much brighter star. With our current instruments, Proxima Centauri b orbits too close to its star to be distinguished. This seemed to close the door on finding out more about our nearest neighbors, until the discovery of a second planet in the system was announced early this year.

Also identified via the radial velocity technique, Proxima Centauri c has a minimum mass of 5.8 Earth masses. It sits further out than its sibling, with a chilly orbit that takes 5.2… Read more