I’ve always liked the image of Proxima Centauri b that the ESO’s Martin Kornmesser has conjured directly below, and have used it in a couple of previous articles about the planet. Indeed, you’ll see it propagated widely when the topic comes up. But like all of these exoplanet artist impressions, it’s made up of educated guesses, as it has to be. We don’t even know, for example, whether the world we see here even has an atmosphere, as depicted.
Whether or not it does is important because it affects the possibilities for life around the star nearest to our own. Twenty times closer to its star than the Earth is to the Sun, Proxima b nonetheless receives roughly the same energy, meaning we could have surface temperatures there that would support liquid water on the surface. But the planet also receives 400 times more X-rays than the Earth, which leads the University of Geneva’s Christophe Lovis to ask:
“Is there an atmosphere that protects the planet from these deadly rays? And if this atmosphere exists, does it contain the chemical elements that promote the development of life (oxygen, for example)? How long have these favourable conditions existed? We’re going to tackle all these questions, especially with the help of future instruments like the RISTRETTO spectrometer, which we’re going to build specially to detect the light emitted by Proxima b, and HIRES, which will be installed on the future ELT 39 m giant telescope that the European Southern Observatory (ESO) is building in Chile.”
Image; This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. © ESO/M. Kornmesser.
RISTRETTO is exciting stuff, but I don’t want to get ahead of myself. Lovis (University of Geneva, or UNIGE) is responsible for data processing on ESPRESSO. The spectrograph that is currently the most accurate of any in our arsenal, ESPRESSO has indeed just confirmed Proxima Centauri b’s existence, the world having been first discovered by a team led by Guillem Anglada-Escudé some four years ago (and boy do I remember when that news came in).
Anglada-Escudé’s work was brilliant, as the ESPRESSO work confirms, for his team was working with the older HARPS spectrograph, a formidable instrument in its own right, but one with three times less precision than ESPRESSO. In fact, the ESPRESSO work that confirmed Proxima b, led by Francesco Pepe at UNIGE, currently works with an accuracy of 30 centimeters per second (cm/s), with the goal of eventually reaching 10 cm/s. By way of comparison, the ELODIE spectrograph that found 51 Peg b, the first main sequence exoplanet discovered, operated with an accuracy of 10 meters per second.
So the news is two-fold. First, we have a solid success from an instrument that is changing the game in terms of radial velocity detections, one that presages great things to come. Second, we have tightened up the numbers on Proxima b, which is now shown to have a minimum mass of 1.17 Earth masses, as opposed to the previous estimate of 1.3, in an orbit of 11.2 days. Both the 2016 discovery and the 2020 confirmation represent radial velocity work at the highest level.
Bubbling interestingly in the mix is the possibility of a second planet in the ESPRESSO results. We do have a signal in the data but the cause remains problematic. Radial velocity studies have to contend with photospheric and chromospheric phenomena on the surface of the star (associated with magnetic fields) that can look much like the signal of a planet. Here I turn to the paper on this work, which will appear in Astronomy & Astrophysics:
We find evidence for a second short period signal with a period of 5.15 days and a semi-amplitude of 0.4 m·s−1. If caused by a planetary companion, it would correspond to a minimum mass of 0.29 ± 0.08 M⊕ at an orbital distance of 0.02895 ± 0.00022 AU, with an equilibrium temperature of 330 ± 30 K. Further ESPRESSO observations will be needed to confirm the presence of the signal and establish its origin. We do not detect any additional companions up to 0.4 M⊕ at orbital distances shorter than the HZ of the star.
Take a look at that mass, less than one-third that of the Earth. If this is a planet, it’s the smallest planet ever measured with radial velocity methods. What an achievement if ESPRESSO can pull that one out of the noise!
The paper is Mascareño et al., “Revisiting Proxima with ESPRESSO,” in process at Astronomy & Astrophysics (preprint).