The existence of an Earth-sized planet around the nearest star in the solar system, Proxima Centauri, has been confirmed by an international team of scientists including researchers from the University of Geneva. The results, published in the journal Astronomy & Astrophysics, reveal that the planet in question, Proxima b, has a mass of 1.17 Earth masses and is located in the habitable zone of its star, which it circles in 11.2 days.
This breakthrough was made possible by unprecedentedly accurate radial velocity measurements using ESPRESSO, a Swiss-made spectrograph - the most accurate currently in use - installed on the Very Large Telescope in Chile. Proxima b was first detected four years ago using an older spectrograph, HARPS - also developed by the Geneva team - which measured a small perturbation in the star’s velocity, suggesting the presence of a companion.
The ESPRESSO spectrograph made radial velocity measurements on the star Proxima Centauri, which is only 4.2 light-years from the Sun, with an accuracy of 30 centimetres per second (cm/s), about three times more precise than that obtained with HARPS, the same type of instrument but of the previous generation. "We were already very satisfied with the performance of HARPS, which has enabled us to discover hundreds of exoplanets over the past 17 years," says Francesco Pepe, from the astronomy department of the University of Geneva and head of ESPRESSO. "We are really happy that ESPRESSO can produce even better measurements, and this is gratifying and just a reward for the teamwork that has lasted almost 10 years". Alejandro Suarez Mascareño, the main author of the article, adds: "Confirming the existence of Proxima b has been an important task, and it is one of the most interesting planets known in the solar neighbourhood". Measurements made by ESPRESSO have clarified that Proxima b has a minimum mass of 1.17 Earth masses (the previous estimate was 1.3) and rotates around its star in only 11.2 days.
ESPRESSO has made it possible to measure the mass of the planet with an accuracy of more than one tenth of the mass of the Earth," says Michel Mayor, winner of the Nobel Prize in Physics in 2019, honorary professor at the Faculty of Science and "architect" of all ESPRESSO-type instruments. "It’s totally new.
And what about life on this planet?
Although Proxima b is about 20 times closer to its star than the Earth is to the Sun, it receives comparable energy, so its surface temperature could mean that water (if any) is in liquid form in places and could therefore harbour life.
That said, although Proxima b is an ideal candidate for biomarker research, we still have a long way to go before we can suggest that life has been able to grow on its surface. In fact, the star Proxima is an active red dwarf that bombards its planet with X-rays, receiving about 400 times more than the Earth.
"Is there an atmosphere that protects the planet from these deadly rays," asks Christophe Lovis, a researcher in the astronomy department of the University of Geneva and responsible for scientific performance and data processing at ESPRESSO. "And if such an atmosphere exists, does it contain the chemical elements that allow the development of life (oxygen, for example)-how long have these favourable conditions existed? We are going to address all these questions, in particular with the help of future instruments such as the RISTRETTO spectrometer, which we are going to build specifically to detect the light emitted by Proxima b, and HIRES, which will be installed on the future giant 39-metre ELT telescope that the European Southern Observatory (ESO) is building in Chile".
Is there a second planet?
In the meantime, the accuracy of ESPRESSO’s measurements could give rise to another surprise. The team found evidence of a second signal in the data, but could not establish the definitive cause. "If the signal was planetary in origin, this other potential planet accompanying Proxima b would have a mass less than a third of the mass of Earth. It would then be the smallest planet ever measured by the radial velocity method," adds Professor Pepe.
It should be noted that ESPRESSO, which became operational in 2017, is still in its infancy and these initial results already open up prospects. The road has been travelled at a frantic pace since the discovery of the first extrasolar planet by Michel Mayor and Didier Queloz. In 1995, the gas giant planet 51Peg b was detected using the ELODIE spectrograph with an accuracy of 10 metres per second (m