Earth-like Planet Around our Nearest Star

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. Image credit: ESO/M. Kornmesser

Humans have been deploying many approaches in the search of Earth-like exoplanets and alien life. Earlier in 2016, a team of researchers has identified a planetary candidate orbiting the habitable zone of our closest neighbour, Proxima Centauri (Gl 551). Also, astronomers not only showed that the nearest star to the Sun could host a planetary system but also that, given the right conditions, it could host a habitable rocky planet. These all totally surprised the scientific community.

A recent discovery by an international team of astronomers including researchers from the University of Geneva observed and revealed that the planet, called Proxima b, is located in the habitable zone of its star and it may a prominent exoplanets candidate on the long going search of habitable exoplanets. This observation shooked the planetary community.

Astrophysicists consider the ability of Proxima Centauri b (Proxima b) to retain water from its formation as the most crucial point in evaluating the planet's present habitability. Proxima b lies only 4.2 light-years from the Sun.

The research finding was published in the journal Astronomy & Astrophysics, on 11 May 2020.

For this new discovery, similar Earth exoplanets, the team basically used the data from Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO).

The star Proxima b was first detected four years ago by using High Accuracy Radial velocity Planet Searcher (HARPS). So this ESPRESSO measurement is just a revisit of the Proxima b by a more advanced tool. Earlier measurement showed a low disturbance in the star’s speed that suggests the presence of a companion. Similar to HARPS but a higher accuracy instrument - ESPRESSO spectrograph, astronomers performed radial velocity measurements on the star Proxima Centauri. ESPRESSO's accuracy is about three times more precise than that obtained with the previous generation HARPS instrument.

Francesco Pepe, a professor in the Astronomy Department in UNIGE’s Faculty of Science and the man in charge of ESPRESSO said, “We were already very happy with the performance of HARPS, which has been responsible for discovering hundreds of exoplanets over the last 17 years. We’re really pleased that ESPRESSO can produce even better measurements, and it’s gratifying and just reward for the teamwork lasting nearly 10 years.”

ESPRESSO is the first ultra-stable high-resolution spectrograph instrument of a new ESO/VLT facility that is designed with the main scientific aims of detecting and characterising Earth twins in the habitable zone of solar-like stars and measuring the potential variation of the constants of the Universe. This new ESPRESSO instrument is installed at the incoherent combined Coudé focus (ICCF) of the VLT.

ESPRESSO mainly consists of three components, the UT Coudé Trains, the Front Ends, and the Spectrograph itself.

Under this new mechanism, light from an astronomical source is redirected from telescopes to the detectors through the UT Coudé Trains (CT), the front end units located in the Combined Coude Lab (CCL), and the spectrograph itself. Each component is designed to have its own functions. The CTs bring the light from each telescope to the CCL and CTs use 13 optical elements like mirrors, lenses, and prisms to do this. The lights that are redirected by CTs are received by four front ends (one for each UT) and feed to the spectrograph entrance fibres. Then in the final step, the fibre link transports the light from the front ends to the vacuum vessel. Once the light gets inside the spectrograph, they are dispersed by an echelle grating and the orders split up into a red and a blue spectrum. Then the dispersed and splited light are recorded on the corresponding science detectors.

The ESPRESSO instrument can be operated in three observing modes: High-Resolution 1-UT (HR), Ultra High-Resolution 1-UT (UHR), and Medium Resolution 4-UT (MR).

Image showing the general mechanism of the Coudé Trains, the Front Ends, and the Spectrograph. Image credit: ESO

The team analysed 63 spectroscopic ESPRESSO observations of Proxima (Gl 551) taken during 2019. 

The radial velocity (RV) measurements were obtained by using a typical radial velocity photon noise of 26 cm·s−1. They combined these data with archival spectroscopic observations and newly obtained photometric measurements to model the stellar activity signals and disentangle them from planetary signals in the radial velocity (RV) data.

Mass of Proxima is just 0.122 Solar mass and a radius of 0.15 Solar radius. It is about 1000 times less luminous than the Sun and being at its close distance it is invisible to the naked eye. The habitable zone of Proxima ranges from distances of 0.05 AU to 0.1 AU. Distinguished characteristics like distance from the Sun, brightness, its frequent and intense flares, and its slow rotation make Proxima a challenging but interesting target for its investigation of Earth like planets and astronomers stuck at its star Proxima b.

The team confirmed the presence of Proxima b independently in the ESPRESSO data and in the combined form from ESPRESSO, HARPS and Ultraviolet and Visual Echelle Spectrograph (UVES) dataset.

The measurement performed by ESPRESSO found that Proxima b has an orbiting period around its star between 11.189 and 11.247 days and a mass between 1.16 and 1.42 Earth-mass [Earth mass = 5.972 × 1024 kg]. Finally, in the combined dataset the measurement showed an orbiting period of 11.18427 ± 0.00070 days and mass of 1.173 ± 0.086 Earth mas.

Michel Mayor, winner of the Nobel Prize for Physics in 2019 and honorary professor in the Faculty of Science and the ‘architect’ of all ESPRESSO-type instruments said, “ESPRESSO has made it possible to measure the mass of the planet with a precision of over one-tenth of the mass of Earth and it’s completely unheard of.”

Proxima showed stellar-induced RV variations larger than the amplitude assigned to Proxima b and modelling these activity induced RV variations is key for the proper extraction of the planetary parameters. To model these activity induced RV variations, astronomers used the Gaussian processes framework which is considered one of the most successful methods.

Astronomers analysed the data and speculate that Proxima b might harbour life. Though Proxima b is about 20 times closer to its star than the Earth is to the Sun, it receives comparable energy and so that its surface temperature could mean that water (if there is any) is in liquid form in places. Liquid water on the surface of Proxima b is not a proven thing till now but we can hope astronomers will certainly reach a firm answer.

The team also evidenced a second short period signal with a period of 5.15 days but to confirm the presence of the signal and establish its origin, further ESPRESSO observations will be needed.

About this companion signal, Pepe said, "If the signal was planetary in origin, this potential other planet accompanying Proxima b would have a mass less than one third of the mass of the Earth. It would then be the smallest planet ever measured using the radial velocity method."

So, although a preliminary analysis showed Proxima b an ideal candidate for biomarker research, there is still a long way to go before we may be determined that life has been able to develop on its surface.

Christophe Lovis who is a researcher in UNIGE’s Astronomy Department and responsible for ESPRESSO’s scientific performance and data processing explained, “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.”