Located at 4.2 light-years, α Centauri is one of the closest stars to us. α Centauri is actually a triple star : its two most massive components, A and B, have masses and luminosity similar to those of the Sun, and the third (the C component) is none other than Proxima Centauri, the star closest to the Earth. For these reasons, it is considered an ideal target for searching for exoplanets that might be in its habitable zone. By "habitable zone", we mean the region around the star where water could be in a liquid state on the surface of a planet that is rocky and has properties similar to Earth.
The NEAR project
For components A and B of the α Centauri system, the radial velocity observation technique was not able to detect a planet. However, it did allow to establish upper limits to their masses : at most 50 times that of the Earth in the inhabitable zone of component A, and 8 times that of the Earth in the inhabitable zone of component B. The NEAR project (acronym for New Earths in the Alpha Centauri Region), as part of the "Breakthrough Watch" program, aimed to obtain deep observations of the environment of these two stars, in search of planets analogous to Earth.
Improvement of the VLT’s VISIR instrument
An international team of researchers has therefore set out to improve the VISIR instrument of the Very Large Telescope (VLT) by equipping it with a coronagraphic system to attenuate stellar light, and to image the very close environment of stars where potential planets, otherwise drowned in the brightness of stars, may be found. The instrument was then installed on the UT4 telescope of the VLT to benefit from its adaptive optics system that corrects atmospheric turbulence, an essential step to improve image quality and increase sensitivity to the weakest objects.
VISIR observes in the mid-infrared spectral range (10-12 microns, comparable to thermal cameras), where planets, colder than their stars, are brighter because they emit light in proportion to their surface temperature.
Two researchers from Paris Observatory - PSL at the Laboratory for Space Studies and Instrumentation in Astrophysics (LESIA) have contributed to this study, in particular concerning the development of the star pointing system on the coronograph, allowing to ensure that the stellar light is attenuated to the maximum during the observations.
Highlighting a source
Images were cumulated over a total of 100 hours of exposure between May and June 2019. Thanks to an advanced phase of data processing, consisting in subtracting the residues of stellar light, as well as various instrumental artifacts, the researchers highlighted a source (noted C1 in the image) which could correspond to a planet the size of Neptune to Saturn.
However, given the level of angular resolution reached, it is not possible for the moment to exclude that it is a cloud of dust in orbit around the star, residue of the formation of planetary systems, or even an instrumental artifact.
Further observations will be needed to confirm these very promising initial results. Indeed, they demonstrate that a larger diameter telescope, such as the future 39-meter Extremely Large European Telescope being built in Chile by ESO (European Southern Observatory) will be able to provide the resolution and sensitivity required to image an Earth-like planet around α Centauri.
Link to publication in Nature Communications