For the search for exoplanets in the solar neighbourhood, there is a prime target: Alpha Centauri A and B, two stars similar to our Sun in size and age, orbiting each other. However, indirect detection methods are not sensitive enough yet to detect rocky planets in the "habitable" zone of these stars, where water could be present in liquid form.
A consortium of European astrophysicists has therefore set out to optimize direct imaging of these exoplanets. To do this, they coupled the VISIR infrared instrument, built under the supervision of the CEA, to one of the telescopes of the VLT (Very Large Telescope) of the European Southern Observatory (ESO), which was recently equipped with an adaptive optics system to correct the effects of atmospheric turbulence.
To detect an exoplanet nearly a million times fainter than its star, at an angular distance of only one arcsecond, they developed a coronagraph that alternately masks Alpha Centauri A and Alpha Centauri B. This allowed them to partly suppress the starlight and reduce all forms of noise from the sky, the camera and the telescope, and, by accumulating observations, to gain sensitivity.
The team observed Alpha Centauri A and B for nearly 100 hours over the course of a month in 2019, collecting over 5 million images. Processing of the raw data revealed a point source reminiscent of a Neptune- or Saturn-sized planet, located at a distance from Alpha Centauri A near the Earth-Sun distance. However, an instrumental artifact of unknown origin cannot be excluded until its presence is confirmed by other observations.
Irfu scientists contributed to the design of the instrument, notably by drawing on their long experience with the VISIR camera. They provided the software to correct systematic errors and select the best images from the 10 terabytes (1013 bytes) of data collected during the campaign.
The next generation of very large telescopes (Extremely Large Telescope, E-ELT) will be able to observe ten times more nearby stars, likely to have "habitable" planets, than the VLT. In this perspective, Irfu participates in the design and construction of the METIS (Mid-infrared ELT Imager and Spectrograph) instrument which will equip the E-ELT telescope of ESO.
This work takes place within the frame of the « Near Earths in the AlphaCen Region (NEAR) » initiative, supported by the gobal Astronomy Program on Research for Earth-like planets Around Nearby Stars called the Breakthrough Watch, and ESO.