Free Astronomy Magazine July-August 2020

10 JULY-AUGUST 2020 EXOPLANETS in Astronomy & Astrophysics in April of this year. A crowded team of researchers led by Raffaele Gratton (National Institute for As- trophysics − INAF, Padua Observatory, Italy) attempted this feat. SPHERE is equipped with a coronograph and a sophisticated adaptive optics system, also designed to characterize extrasolar planets at visible and near-infrared wave- lengths. Starting in 2014, this instrument has revealed the existence of several proto- planetary disks around distant stars and, for a period of four years, it was the fulcrum of the SHINE survey, during which it pho- tographed about 600 nearby stars. One of these stars was Proxima Centauri, around which the existence of Proxima c was not yet suspected. Gratton’s team searched for traces of this potential planet in the SHINE survey images, trusting that if it had emitted a fairly in- tense infrared signal, SPHERE might have detected it. It had been calculated that the maximum projected separation between planet and star, or the an- gular distance to the or- bital quadratures, could ex- ceed 1 arcsecond, an en- couraging value consider- ing the resolving power of the photographic instru- ment. In order to account for the expected large orbital mo- tion of the planet, the re- searchers used a method that assumes a circular orbit obtained from radial velocities and exploits the sequence of observations acquired close to quadra- ture in the orbit. Unfortu- nately, the images acquired by SPHERE did not detect any clear trace of Proxima c. The best candidate was a small and faint patch of light with a signal-to-noise ratio of 6-to-1. Statistically, the probability that this de- tection is due to a random fluctuation is less than 1%, a value that would be encouraging if the noise distribution in the images was uni- form, which is not the case. However, Grat- ton and colleagues point out that the position and orientation of the candidate planet’s possible orbital plane is well-suited to the arrangement of an external belt of residual material imaged by ALMA. On the other hand, the position and the orbital mo- tion of the detected signal are not consis- tent with what was observed by the Gaia R affaele Grat- ton, the team leader who at- tempted to con- firm the existence of Proxima c through the im- ages acquired by the SHINE sur- vey. Below, the SPHERE spec- tropolarimeter, with which the SHINE survey was conducted. Here we see it installed on the UT3 unit of ESO’s Very Large Tele- scope. [SPHERE/ ONERA/ JF Sauvage]