A low-mass planet candidate orbiting Proxima Centauri at a distance of 1.5 AU
Our nearest neighbor, Proxima Centauri, hosts a temperate terrestrial planet. We detected in radial velocities evidence of a possible second planet with minimum mass mc sin ic = 5.8 ± 1.9M⊕ and orbital period P c = 5.21 −0.22 +0.26 years. The analysis of photometric data and spectro-scopic activity diagnostics does not explain the signal in terms of a stellar activity cycle, but follow-up is required in the coming years for confirming its planetary origin. We show that the existence of the planet can be ascertained, and its true mass can be determined with high accuracy, by combining Gaia astrometry and radial velocities. Proxima c could become a prime target for follow-up and characterization with next-generation direct imaging instrumentation due to the large maximum angular separation of ~1 arc second from the parent star. The candidate planet represents a challenge for the models of super-Earth formation and evolution. ; Progetto Premiale 2015 FRONTIERA funding scheme of the Italian Ministry of Education, University, and Research Italian Space Agency INAF Italian Space Agency Science & Technology Facilities Council (STFC) Comisión Nacional de Investigación Científica y Tecnológica (CONICYT) CONICYT FONDECYT CONICYT project Basal Spanish MINECO State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award Science & Technology Facilities Council (STFC) European Union (EU)