A candidate super-Earth planet orbiting near the snow line of Barnard's star
Rebolo, R.; Amado, P. J.; Caballero, J. A.; Pallé, E.; Reiners, A.; Ribas, I.; Quirrenbach, A.; Anglada-Escudé, G.; Béjar, V. J. S.; Kaminski, A.; Kürster, M.; Montes, D.; Morales, J. C.; Tal-Or, L.; Zechmeister, M.; Schweitzer, A.; Cortés-Contreras, M.; Azzaro, M.; Dreizler, S.; Jeffers, S. V.; Lafarga, M.; Burt, J.; Butler, R. P.; Rodríguez, E.; Henning, Th.; González Hernández, J. I.; Suárez Mascareño, A.; Rodríguez-López, C.; Toledo-Padrón, B.; Seifert, W.; Barnes, J. R.; Jones, H. R. A.; Jenkins, J.; Street, R. A.; Tsapras, Y.; Guinan, E. F.; Murgas, F.; Engle, S. G.; Herrero, E.; Reffert, S.; Holden, B.; Perger, M.; Rosich, A.; Tuomi, M.; Trifonov, T.; Hatzes, A.; Ofir, A.; Morin, J.; Haswell, C. A.; Staab, D.; Berdiñas, Z. M.; López-González, M. J.; Wang, S. X.; Teske, J.; Feng, F.; Strachan, J. B. P.; Lee, M. H.; Crane, J.; Shectman, S. A.; Nelson, R. P.; Vogt, S. S.; Kiraga, M.; Coleman, G.
Spain, United Kingdom, Germany, United States, Israel, Chile, Switzerland, Poland, Hong Kong SAR, France
Abstract
Barnard's star is a red dwarf, and has the largest proper motion (apparent motion across the sky) of all known stars. At a distance of 1.8 parsecs1, it is the closest single star to the Sun; only the three stars in the α Centauri system are closer. Barnard's star is also among the least magnetically active red dwarfs known2,3 and has an estimated age older than the Solar System. Its properties make it a prime target for planetary searches; various techniques with different sensitivity limits have been used previously, including radial-velocity imaging4-6, astrometry7,8 and direct imaging9, but all ultimately led to negative or null results. Here we combine numerous measurements from high-precision radial-velocity instruments, revealing the presence of a low-amplitude periodic signal with a period of 233 days. Independent photometric and spectroscopic monitoring, as well as an analysis of instrumental systematic effects, suggest that this signal is best explained as arising from a planetary companion. The candidate planet around Barnard's star is a cold super-Earth, with a minimum mass of 3.2 times that of Earth, orbiting near its snow line (the minimum distance from the star at which volatile compounds could condense). The combination of all radial-velocity datasets spanning 20 years of measurements additionally reveals a long-term modulation that could arise from a stellar magnetic-activity cycle or from a more distant planetary object. Because of its proximity to the Sun, the candidate planet has a maximum angular separation of 220 milliarcseconds from Barnard's star, making it an excellent target for direct imaging and astrometric observations in the future.