A candidate super-Earth planet orbiting near the snow line of Barnard’s star

Abstract

At a distance of 1.8 parsecs 1 , Barnard's star (Gl 699) is a red dwarf with the largest apparent motion of any known stellar object. It is the closest single star to the Sun, second only to the a Centauri triple stellar system. Barnard's star is also among the least magnetically active red dwarfs known 2,3 and has an estimated age older than our Solar System. Its properties have made it a prime target for planet searches employing techniques such as radial velocity 4,5,6 , astrometry 7,8 , and direct imaging 9 , all with different sensitivity limits but ultimately leading to disproved or null results. Here we report that the combination of numerous measurements from high-precision radial velocity instruments reveals the presence of a low-amplitude but significant periodic signal at 233 days. Independent photometric and spectroscopic monitoring, as well as the analysis of instrumental systematic effects, show 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 Earth masses orbiting near its snow-line. The combination of all radial velocity datasets spanning 20 years additionally reveals a long-term modulation that could arise from a magnetic activity cycle or from a more distant planetary object. Because of its proximity to the Sun, the proposed planet has a maximum angular separation of 220 milliarcseconds from Barnard's star, making it an excellent target for complementary direct imaging and astrometric observations.Barnard's star is the second closest red dwarf to the Solar System, after Proxima Centauri, and thus an ideal target to search for exoplanets with potential for further characterisation 10 . Its very low X-ray flux, lack of Ha emission, low chromospheric emission indices, slow rotation rate, slightly sub-solar metallicity, and membership of the thick disc kinematic population indicate an extremely low magnetic activity level and suggest an age older than the Sun. Because of its apparent brightness and very low variability, Barnard's star is often regarded as a benchmark for intermediate M-type dwarfs. Its basic properties are summarized in Table 1.An early analysis of archival radial velocity datasets of Barnard's star up to 2015 indicated the presence of at least one significant signal with a period of ~230 days but with rather poor sampling. To elucidate its presence and nature we undertook an intensive monitoring campaign with the CARMENES spectrometer 11 , collecting precise radial velocity measurements on every possible night during 2016-2017, and we obtained overlapping observations with the ESO/HARPS and HARPS-N instruments. The combined Doppler monitoring effort of Barnard's star, including archival and newly acquired observations, resulted in 771 radial velocity epochs (nightly averages) with typical individual precisions of 0.9 to 1.8 m s -1 , obtained over a timespan exceeding 20 years from seven different facilities and yielding eight independent datasets (ED Table 1).While e...

Public HARPS radial velocity database corrected for systematic errors

“…The stars within 100 pc represent ∼ 67% of our HARPS sample. This collection of stars, are representative of a volume-limited, long-lasting HARPS surveys dedicated to solar-mass G-dwarf stars (Pepe et al 2004;Lo Curto et al 2010;Moutou et al 2011;Lo Curto et al 2013;Udry et al 2019), low-mass M-dwarfs Trifon Trifonov et al: A public HARPS radial velocity database corrected for systematic errors (Bonfils et al 2005;Mayor et al 2009;Forveille et al 2009;Bonfils et al 2013;Anglada-Escudé et al 2016;Astudillo-Defru et al 2017;Ribas et al 2018) and metal-poor stars (Santos et al 2011;Faria et al 2016;Mortier et al 2016), which all target nearby stars. The remaining ∼ 28% of the HARPS sample (with distance > 120 pc) are typically bright main sequence stars of spectral types A0 to F6, some fainter and more distant transiting planet hosts observed by more recent RV follow-up campaigns of transit planet candidates from the HATSouth (Bakos et al 2013;Brahm et al 2016;Henning et al 2018;Espinoza et al 2019), WASP-south (Pollacco et al 2006;Gillon et al 2009;Nielsen et al 2019), and the K2 extended mission (Howell et al 2014;Grziwa et al 2016;Johnson et al 2018), or evolved subgiant and giant branch stars of spectral types G8 IV − K4 III.…”

Formation of compact systems of super-Earths via dynamical instabilities and giant impacts

“…Nonetheless, there are hints in the data that super-Earths do not always come as members of compact multiple systems. For example, the recently discovered super-Earth orbiting with a period of 233 days around Barnard's star indicates a lack of close orbiting planets with similar masses in that system (Ribas et al 2018), and certainly none that became anchored at the inner edge of the protoplanetary disc during their formation, as often occurs in N-body simulations of planet formation that involve pebble drift or planet migration. Similarly, Proxima b, orbiting with a period of 11 days does not appear to have closely neighbouring planets of similar masses (Anglada-Escudé et al 2016).…”

Self-induced dust traps around snow lines in protoplanetary discs

“…Observations show that the water snow line also potentially affects the occurence of giant planets (Fernandes et al 2019). The recent discovery of a super-Earth orbiting Barnard's Star b at the location of the water snow line (Ribas et al 2018) provides further evidence of the importance of the ice line in planet formation.…”