Earth’s new neighbor
Barnard’s Star is the closest single star to the Sun and has the fastest apparent motion across the sky. Since the 1960s all the controversial claims that have been made about the existence of exoplanets around the approximately 10-billion-year-old red dwarf have been debunked or found to be inconclusive (see the article by John Johnson, Physics Today, March 2014, page 31 ). Now a collaboration led by Ignasi Ribas at the Institute of Space Sciences of Catalonia in Barcelona, Spain, and Guillem Anglada-Escudé at Queen Mary University of London has found convincing evidence of an exoplanet orbiting the Sun’s older neighbor. That exoplanet, Barnard’s Star b, is at least 3.2 times the mass of Earth and orbits the star every 233 days at a distance of about 0.4 AU.
The researchers detected the elusive exoplanet using the radial-velocity method, also known as Doppler spectroscopy. When a star has an orbiting planet, it also traverses a small orbit of its own because of the planet’s gravitational pull. As the star’s velocity relative to Earth changes along that orbit, its spectrum varies periodically due to the Doppler effect. For small planets with large orbits, such shifts are tiny. The peak radial velocity of Barnard’s Star b is 1.2 m/s, which is close to the lower limit for the Doppler technique, especially for a planet with such a long orbital period. To confirm their finding, Ribas, Anglada-Escudé, and colleagues had to stitch together information from eight data sets, shown in the figure; the data spanned 20 years, for a total of 771 nightly averages. The archival data most notably came from the HIRES instrument at the Keck Observatory, and intensive data collection was done in 2016–17 by three spectrometers: CARMENES at the Calar Alto Observatory in Spain, HARPS at La Silla Observatory in Chile, and HARPS-N at Roque de los Muchachos Observatory on La Palma in the Canary Islands.
The newly discovered exoplanet’s orbit is near the snow line, the minimum distance from a star at which volatile compounds can condense. Some models predict that the region around the snow line is hospitable for planet formation, and subsequent observations from a range of techniques could help inform and constrain those models. (I. Ribas et al., Nature 563, 365, 2018 ; thumbnail illustration by ESO/M. Kommesser.)