The red dwarf Barnard’s Star is only around six light-years away from us – only Proxima and Alpha Centauri are closer. Now astronomers have detected a small exoplanet around this cool, little dwarf star. This planet, named Barnard b, is only about a third the mass of our Earth and orbits its star within 3.15 days. This means that its orbit lies starward of the habitable zone; the temperature on its surface is probably around 125 degrees Celsius, as astronomers report. In addition to Barnard b, they discovered the possible signatures of three other planet candidates in close orbits around Barnard’s star. However, they were unable to confirm a super-Earth postulated in 2018 with an orbital period of 233 days.
Red dwarf stars are the most common type of star in our Milky Way. Observations show that these small, relatively cool stars make up around 80 percent of all stars in the orbit of our solar system. Astronomers have already discovered exoplanets around some of these neighboring stars. Our nearest neighbor, the red dwarf Proxima Centauri, which is around four light-years away, is orbited by three planets; the red dwarf TRAPPIST-1, around 40 light-years away, even has seven Earth-like satellites. Most of these planets were discovered using the so-called radial velocity method. Periodic shifts in the star’s light spectrum reveal that its movement is influenced by the gravity of one or more companions. “Because M dwarfs are cooler, less bright and less massive than sun-like stars and their habitable zone is also closer to the star, they are ideal targets for planet searches using radial velocity,” explain Jonay González Hernández from the Astrophysical Institute of the Canary Islands and his colleagues.
Does the single star closest to the sun have planets?
Despite these actually favorable conditions, it has proven difficult to detect planets around Barnard’s Star. This is the closest single star to the Sun and the second closest star after the Proxima Alpha Centauri triple system. In 2018, astronomers finally seemed to have found what they were looking for: They discovered a recurring shift in the star’s spectrum that indicated the presence of a super-Earth with an orbital period of around 233 days. However, further observations could not clearly confirm this finding. In addition, it could not be ruled out that the observed spectral signature was due only to the activity of the red dwarf rather than to the gravitational influence of an orbiting planet. To provide greater clarity, González Hernández and his team have now taken another look at Barnard’s star using the high-resolution ESPRESSO spectrograph on the Very Large Telescope (VLT) at the European Southern Observatory (ESO). To obtain the most precise data possible, light from up to four of the observatory’s large reflecting telescopes can be synchronized and fed into this instrument.
For their study, the astronomers evaluated a total of 157 ESPRESSO observations that were made of Barnard’s star between May 2019 and July 2023. They also used data from other astronomical instruments at the La Silla Observatory in Chile and the Calar Alto Observatory in Spain. “Even though it took a long time, we were always confident that we would find something,” says González Hernández. In fact, the computer-aided evaluations showed several striking signals in the light spectrum of Barnard’s star. The most significant periodic shifts were repeated at intervals of around 3.15 days. After closer inspection and comparison with the activity of the star, the astronomers come to the conclusion that this signal must come from an exoplanet. “We confirmed it as a planetary signal,” write González Hernández and his team.
(Video: ESO)
A planetary lightweight and three other candidates
According to astronomers’ calculations, the planet, named Barnard b, is around a third as heavy as Earth. “Barnard b is one of the lowest-mass exoplanets known and one of the few whose mass is less than that of Earth,” explains González Hernández. However, this small rocky planet is probably not friendly to life. With an orbital period of just 3.15 days and a distance of around 0.019 astronomical units from the star, it is around 20 times closer to it than Mercury to the sun. “Even if the star is about 2,500 degrees cooler than our Sun, the planet is too hot to support liquid water on the surface,” says González Hernández. The team estimates that the average surface temperature on Barnard b is around 125 degrees Celsius – water would therefore evaporate.
But this newly discovered exoplanet is probably not the only satellite around Barnard’s star: astronomers have discovered three other potential planetary signatures in the red dwarf’s spectrum. They suggest exoplanets with orbital periods of 2.34 days, 4.12 days and 6.74 days. However, González Hernández and his team have not yet been able to confirm these planet candidates with their measurements. “To confirm the presence of such a compact four-planet system around Barnard’s Star, we would need many more ESPESSO observations,” they explain. On the other hand, their review of the potential super-Earth with an orbital period of 233 days, which caused a stir in 2018, was clearer. “The quality of the ESPRESSO radial velocity data should have allowed us to clearly detect the strong signal from such a planet,” the astronomers explain. But that wasn’t the case. They therefore rule out the existence of such a super-Earth in the wide orbit around Barnard’s star – according to them, the previously detected spectral shifts are due to stellar activity.
Source: Jonay González Hernández (Instituto de Astrofísica de Canarias, Tenerife) et al., Astronomy and Astrophysics, doi: 10.1051/0004-6361/202451311