The nearest single star to Earth has four small planets

The nearest single star to the sun, Barnard’s star, has a brood of planets all its own. The red dwarf star, about six light-years from Earth, hosts four close-in planets each about two to three times the mass of Mars, astronomers report in the March 20 Astrophysical Journal Letters.

“Barnard’s star has a long history of claimed detections, but none of them could be confirmed for a long time,” says astronomer Ritvik Basant of the University of Chicago. “It’s pretty exciting to know what’s orbiting the nearest stars.”

From 2021 to 2023, Basant and his colleagues observed Barnard’s star 112 times using the MAROON-X spectrograph on the Gemini North telescope in Hawaii. “I was starting to see these signals in the dataset,” Basant says. But because he was still calibrating the instrument, “we ignored it for that time.”

Then last October, a team using the ESPRESSO spectrograph on a telescope in Chile reported evidence of one planet orbiting Barnard’s star, dubbed simply b, and hints of three others — designated c, d and e. With these hints in mind, Basant and colleagues reexamined their data,  confirming three of the planets.

“I am very happy to see that new MAROON-X data provide an independent confirmation of the planet b and candidates c and d,” says astrophysicist Jonay González Hernández of the Instituto de Astrofísica de Canarias in Tenerife, Spain, who led the ESPRESSO team. “Together with the ESPRESSO data, the analysis makes the detection significantly more robust.”

Combining the MAROON-X and ESPRESSO data also confirmed the existence of planet e.

All four planets are so close to Barnard’s star, which has about one-seventh the mass of the sun, that they orbit it in less than a week. That means they’re too hot to be habitable, Basant says. In fact, the new observations probably rule out the presence of any habitable-zone planets.

Both studies employed the radial velocity method, which detects changes in the light from Barnard’s star caused by the gravitational pull of the orbiting planets. This technique was responsible for finding the first known exoplanet in 1995 and has since identified hundreds of planets.

But all those planets were more massive than Earth. Thanks to larger telescopes and improved control over temperature and pressure variations in the instruments, the MAROON-X and ESPRESSO teams successfully detected much smaller planets. The four planets range from at least 0.19 to 0.34 times the mass of Earth, Basant and his colleagues found.

“Previous instruments were not that great at finding sub-Earth mass planets,” Basant says. “This is a big technological break. And a glimpse of what these next generation instruments can do.”