Four Tiny Planets Discovered Near Earth after Century-Long Search

Using the high-precision MAROON-X instrument on the Gemini North telescope, researchers confirmed these planets using the radial velocity technique, marking a milestone in detecting small planets. These rocky worlds, among the smallest ever found, challenge previous planet-hunting efforts that had often yielded uncertain results. The discovery hints at a new era of exploring smaller exoplanets, which could offer vital clues about planetary formation and the possibility of habitability, the Astrophysical Journal Letters reported.

Astronomers have discovered four sub-Earth exoplanets orbiting Barnard’s Star, the closest single-star system to Earth. The discovery was made in part using the Gemini North telescope, which is part of the International Gemini Observatory. Notably, one of the newly identified planets is the least massive exoplanet ever detected using the radial velocity technique, setting a new benchmark for finding smaller planets around nearby stars.

For over a century, scientists have studied Barnard’s Star in hopes of finding planets. First identified by E. E. Barnard at Yerkes Observatory in 1916, it is the nearest single star system to Earth. Barnard’s Star is a red dwarf, a type of low-mass star that often hosts compact planetary systems with multiple rocky planets. Since red dwarfs are the most common type of star in the universe, understanding the planets they host is key to broader astronomical research.

One major effort in this search was led by Jacob Bean from the University of Chicago. His team developed MAROON-X, a highly sensitive instrument designed specifically to detect distant planets around red dwarf stars. MAROON-X is mounted on the Gemini North telescope, enhancing the observatory’s capability to identify small exoplanets.

MAROON-X uses the radial velocity technique to find exoplanets, which detects the tiny back-and-forth motion of a star caused by the gravitational pull of its orbiting planets. This motion results in minuscule shifts in the star’s light wavelength. MAROON-X is so precise that it can measure these subtle changes to determine the number and mass of the planets affecting the star, allowing astronomers to confirm their existence with remarkable accuracy.

After rigorously calibrating and analyzing data taken during 112 nights over a period of three years, the team found solid evidence for three exoplanets around Barnard’s Star, two of which were previously classified as candidates. The team also combined data from MAROON-X with data from a 2024 study done with the ESPRESSO instrument at the European Southern Observatory’s Very Large Telescope in Chile to confirm the existence of a fourth planet, elevating it as well from candidate to bona fide exoplanet.

“It’s a really exciting find — Barnard’s Star is our cosmic neighbor, and yet we know so little about it,” says Ritvik Basant, PhD student at the University of Chicago and first author of the paper appearing in The Astrophysical Journal Letters. “It’s signaling a breakthrough with the precision of these new instruments from previous generations.”

The newly discovered planets are most likely rocky planets, rather than gas planets like Jupiter. However, this will be difficult to pin down with certainty since, because of the angle we observe them from Earth, the planets do not cross in front of their star, which is the usual method for determining a planet’s composition. But with information from similar planets around other stars, the team will be able to make better estimates of their makeup.

They were, however, able to rule out with a fair degree of certainty the existence of other exoplanets with masses comparable to Earth in Barnard Star’s habitable zone — the region of space around a star that is just right to allow liquid water on an orbiting planet’s surface.

Barnard’s Star has been called the “great white whale” for planet hunters; several times over the past century, groups have announced evidence that suggested planets around Barnard’s Star, only for them to be subsequently disproved. But these latest findings give a much larger degree of confidence than any previous result.

“We observed at different times of night on different days. They’re in Chile; we’re in Hawai‘i. Our teams didn’t coordinate with each other at all,” says Basant. “That gives us a lot of assurance that these aren’t phantoms in the data.”

The four planets, each only about 20 to 30% of the mass of Earth, are so close to their home star that they zip all the way around it in a matter of days. The fourth planet is the least massive planet discovered to date using the radial velocity technique. The team hopes this will spark a new era of finding more and more sub-Earth exoplanets in the Universe.

Most rocky planets found so far are much larger than Earth, and they appear to be fairly similar throughout the Milky Way Galaxy. However, there are reasons to think that smaller exoplanets have more widely varied compositions. As scientists find more of them, they can begin to tease out more information about how these planets form and what makes them likely to have habitable conditions.

“The U.S. National Science Foundation is collaborating with the astronomy community on an adventure to look deeper into the Universe to detect planets with environments that might resemble Earth’s,” says Martin Still, NSF program director for the International Gemini Observatory. “The planet discoveries provided by MAROON-X mounted on Gemini North provide a significant step along that journey.”

MAROON-X is still a visiting instrument at Gemini North. Given its outstanding performance and popularity with the user community, it is in the process of being converted to a permanent facility instrument.

“This result demonstrates the competitive, state-of-the-art capabilities that Gemini offers its user community. The observatory is in the middle of rejuvenating its instrumentation portfolio and MAROON-X is part of the first wave of new instruments, alongside GHOST on Gemini South and IGRINS-2 on Gemini North,” says Andreas Seifahrt, Associate Director of Development for the International Gemini Observatory, co-author of the paper, and member of the team who designed and built MAROON-X.

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