Astronomers using ESA’s Gaia star-mapping satellite have discovered two substellar companions — named Gaia-4b and Gaia-5b — orbiting two low-mass stars.
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An artist’s impression of the super-Jupiter exoplanet Gaia-4b. Image credit: ESA / Gaia / DPAC / M. Marcussen.
Gaia-4b is a ‘super-Jupiter’ exoplanet (11.8 Jupiter masses) orbiting an M- or K-type dwarf star called Gaia-4. The system resides approximately 244 light-years away from Earth.
Gaia-5b is a brown dwarf orbiting a nearby mid M dwarf, Gaia-5, which is around 134 light-years away from our planet.
“With an orbital period of 571 days, Gaia-4b is a relatively cold gas giant planet,” said Dr. Guðmundur Stefánsson, an astronomer at the University of Amsterdam.
“With a mass of around 21 Jupiters, Gaia-5b is a brown dwarf, more massive than a planet but too light to sustain nuclear fusion to be a star.”
According to the team, Gaia-4b is one of the first exoplanets discovered via the astrometric technique, and is one of the most massive planets known to orbit a low-mass star.
“A planet in orbit around a star creates a tiny gravitational ‘tug’ that makes the star ‘wobble’ around its center of mass and travel in a corkscrew-like motion across the sky,” the astronomers said.
“The easiest objects to discover using astrometry are massive and in distant orbits around their parent star.”
“Previously, a few massive brown dwarfs were confirmed to exist by other telescopes who observed their faint glow next to bright stars for which Gaia had detected such a wobble.”
“That’s in contrast to the transit method, which detects planets as they pass in front of their star and is most likely to find planets in a close orbit.”
“And although detecting a wobble suggests a star might have a planet, there are other potential causes, so astrometric discoveries must be confirmed using other methods.”
“Gaia was repeatedly scanning these stars, building up an increasingly detailed picture over time,” Dr. Guðmundur said.
“In 2022, Gaia Data Release 3 included a list of stars that appear to be moving as though pulled by an exoplanet.”
“Using ground-based spectroscopic data and the radial velocity technique to investigate these stars, we confirmed our first planet and our first brown dwarf.”
The findings were published this week in the Astronomical Journal.
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Gudmundur Stefánsson et al. 2025. Gaia-4b and 5b: Radial Velocity Confirmation of Gaia Astrometric Orbital Solutions Reveal a Massive Planet and a Brown Dwarf Orbiting Low-mass Stars. AJ 169, 107; doi: 10.3847/1538-3881/ada9e1
