Regulus, the brightest star in the constellation Leo, is known to float through space with three companion stars. New research shows that it may have yet another member in its stellar entourage.
The Growing Regulus Family

The constellation Leo, showing the location of Regulus. Click to enlarge. [IAU/Sky & Telescope; CC BY 4.0]
Using these techniques, astronomers have discovered that Regulus, the nearest B-type star to Earth, has three companions. First to be identified was Regulus B, a 0.8-solar-mass K-type star. Regulus B itself was discovered to have a companion, a 0.32-solar-mass M-type star named Regulus C, in 1873.
In 2008, technological advances allowed for the discovery of an even smaller companion: a 0.31-solar-mass stellar core that sweeps around Regulus every 40 days. Although this companion has yet to be imaged directly, it was detected spectroscopically only a few years ago.
Now, researchers may have tracked down Regulus’s smallest companion yet.
A Potential Companion
SDSS J100711.74 +193056.2, or SDSS J1007+1930 for short, is a brown dwarf in Regulus’s vicinity. Brown dwarfs lie in between stars and planets, massive enough to temporarily experience fusion of deuterium and sometimes lithium, but not massive enough to initiate hydrogen fusion. SDSS J1007+1930’s mass is estimated to be just 0.06 solar mass. The brown dwarf sits 12.6 light-years from Regulus, and its proper motion across the sky is similar to that of Regulus, suggesting that the two objects might be associated.

A near-infrared spectrum of SDSS J1007+1930 (gray is the high-resolution spectrum, black is the smoothed spectrum) along with the spectrum of an L9-type brown dwarf standard (purple). Click to enlarge. [Adapted from Mamajek & Burgasser 2025]
Assembling the Clues
Mamajek and Burgasser found that SDSS J1007+1930 bears a close resemblance to members of the Regulus system in several ways. The brown dwarf is less metal-rich than the Sun, with a metallicity similar to that of Regulus B. Its radial velocity is also similar to that of Regulus B and Regulus itself. Finally, spectral analysis suggests that SDSS J1007+1930 is unlikely to be young, and its age may instead be similar to the 1–2 billion year age of the system.
These clues leave open the possibility that SDSS J1007+1930 is associated with Regulus — but is it? So far, the available evidence isn’t conclusive, but it does suggest that SDSS J1007+1930 formed either within the Regulus system or in the same natal star cluster. Even if SDSS J1007+1930 is currently linked to the Regulus system, the association might be short lived. Given the brown dwarf’s distance from Regulus, it’s possible that the object is no longer gravitationally bound to the system, and a future interaction with a passing star or even a massive gas cloud could steal the brown dwarf away.
Citation
“SDSS J100711.74+193056.2: A Candidate Common Motion Substellar Companion to the Nearest B-Type Star Regulus,” Eric E. Mamajek and Adam J. Burgasser 2025 AJ 169 77. doi:10.3847/1538-3881/ad991b