66 Leonis hosts a
super-Jupiter exoplanet, discovered combining
direct imaging from the CHARIS instrument at the
Subaru Telescope and
astrometry data from the
Hipparcos and
Gaia spacecrafts. Its discovery was published in 2025, making it the third planet to have been discovered using both direct imaging and astrometry, after
AF Leporis b and
HIP 99770 b. The planet has an estimated radius of and an
effective temperature of . It takes around 90 years to complete an orbit around 66 Leonis and has a
semi-major axis of 25
astronomical units, similar to the distance of Neptune to the Sun (30.1 au). Its mass, estimated from the astrometric observations, is , which is higher than the traditional boundary between planets and brown dwarfs of . Based on this boundary, would be a brown dwarf, but its position in the masssemimajor axis diagram and its low mass ratio relative to the host star are similar to that of other planets and discrepant with more massive brown dwarfs, supporting its classification as a planet. Furthermore, the mass required for an object to burn deuterium also depends on its helium abundance, which cannot be reliably measured, and multiple studies have rejected deuterium burning as a delimiter between planets and brown dwarfs based on demographic grounds. ==References==