| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Leo[1] |
| Right ascension | 11h 09m 13.6587s[2] |
| Declination | −01° 19′ 59.736″[2] |
| Apparent magnitude (V) | 6.80±0.01[3] |
| Characteristics | |
| Evolutionary stage | Main sequence[4] |
| Spectral type | A5V[4] |
| Astrometry | |
| Radial velocity (Rv) | +12.81±0.30[2] km/s |
| Proper motion (μ) | RA: −65.160mas/yr[2] Dec.: −5.525mas/yr[2] |
| Parallax (π) | 12.0377±0.0292 mas[2] |
| Distance | 270.9 ± 0.7 ly (83.1 ± 0.2 pc) |
| Absolute magnitude (MV) | 2.21[4] |
| Details | |
| Mass | 1.90+0.19 −0.20[4] M☉ |
| Radius | 1.65±0.05[5] R☉ |
| Luminosity | 10.5±0.4[5] L☉ |
| Surface gravity (log g) | 4.30±0.07[5] cgs |
| Temperature | 8,090±120[5] K |
| Age | 115+85 −92[4] Myr |
| Other designations | |
| BD−00°2409,HD 96855,HIP 54515,TYC 4921-1206-1[6] | |
| Database references | |
| SIMBAD | data |
66 Leonis is a single, white-hued star in the constellation ofLeo. With anapparent magnitude of +6.80, it is far too faint to be viewed to thenaked eye under normal conditions, and can only be faintly seen in ideal conditions underdark skies. Based onparallax measurements by theGaia spacecraft,[2] it lies at a distance of 271light-years (83.1parsecs). It is moving away from the Sun at a velocity of 12.8 km/s.
The star has astellar classification of A5V, with theluminosity class V indicating that it is in themain sequence. It is estimated to be 100 million years old, has 1.9 times themass of the Sun,[4] 1.65 times theSun's radius, and 10.5 times theSun's luminosity. Theeffective temperature is 8,090 K, giving it the typical white hue of anA-type star.[7]
66 Leonis hosts asuper-Jupiter exoplanet, discovered combiningdirect imaging from the CHARIS instrument at theSubaru Telescope andastrometry data from theHipparcos andGaia spacecrafts. Its discovery was published in 2025, making it the third planet to have been discovered using both direct imaging and astrometry, afterAF Leporis b andHIP 99770 b.[4]
The planet has an estimated radius of1.1 RJ and aneffective temperature of2,348±218 K. It takes around 90 years to complete an orbit around 66 Leonis and has asemi-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, is17.7+7.6
−4.9 MJ, which is higher than the traditional boundary between planets and brown dwarfs of13 MJ. Based on this boundary,66 Leo b would be a brown dwarf, but its position in the mass—semimajor 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 demographical grounds.[4]
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (years) | Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b | 17.7+7.6 −4.9 MJ | 24.8+7.2 −4.7 | 89+41 −24 | 0.42+0.13 −0.14 | 129.6+6.7 −5.9° | 1.08±0.23 RJ |
