66 Leonis

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66 Leonis
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: −65.160 mas/yr [2]
Dec.: −5.525 mas/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 of Leo. With an apparent magnitude of +6.80, it is far too faint to be viewed to the naked eye under normal conditions, and can only be faintly seen in ideal conditions under dark skies. Based on parallax measurements by the Gaia spacecraft, [2] it lies at a distance of 271 light-years (83.1 parsecs ). It is moving away from the Sun at a velocity of 12.8 km/s.

The star has a stellar classification of A5V, with the luminosity class V indicating that it is in the main sequence. It is estimated to be 100 million years old, has 1.9 times the mass of the Sun, [4] 1.65 times the Sun's radius, and 10.5 times the Sun's luminosity. The effective temperature is 8,090  K, giving it the typical white hue of an A-type star. [7]

Planetary system

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. [4]

The planet has an estimated radius of 1.1 RJ and an effective temperature of 2,348±218  K . 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 17.7+7.6
−4.9  MJ , which is higher than the traditional boundary between planets and brown dwarfs of 13 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 abudance, 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]

The 66 Leonis planetary system [4]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(years)		 Eccentricity Inclination Radius
b17.7+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

References

  1. ↑ Anderson, E.; Francis, Ch. (2012). "XHIP: An extended hipparcos compilation". Astronomy Letters . 38 (5): 331. arXiv: 1108.4971 . Bibcode:2012AstL...38..331A. doi:10.1134/S1063773712050015. XHIP record for this object at VizieR.
  2. 1 2 3 4 5 6 7 Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv: 2208.00211 . Bibcode:2023A&A...674A...1G. doi: 10.1051/0004-6361/202243940 . S2CID   244398875. Gaia DR3 record for this source at VizieR.
  3. ↑ Høg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27 –L30. Bibcode:2000A&A...355L..27H. ISSN   0004-6361.
  4. 1 2 3 4 5 6 7 8 9 Currie, Thayne; Li, Yiting; El Morsy, Mona; Lacy, Brianna; Vincent, Maria; Tobin, Taylor L.; Kuzuhara, Masayuki; Chilcote, Jeffrey; Guyon, Olivier; Gu, Ziying; Bovie, Danielle; Peng, Dillon; An, Qier; Brandt, Timothy D; Deo, Vincent (2025-12-03). "SCExAO/CHARIS and Gaia Direct Imaging and Astrometric Discovery of a Superjovian Planet 3--4 lambda/D from the Accelerating Star HIP 54515". The Astronomical Journal. 171 (1): 5. arXiv: 2512.02159 . doi: 10.3847/1538-3881/ae1a82 . ISSN   0004-6256.
  5. 1 2 3 4 Stassun, Keivan G.; Oelkers, Ryan J.; Paegert, Martin; Torres, Guillermo; Pepper, Joshua; De Lee, Nathan; Collins, Kevin; Latham, David W.; Muirhead, Philip S.; Chittidi, Jay; Rojas-Ayala, Bárbara; Fleming, Scott W.; Rose, Mark E.; Tenenbaum, Peter; Ting, Eric B. (2019-10-01). "The Revised TESS Input Catalog and Candidate Target List". The Astronomical Journal. 158 (4): 138. arXiv: 1905.10694 . Bibcode:2019AJ....158..138S. doi: 10.3847/1538-3881/ab3467 . ISSN   0004-6256.
  6. ↑ "66 Leonis". SIMBAD . Centre de données astronomiques de Strasbourg.
  7. ↑ "The Colour of Stars". Australia Telescope, Outreach and Education. Commonwealth Scientific and Industrial Research Organisation. December 21, 2004. Archived from the original on 2013-12-03. Retrieved 2012-01-16.
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