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81 Ceti
Observation data; Epoch J2000.0 Equinox J2000.0
Constellation	Cetus
Right ascension	02h 37m 41.8018s
Declination	–03° 23′ 46.2267″
Apparent magnitude (V)	5.65
Characteristics
Evolutionary stage	horizontal branch
Spectral type	K0III
B−V color index	1.021±0.001
Variable type	None
Astrometry
Radial velocity (Rv)	+8.76±0.28 km/s
Proper motion (μ)	RA: 42.066±0.190 mas/yr ; Dec.: −43.331±0.197 mas/yr
Parallax (π)	9.8489 ± 0.1183 mas
Distance	331 ± 4 ly ; (102 ± 1 pc)
Absolute magnitude (MV)	0.82
Details
Mass	1.6±0.2 M☉
Radius	11.1±0.3 R☉
Luminosity	60.0±0.8 L☉
Surface gravity (log g)	2.5±0.1 cgs
Temperature	4,825±41 K
Metallicity [Fe/H]	0.00±0.04 dex
Rotational velocity (v sin i)	1.20±0.47 km/s
Age	2.5±0.9 Gyr
Other designations
	81 Cet, BD−04°436, GC 3158, HD 16400, HIP 12247, HR 771, SAO 130026
Database references
SIMBAD	data
Exoplanet Archive	data

81 Ceti is a star located approximately 331 light years away from the Sun in the equatorial constellation of Cetus. 81 Ceti is the Flamsteed designation for this object. It is visible to the naked eye as a dim, yellow-hued point of light with an apparent visual magnitude of 5.65.^[2] The star is drifting further away from the Earth with a heliocentric radial velocity of +9 km/s.^[2]

This is an aging K-type giant star with a stellar classification of K0III,^[3] having exhausted the supply of hydrogen at its core and expanded to 11^[4] times the Sun's radius. It is a red clump giant,^[6] which indicates it is on the horizontal branch and is generating energy from core helium fusion. The star is now 2.5 billion years old with 1.6 times the mass of the Sun.^[4] It is radiating 60 times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 4,825 K.^[4]

Planetary system

In July 2008, the planet 81 Ceti b was announced by Sato and collaborators, along with 14 Andromedae b and 6 Lyncis b. The planet was found to be a super-Jupiter, with 5.3 times the mass of Jupiter. It takes 953 days for it to complete its orbit around the star.^[7]

The 81 Ceti planetary system^[7]
Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	≥5.3 $M$ _J	2.5	952.7 ± 8.8	0.206 ± 0.029	—	—

References

^ ^a ^b ^c ^d ^e Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
^ ^a ^b ^c ^d ^e ^f 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. S2CID 119257644.
^ ^a ^b Houk, N.; Swift, C. (1999). "Michigan catalogue of two-dimensional spectral types for the HD Stars". Michigan Spectral Survey. 5. Bibcode:1999MSS...C05....0H.
^ ^a ^b ^c ^d Bonfanti, A.; et al. (2015). "Revising the ages of planet-hosting stars". Astronomy and Astrophysics. 575. A18. arXiv:1411.4302. Bibcode:2015A&A...575A..18B. doi:10.1051/0004-6361/201424951. S2CID 54555839.
^ ^a ^b Jofré, E.; et al. (2015). "Stellar parameters and chemical abundances of 223 evolved stars with and without planets". Astronomy & Astrophysics. 574: A50. arXiv:1410.6422. Bibcode:2015A&A...574A..50J. doi:10.1051/0004-6361/201424474. S2CID 53666931.
^ Mishenina, T. V.; et al. (September 2006), "Elemental abundances in the atmosphere of clump giants", Astronomy and Astrophysics, 456 (3): 1109–1120, arXiv:astro-ph/0605615, Bibcode:2006A&A...456.1109M, doi:10.1051/0004-6361:20065141, S2CID 18764566
^ ^a ^b Sato, Bun'ei; et al. (2008). "Planetary Companions to Evolved Intermediate-Mass Stars: 14 Andromedae, 81 Ceti, 6 Lyncis, and HD167042". Publications of the Astronomical Society of Japan. 60 (6): 1317–1326. arXiv:0807.0268. Bibcode:2008PASJ...60.1317S. doi:10.1093/pasj/60.6.1317. S2CID 67841762.