XO-5

XO-5 is a likely binary star system, made up of a G-type dwarf and a red dwarf companion, located approximately 893 light-years away from Earth in the Lynx constellation. It has a magnitude of about 12 and cannot be seen with the naked eye but is visible through a small telescope.^[3][7]

XO-5 / Absolutno

Observation data Epoch J2000      Equinox J2000
Constellation	Lynx[1]
Right ascension	07h 46m 51.9615s[2]
Declination	+39° 05′ 40.461″[2]
Apparent magnitude (V)	12.13±0.03[3]
Characteristics
Spectral type	G8V[3] + M[4]
Apparent magnitude (V)	12.13±0.03[3]
Apparent magnitude (R)	11.844[5]
Apparent magnitude (J)	10.774±0.019[6]
Apparent magnitude (H)	10.443±0.021[6]
Apparent magnitude (K)	10.345±0.018[6]
Astrometry
Radial velocity (Rv)	−9.47±0.65[2] km/s
Proper motion (μ)	RA: −21.509(18) mas/yr[2] Dec.: −23.910(13) mas/yr[2]
Parallax (π)	3.6517±0.0173 mas[2]
Distance	893 ± 4 ly (274 ± 1 pc)
Absolute magnitude (MV)	5.06±0.12[7]
Details
Mass	1.04±0.03[8] M☉
Radius	1.13±0.03[8] R☉
Luminosity	0.88±0.09[7] L☉
Surface gravity (log g)	4.35±0.02[8] cgs
Temperature	5430±70[8] K
Metallicity [Fe/H]	+0.05±0.06[7] dex
Rotational velocity (v sin i)	0.7±0.5[7] km/s
Age	7.0±7.0[9] Gyr
Other designations
Absolutno, TOI-5373, TIC 9155187, GSC 02959-00729, 2MASS J07465196+3905404, UCAC2 45499774[10]
Database references
SIMBAD	data
Exoplanet Archive	data

XO-5 has a suspected red dwarf companion with a temperature of 3500⁺²⁵⁰
₋₁₅₀ K, on a wide orbit.^[4] A 2024 study also identified it as a very likely binary star, with 94% probability.^[11]

The star XO-5 is named Absolutno. The name was selected in the NameExoWorlds campaign by the Czech Republic, during the 100th anniversary of the IAU. Absolutno is a fictional miraculous substance in the Czech sci-fi novel Továrna na absolutno (The Factory for the Absolute).^[12][13]

Planetary system

Size comparison of XO-5 b and Jupiter

The exoplanet XO-5b was discovered by the XO Telescope using the transit method in 2008. This planet is classified as a hot Jupiter.^[3] A search for transit timing variations caused by additional planets was negative.^[14]

The XO-5 planetary system^{[8][note 1]}

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b / Makropulos	1.19±0.03 MJ	0.0515±0.0005	4.1877558(6)	0	86.8±0.2°	1.14±0.03 RJ

Notes

1. eccentricity is insignificant compared to its uncertainties^[7]