XO-4

Not to be confused with the OLPC XO-4 from the One Laptop per Child (OLPC) initiative or the Martin XO-4.

XO-4 is a star located approximately 863 light-years away from Earth in the Lynx constellation. It has a magnitude of about 11 and cannot be seen with the naked eye but is visible through a small telescope.^[3] A search for a binary companion star using adaptive optics at MMT Observatory was negative.^[7]

XO-4 / Koit

Observation data Epoch J2000      Equinox J2000
Constellation	Lynx[1]
Right ascension	07h 21m 33.1602s[2]
Declination	+58° 16′ 05.110″[2]
Apparent magnitude (V)	10.674 ± 0.019[3]
Characteristics
Spectral type	F5V[3]
Apparent magnitude (B)	11.240 ± 0.029[3]
Apparent magnitude (V)	10.674 ± 0.019[3]
Apparent magnitude (J)	9.667 ± 0.021[4]
Apparent magnitude (H)	9.476 ± 0.022[4]
Apparent magnitude (K)	9.406 ± 0.023[4]
Astrometry
Radial velocity (Rv)	1.74±0.33[2] km/s
Proper motion (μ)	RA: −17.008(12) mas/yr[2] Dec.: 5.463(12) mas/yr[2]
Parallax (π)	3.7812±0.0156 mas[2]
Distance	863 ± 4 ly (264 ± 1 pc)
Details[3][5]
Mass	1.32±0.02 M☉
Radius	1.56±0.05 R☉
Surface gravity (log g)	4.18±0.07 cgs
Temperature	6397±70 K
Metallicity [Fe/H]	−0.04±0.03 dex
Rotational velocity (v sin i)	8.8±0.5 km/s
Age	2.1±0.6 Gyr
Other designations
Koit, TOI-1721, TIC 51234631, TYC 3793-1994-1, GSC 03793-01994, 2MASS J07213317+5816051[6]
Database references
SIMBAD	data
Exoplanet Archive	data

The star XO-4 is named Koit. The name was selected in the NameExoWorlds campaign by Estonia, during the 100th anniversary of the IAU. Koit is Estonian for dawn, and was named for a character in a folk tale written by Friedrich Robert Faehlmann.^[8][9][10]

Planetary system

One known exoplanet, XO-4b, which is classified as a hot Jupiter, orbits XO-4. This exoplanet was discovered in 2008 by the XO Project using the transit method.^[3] It has been named Hämarik, meaning dusk, and referring to a character from the same Faehlmann story featuring Koit.^[11] The planetary orbit is misaligned with the stellar rotation.^[12]

The XO-4 planetary system^{[5][note 1]}

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b / Hämarik	1.612+0.027 −0.028 MJ	0.05524+0.00027 −0.00028	4.1250823(39)	<0.0039	88.7±1.1°	1.34±0.05 RJ

Notes

1. eccentricity approximately equal to zero is expected theoretically and is consistent with the radial velocities and secondary eclipse timing^[3][13]