HD 220689

HD 220689 is a single^[8] star in the equatorial constellation of Aquarius. It is a challenge to view with the naked eye, having an apparent visual magnitude of +7.74,^[1] but is readily viewed with a pair of binoculars.^[9] The star is located at a distance of 153 light years from the Sun based on parallax, and is drifting further away with a radial velocity of +12 km/s.^[2] A survey in 2015 has ruled out the existence of any additional stellar companions at projected distances from 26 to 305 astronomical units.^[8]

HD 220689

Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Aquarius[1]
Right ascension	23h 25m 52.99170s[2]
Declination	−20° 36′ 57.6998″[2]
Apparent magnitude (V)	+7.74[1]
Characteristics
Spectral type	G3V[3]
B−V color index	+0.603±0.002[1]
Astrometry
Radial velocity (Rv)	+12.30±0.14[2] km/s
Proper motion (μ)	RA: −15.328 mas/yr[2] Dec.: −177.593 mas/yr[2]
Parallax (π)	21.3074±0.0236 mas[2]
Distance	153.1 ± 0.2 ly (46.93 ± 0.05 pc)
Absolute magnitude (MV)	4.50[1]
Details[4]
Mass	1.016±0.048 M☉
Radius	1.068±0.047 R☉
Luminosity	1.491±0.005[5] L☉
Surface gravity (log g)	4.360±0.045 cgs
Temperature	5,921±26 K
Metallicity [Fe/H]	−0.07±0.10 dex
Rotation	~29 d
Rotational velocity (v sin i)	5.5[6] km/s
Age	4.586±2.487 Gyr
Other designations
BD−21°6419, GC 32591, HD 220689, HIP 115662, SAO 191898, G 273-43[7]
Database references
SIMBAD	data
Exoplanet Archive	data

The stellar classification of HD 220689 is G3V,^[3] matching a yellow hued G-type main-sequence star that is generating energy through core hydrogen fusion. It is roughly 4.6 billion years old and is spinning with a projected rotational velocity of 5.5 km/s,^[6] giving it a rotation period of around 29 days.^[4] The star has a similar size, mass, and elemental abundances as the Sun. It is radiating 1.5^[5] times the luminosity of the Sun from its photosphere at an effective temperature of 5,921 K.^[4]

Planetary system

From 1998 to 2012, the star was under observation from the CORALIE echelle spectrograph at La Silla Observatory. In 2012, a long-period, wide-orbiting exoplanet was deduced from radial velocity variations. This was published in November.^[10] The maximum orbital period allowing for dynamic stability of a hypothetical inner planet is 3.9 years.^[4] In 2023, the inclination and true mass of HD 220689 b were determined via astrometry.^[11]

The HD 220689 planetary system^[11]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (years)	Eccentricity	Inclination	Radius
b	1.2+0.22 −0.11 MJ	3.433+0.065 −0.064	6.23+0.15 −0.14	0.053+0.06 −0.037	71+13 −18 or 109+18 −13°	—