HD 203473

HD 203473 is a star in the equatorial constellation Equuleus. With an apparent magnitude of 8.23,^[2] it’s only visible by using an amateur telescope. The star is located at a distance of 237 light years^[1] based on its parallax shift but is drifting closer at a high rate of 61.7 km/s.^[1] As of 2014, no stellar companions have been detected around the star.^[7]

HD 203473

Observation data Epoch J2000      Equinox J2000.0 (ICRS)
Constellation	Equuleus
Right ascension	21h 22m 18.87390s[1]
Declination	+05° 01′ 24.9072″[1]
Apparent magnitude (V)	8.23[2]
Characteristics
Spectral type	G6 V[3]
B−V color index	+0.66[2]
Astrometry
Radial velocity (Rv)	−61.70±0.13[1] km/s
Proper motion (μ)	RA: 180.093±0.035 mas/yr[1] Dec.: 0.098±0.031 mas/yr[1]
Parallax (π)	13.7402±0.0370 mas[1]
Distance	237.4 ± 0.6 ly (72.8 ± 0.2 pc)
Absolute magnitude (MV)	+4.2[2]
Details
Mass	0.82[4] M☉
Radius	1.48+0.05 −0.02[5] R☉
Luminosity	2.31±0.01[5] L☉
Surface gravity (log g)	4.25[4] cgs
Temperature	5847+35 −94[5] K
Metallicity [Fe/H]	+0.18[4] dex
Rotational velocity (v sin i)	1[6] km/s
Age	5.2[4] or 8.5[6] Gyr
Other designations
AG+04° 2898, BD+04°4656, HD 203473, HIP 105521, SAO 126740, GSC 00536-00696
Database references
SIMBAD	data
Exoplanet Archive	data

HD 203473 is an ordinary G-type main-sequence star with 82% the mass of the Sun,^[4] but is 48% larger than the latter.^[5] This star is over luminous and hot for its class, with it radiating at 2.31 the luminosity of the Sun^[5] and an effective temperature of 5,847 K.^[5] HD 203473 has different age estimates, either being 5^[4] or 8^[6] billion years old. The higher luminosity and low projected rotational velocity of 1 km/s^[6] favors the older age estimate. Like many planetary hosts, HD 203473 has an enhanced metallicity, with an iron abundance 1.51 times that of the Sun.^[4]

Companion

In 2018, the N2K project discovered an object, initially thought to be a planet, orbiting the star via Doppler spectroscopy. Due to the detection method, its inclination and true mass were initially unknown.^[8] In 2022, the inclination and true mass of this companion were measured via astrometry, revealing it to be 96 M_J and thus either a massive brown dwarf or low-mass star. The companion's orbital period was also found to be twice as long as originally thought.^[9]

The HD 203473 planetary system^[9]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (years)	Eccentricity	Inclination	Radius
b	95.886+8.523 −8.864 MJ	4.161+0.172 −0.190	8.103+0.014 −0.016	0.404±0.007	141.240+0.949 −0.909°	—