HD 121056

HD 121056, or HIP 67851, is an aging giant star with a pair of orbiting exoplanets located in the southern constellation of Centaurus. This star is dimly visible to the naked eye with an apparent visual magnitude of 6.17.^[3] It is located at a distance of 209 light-years from the Sun, based on parallax measurements, and is drifting further away with a radial velocity of 5.6 km/s.^[2]

HD 121056

Observation data Epoch J2000      Equinox J2000
Constellation	Centaurus[1]
Right ascension	13h 53m 52.06131s[2]
Declination	−35° 18′ 51.6918″[2]
Apparent magnitude (V)	6.17[3]
Characteristics
Evolutionary stage	red giant branch[2]
Spectral type	K0 III[4]
B−V color index	1.01[5]
Astrometry
Radial velocity (Rv)	5.60±0.13[2] km/s
Proper motion (μ)	RA: −295.304 mas/yr[2] Dec.: −68.458 mas/yr[2]
Parallax (π)	15.6261±0.0418 mas[2]
Distance	208.7 ± 0.6 ly (64.0 ± 0.2 pc)
Absolute magnitude (MV)	+2.08[1]
Details
Mass	1.394±0.052[6] M☉
Radius	5.948±0.077[6] R☉
Luminosity	18.2+2.0 −1.8[6] L☉
Surface gravity (log g)	3.034±0.005[6] cgs
Temperature	4,867±49[3] K
Metallicity	[Mg/H] 0.286±0.05 dex[3]
Metallicity [Fe/H]	0.020±0.031[3] dex
Rotational velocity (v sin i)	2.38±0.51[3] km/s
Age	5.46[7] Gyr
Other designations
CD−34 9223, GJ 532.1, HD 121056, HIP 67851, HR 5224, TYC 7287-1874-1, 2MASS J13535209-3518517[8]
Database references
SIMBAD	data
Exoplanet Archive	data

The spectrum of HD 121056 presents as an evolved K-type giant star with a stellar classification of K0 III.^[9] It is presently ascending the red-giant branch, having exhausted the supply of hydrogen at its core.^[5] The star is about 5.5^[7] billion years old and is spinning with a projected rotational velocity of 2.4 km/s. HD 121056’s concentration of heavy elements is similar to the Sun, with a metallicity Fe/H index of 0.020±0.031, although the star is enriched in lighter rock-forming elements like magnesium and aluminum.^[3] It has 1.6^[10] times the mass of the Sun and has expanded to 5.72 times the Sun's radius.^[7] The star is radiating 15.8^[7] times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 4,867 K.^[3]

Planetary system

In 2014, two planets orbiting HD 121056 were discovered by the radial velocity method,^[5] and were confirmed a few months later.^[9] The orbits of these planets are stable on astronomical timescales,^[11] although the periods are not in orbital resonance.^[12] In 2022, the inclination and true mass of HD 121056 c were measured via astrometry, indicating a nearly edge-on orbit.^[13] Subsequent studies have provided further updates to the planetary parameters.^[6][14]

The planetary system configuration is favorable for direct imaging of exoplanets in the near future, being included in the top ten easiest targets in 2018.^[15]

The HD 121056 planetary system^[14][a]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	≥1.15±0.04 MJ	0.42±0.0002	89.23±0.06	0.13±0.04	—	—
c	4.97+0.22 −0.21 MJ	4.55±0.05	3,128.41+52.09 −47.13	0.30±0.03	89.86+28.38 −24.73[13]°	—

Notes

1. Most parameters are taken from the most recent study of the system, Fontanet et al. 2025. This does not incorporate astrometry, hence the mass of planet c is technically a minimum mass, but since the inclination measured by Feng et al. 2022 is nearly edge-on, the true mass is about the same as this value.