HD 20781

HD 20781 is a star which is part of a wide binary system with HD 20782. The companion star has a very large angular separation of 252 arcsec, corresponding to 9080 AU at the distance of HD 20782.^[5] Both stars possess their own planetary systems in S type orbits, with a total of five known planets around both stars.^[3][6] This is the first known example of planets being found orbiting both components of a wide binary system.^[5][7] HD 20781 has no noticeable starspot activity.^[3]

HD 20781

Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Fornax[1]
Right ascension	03h 20m 02.94286s[2]
Declination	−28° 47′ 01.7905″[2]
Apparent magnitude (V)	8.48[3]
Characteristics
Spectral type	K0V[3]
B−V color index	0.82[3]
Astrometry
Radial velocity (Rv)	40.27±0.12[2] km/s
Proper motion (μ)	RA: 348.869±0.015 mas/yr[2] Dec.: −66.614±0.019 mas/yr[2]
Parallax (π)	27.8123±0.0239 mas[2]
Distance	117.3 ± 0.1 ly (35.96 ± 0.03 pc)
Absolute magnitude (MV)	5.70[3]
Details[3]
Mass	0.7 M☉
Luminosity	0.49±0.04 L☉
Surface gravity (log g)	4.37±0.05 cgs
Temperature	5256±29 K
Metallicity [Fe/H]	−0.11±0.02 dex
Rotation	46.8±4.4 d
Rotational velocity (v sin i)	1.1 km/s
Other designations
CD–29° 1229, HIP 15526, LTT 1581, SAO 168468, 2MASS J03200291-2847016, Gaia DR1 5060105892897388288[4]
Database references
SIMBAD	data

Planetary system

In 2011, a pair of Neptune-mass gas giants were detected with the radial velocity method.^[7] In 2017, these planets were confirmed and an additional two inner super-Earths were detected, with periods of 5.3 and 13.9 days respectively.^[3]

The HD 20781 planetary system^[3]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	≥1.93+0.39 −0.36 M🜨	0.0529+0.0024 −0.0027	5.3135±0.0010	0.10+0.11 −0.07	—	—
c	≥5.33+0.70 −0.67 M🜨	0.1004+0.0046 −0.0051	13.8905+0.0033 −0.0034	0.09+0.09 −0.06	—	—
d	≥10.61+1.20 −1.19 M🜨	0.1647+0.0076 −0.0083	29.1580+0.0102 −0.0100	0.11+0.05 −0.06	—	—
e	≥14.03±1.56 M🜨	0.3374+0.0155 −0.0170	85.5073+0.0983 −0.0947	0.06+0.06 −0.04	—	—

See also

Other systems with multiple planet-hosting stars:

• HD 133131
• WASP-94
• XO-2