40 Eridani is a triple star system in the constellation of Eridanus, abbreviated 40 Eri. It has the Bayer designation Omicron² Eridani, which is Latinized from ο² Eridani and abbreviated Omicron² Eri or ο² Eri. Based on parallax measurements taken by the Gaia mission, it is about 16.3 light-years from the Sun.

Quick Facts Apparent magnitude (V), Characteristics ...

40 Eridani/Keid

Location of 40 Eridani (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Eridanus
40 Eridani A
Right ascension	04h 15m 16.31962s[1]
Declination	−07° 39′ 10.3308″[1]
Apparent magnitude (V)	4.43[2]
40 Eridani B
Right ascension	04h 15m 21.79572s[3]
Declination	−07° 39′ 29.2040″[3]
Apparent magnitude (V)	9.52[4]
40 Eridani C
Right ascension	04h 15m 21.53600s[5]
Declination	−07° 39′ 20.6946″[5]
Apparent magnitude (V)	11.17[4]
Characteristics
40 Eridani A
Spectral type	K0.5V[6]
U−B color index	+0.45[4]
B−V color index	+0.82[2]
40 Eridani B
Spectral type	DA4[4]
U−B color index	+0.45[4]
B−V color index	+0.03[4]
40 Eridani C
Spectral type	M4.5eV[7]
U−B color index	+0.83[4]
B−V color index	+1.67[4]
Variable type	Flare star[8]
Astrometry
40 Eridani A
Radial velocity (Rv)	−42.47±0.12[1] km/s
Proper motion (μ)	RA: −2,240.085 mas/yr[1] Dec.: −3,421.809 mas/yr[1]
Parallax (π)	199.6080 ± 0.1208 mas[1]
Distance	16.340 ± 0.010 ly (5.010 ± 0.003 pc)
Absolute magnitude (MV)	5.93[9]
40 Eridani B
Radial velocity (Rv)	−21[10] km/s
Proper motion (μ)	RA: −2,236.169 mas/yr[3] Dec.: −3,338.955 mas/yr[3]
Parallax (π)	199.6911 ± 0.0512 mas[3]
Distance	16.333 ± 0.004 ly (5.008 ± 0.001 pc)
40 Eridani C
Radial velocity (Rv)	−44.06±0.20[5] km/s
Proper motion (μ)	RA: −2,247.183 mas/yr[5] Dec.: −3,409.824 mas/yr[5]
Parallax (π)	199.4516 ± 0.0692 mas[5]
Distance	16.353 ± 0.006 ly (5.014 ± 0.002 pc)
Orbit
Primary	40 Eridani A
Companion	40 Eridani BC
Period (P)	~8,000[11] yr
Semi-major axis (a)	~400[12] AU
Orbit[13]
Primary	40 Eridani B
Companion	40 Eridani C
Period (P)	230.30±0.68 yr
Semi-major axis (a)	6.930±0.050" (~35 AU)
Eccentricity (e)	0.4294±0.0027
Inclination (i)	107.56±0.29°
Longitude of the node (Ω)	151.44±0.12°
Periastron epoch (T)	1847.7±1.1
Argument of periastron (ω) (secondary)	318.4±1.1°
Details
40 Eridani A
Mass	0.78±0.08[14] M☉
Radius	0.812±0.017[14] R☉
Luminosity	0.457±0.002[15] L☉
Surface gravity (log g)	4.45±0.19[14] cgs
Temperature	5072±53[14] K
Metallicity [Fe/H]	−0.42±0.04[14] dex
Rotation	~37–43[16] days
Rotational velocity (v sin i)	1.23 ± 0.28[16] km/s
Age	6.9±4.7[14] Gyr
40 Eridani B
Mass	0.573±0.018[13] M☉
Radius	0.014[17] R☉
Luminosity	0.013[18] L☉
Temperature	16,500[19] K
40 Eridani C
Mass	0.2036±0.0064[13] M☉
Radius	0.31[20] R☉
Luminosity	0.008[note 1] L☉
Temperature	3,100[21] K
Age	5.6[22] Gyr 1.8[23] Gyr
Other designations
ο2 Eri, 40 Eri, GJ 166, ADS 3093, CCDM J04153-0739
A: Keid[24], BD−07° 780, HD 26965, HIP 19849, HR 1325, SAO 131063, LHS 23, LTT 1907
B: BD−07° 781, HD 26976, SAO 131065, G 160-060, LHS 24, LTT 1908
C: DY Eri, BD−07°781 C, LHS 25, LTT 1909
Database references
SIMBAD	A
	B
	C

Close

The primary star of the system, designated 40 Eridani A and named Keid,^[24] is easily visible to the naked eye. It is orbited by a binary pair whose two components are designated 40 Eridani B and C, and which were discovered on January 31, 1783, by William Herschel.^[25]: p73 It was again observed by Friedrich Struve in 1825 and by Otto Struve in 1851.^[11][26]

In 1910, it was discovered that although component B was a faint star, it was white in color. This meant that it had to be a small star; in fact it was a white dwarf, the first discovered.^[27] Although it is neither the closest white dwarf, nor the brightest in the night sky, it is by far the easiest to observe; it is nearly three magnitudes brighter than Van Maanen's Star, the nearest solitary white dwarf, and unlike the companions of Procyon and Sirius it is not swamped in the glare of a much brighter primary.^[18]

40 Eridani is the system's Flamsteed designation and ο² Eridani (Latinised to Omicron² Eridani) its Bayer designation. The designations of the sub-components – 40 Eridani A, B and C – derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).^[28] 40 Eridani C also bears the variable star designation DY Eridani.

The system bore the traditional name Keid derived from the Arabic word قيض (alqayḍ) meaning "the eggshells," alluding to its neighbour Beid (Arabic "egg").^[29] In 2016, the IAU organized a Working Group on Star Names (WGSN)^[30] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.^[31] It approved the name Keid for the component 40 Eridani A on 12 September 2016 and it is now so included in the List of IAU-approved Star Names.^[24]

40 Eridani A is a main-sequence dwarf of spectral type K1, 40 Eridani B is a 9th magnitude white dwarf of spectral type DA4, and 40 Eridani C is an 11th magnitude red dwarf flare star of spectral type M4.5e. When component B was a main-sequence star, it is thought to have been the most massive member of the system, but ejected most of its mass before it became a white dwarf.^[23] B and C orbit each other approximately 400 AU from the primary star, A.^[12] Their orbit has a semimajor axis of 35 AU and is rather elliptical with an orbital eccentricity of 0.410).^[11]

As seen from the 40 Eridani system, the Sun is a 3.4-magnitude star in Hercules, near the border with Serpens Caput.^{[note 2]}

40 Eridani A shows periodic radial velocity variations, which may be caused by a planetary companion. The 42-day period is close to the stellar rotation period, making the planetary nature of the signal difficult to confirm.^[16] A 2018 study found that most evidence supports a planetary origin for the signal,^[14] but this has remained controversial, with a 2021 study characterizing the signal as a false positive.^[32] As of 2022, the cause of the radial velocity variations remained inconclusive.^[33]

A study in 2023 concluded that the radial velocity signal very likely does originate from stellar activity, and not from a planet.^{[34]: 23–24, 44}

The candidate planet would have a minimum mass of 8.47±0.47 M_🜨, and lie considerably interior to the habitable zone, receiving nine times more stellar flux than Earth, which is an even greater amount than Mercury, the innermost planet in the Solar System, on average receives from the Sun.^[14]

• 40 Eridani in fiction