The primary star of the system, designated 40 Eridani A and namedKeid,[22] is easily visible to the naked eye. It is orbited by abinary pair whose two components are designated 40 Eridani B and C, and which were discovered on January 31, 1783, byWilliam Herschel.[23]: p73 It was again observed byFriedrich Struve in 1825 and byOtto Struve in 1851.[12][24]
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 awhite dwarf, the first discovered.[25] 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 thanVan Maanen's Star, the nearest solitary white dwarf, and unlike the companions ofProcyon andSirius it is not swamped in the glare of a much brighter primary.[26]
The system bore the traditional nameKeid derived from theArabic word قيض (alqayḍ) meaning "theeggshells", alluding to its neighbourBeid (Arabic "egg").[28] In 2016, the IAU organized aWorking Group on Star Names (WGSN)[29] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entiremultiple systems.[30] It approved the nameKeid for the component40 Eridani A on 12 September 2016 and it is now so included in the List of IAU-approved Star Names.[22]
40 Eridani A is amain-sequencedwarf ofspectral type K1, 40 Eridani B is a 9th magnitudewhite dwarf of spectral type DA4, and 40 Eridani C is an 11th magnitudered dwarfflare 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 at1.8 M☉, but ejected much of its mass before it became a white dwarf.[18] B and Corbit each other approximately 400AU from the primary star, A.[13] Their orbit has a semimajor axis of 35AU and is rather elliptical with anorbital eccentricity of 0.410.[12]
As seen from the 40 Eridani system, the Sun is a 3.4-magnitude star inHercules, near the border withSerpens Caput.[note 1]
Thehabitable zone of40 Eridani A, where a planet could exist with liquid water, is near 0.68 AU from A. At this distance a planet would complete a revolution in 223 Earth days (according to the third ofKepler's laws) and40 Eridani A would appear nearly 20%[note 2] wider than the Sun does on Earth. An observer on a planet in the40 Eridani A system would see the B-C pair as unusually bright white and reddish-orange stars in the night sky –magnitudes −8 and −6, slightly brighter than the appearance ofVenus seen from Earth as theevening star.
It is unlikely that habitable planets exist around40 Eridani B because they would have been sterilized by its evolution into a white dwarf. As for40 Eridani C, it is prone to flares, which cause large momentary increases in the emission ofX-rays as well as visible light. This would be lethal toEarth-type life on planets near the flare star.[13]
40 Eridani A shows periodicradial velocity variations, which were suggested to be caused by a planetary companion. The 42-day period is close to the stellar rotation period, which made the possible planetary nature of the signal difficult to confirm.[17] A 2018 study found that most evidence supports a planetary origin for the signal,[15] but this was controversial, with a 2021 study characterizing the signal as a false positive,[31] and a 2022 study getting inconclusive results.[32] Further studies in 2023[33]: 23–24, 44 and 2024 concluded that the radial velocity signal very likely does originate from stellar activity, and not from a planet.[34]
The candidate planet would have had aminimum mass of8.47±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 thanMercury, the innermost planet in the Solar System, on average receives from the Sun.[15]
In theStar Trek franchise, the planetVulcan orbits 40 Eridani A.[35] Vulcan has been referenced in relation to the real-life search for exoplanets in this system.[36][34] The hypothetical planet 40 Eridani A b is also mentioned in the bookProject Hail Mary as the home of the eponymous Eridian species.[37] In the Bobiverse series byDennis E. Taylor, a double planet is found orbiting 40 Eridani A,[38] and named Vulcan and Romulus by an uploaded intelligence in avon Neumann probe.
^From 40 Eridani the Sun would appear on the diametrically opposite side of the sky at the coordinates RA=16h 15m 16.32s, Dec=07° 39′ 10.34″, which is located near the border ofHercules (constellation) andSerpens Caput, the closest bright star beingAlpha Serpentis. The absolute magnitude of the Sun is 4.85, so, at a distance of 5.04 parsecs, the Sun would have an apparent magnitude.
^From whereh is the apparent height,d is the distance of the object, anda is the actual size of the object.
^Gray, R. O.; Corbally, C. J.; Garrison, R. F.; McFadden, M. T.; Bubar, E. J.; McGahee, C. E.; O'Donoghue, A. A.; Knox, E. R. (2006). "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample".The Astronomical Journal.132 (1):161–170.arXiv:astro-ph/0603770.Bibcode:2006AJ....132..161G.doi:10.1086/504637.S2CID119476992.
^Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)".VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S.1: B/GCVS.Bibcode:2009yCat....102025S.
^Evans, D. S (1967). "The Revision of the General Catalogue of Radial Velocities".Determination of Radial Velocities and Their Applications.30: 57.Bibcode:1967IAUS...30...57E.
^abCifuentes, C.; Caballero, J. A.; Cortés-Contreras, M.; Montes, D.; Abellán, F. J.; Dorda, R.; Holgado, G.; Zapatero Osorio, M. R.; Morales, J. C.; Amado, P. J.; Passegger, V. M.; Quirrenbach, A.; Reiners, A.; Ribas, I.; Sanz-Forcada, J. (2020-10-01). "CARMENES input catalogue of M dwarfs. V. Luminosities, colours, and spectral energy distributions".Astronomy and Astrophysics.642: A115.arXiv:2007.15077.Bibcode:2020A&A...642A.115C.doi:10.1051/0004-6361/202038295.ISSN0004-6361.
^Kemmer, J.; Lafarga, M.; Fuhrmeister, B.; Shan, Y.; Schöfer, P.; Jeffers, S. V.; Caballero, J. A.; Quirrenbach, A.; Amado, P. J. (2025-04-11). "The CARMENES search for exoplanets around M dwarfs. Cluster analysis of signals from spectral activity indicators to search for shared periods".Astronomy and Astrophysics.697.arXiv:2504.08363.Bibcode:2025A&A...697A.225K.doi:10.1051/0004-6361/202347056.
^Van Den Bos, W. H. (1926). "The orbit and the masses of 40 Eridani BC".Bulletin of the Astronomical Institutes of the Netherlands.3: 128.Bibcode:1926BAN.....3..128V.
^White Dwarfs, E. Schatzman, Amsterdam: North-Holland, 1958., p. 1
^Hessman, F.V.; Dhillon, V.S.; Winget, D.E.; Schreiber, M.R.; Horne, K.; Marsh, T.R.; et al. (2010). "On the naming convention used for multiple star systems and extrasolar planets".arXiv:1012.0707 [astro-ph.SR].
