Periodic changes in Epsilon Eridani'sradial velocity haveyielded evidence of agiant planet orbiting it, designatedEpsilon Eridani b.[20] The discovery of the planet was initially controversial,[21] but most astronomers now regard the planet as confirmed. In 2015 the planet was given theproper nameAEgir [sic].[22] The Epsilon Eridaniplanetary system also includes adebris disc consisting of aKuiper belt analogue at 70 au from the star and warm dust between about 3 au and 20 au from the star.[23][24] The gap in the debris disc between 20 and 70 au implies the likely existence of outer planets in the system.
The planet and its host star were selected by theInternational Astronomical Union (IAU) as part of theNameExoWorlds competition for giving proper names to exoplanets and their host stars, for some systems that did not already have proper names.[29][30] The process involved nominations by educational groups and public voting for the proposed names.[31] In December 2015, the IAU announced the winning names wereRan for the star andAEgir [sic] for the planet.[22] Those names had been submitted by the pupils of the8th Grade at Mountainside Middle School inColbert, Washington, United States. Both names derive fromNorse mythology:Rán is the goddess of the sea andÆgir, her husband, is the god of the ocean.[32]
In 2016, the IAU organised aWorking Group on Star Names (WGSN)[33] to catalogue and standardise proper names for stars. In its first bulletin of July 2016,[34] the WGSN explicitly recognised the names of exoplanets and their host stars that were produced by the competition. Epsilon Eridani is now listed as Ran in the IAU Catalog of Star Names.[17] Professional astronomers have mostly continued to refer to the star as Epsilon Eridani.[35]
Above, the northern section of the Eridanus constellation is delineated in green, whileOrion is shown in blue. Below, an enlarged view of the region in the white box shows the location of Epsilon Eridani at the intersection of the two lines.
Epsilon Eridani has been known to astronomers since at least the 2nd century AD, whenClaudius Ptolemy (aGreek astronomer fromAlexandria,Egypt) included it in his catalogue of more than a thousand stars. The catalogue was published as part of his astronomical treatise theAlmagest. The constellationEridanus was named by Ptolemy –Ποταμού (Ancient Greek for 'River'), and Epsilon Eridani was listed as its thirteenth star. Ptolemy called Epsilon Eridaniό τών δ προηγούμενος (Ancient Greek for 'a foregoing of the four') (hereδ is the number four). This refers to a group of four stars in Eridanus:γ,π,δ and ε (10th–13th in Ptolemy's list). ε is the most western of these, and thus the first of the four in the apparent daily motion of the sky from east to west. Modern scholars of Ptolemy's catalogue designate its entry as"P 784" (in order of appearance) and"Eri 13". Ptolemy described the star'smagnitude as 3.[38][39]
Epsilon Eridani was included in several star catalogues ofmedieval Islamic astronomical treatises, which were based on Ptolemy's catalogue: inAl-Sufi'sBook of Fixed Stars, published in 964,Al-Biruni'sMas'ud Canon, published in 1030, andUlugh Beg'sZij-i Sultani, published in 1437. Al-Sufi's estimate of Epsilon Eridani's magnitude was 3. Al-Biruni quotes magnitudes from Ptolemy and Al-Sufi (for Epsilon Eridani he quotes the value 4 for both Ptolemy's and Al-Sufi's magnitudes; original values of both these magnitudes are 3). Its number in order of appearance is 786.[40] Ulugh Beg carried out new measurements of Epsilon Eridani's coordinates inhis observatory atSamarkand, and quotes magnitudes from Al-Sufi (3 for Epsilon Eridani). The modern designations of its entry in Ulugh Beg's catalogue are"U 781" and"Eri 13" (the latter is the same as Ptolemy's catalogue designation).[38][39]
In 1598 Epsilon Eridani was included inTycho Brahe's star catalogue, republished in 1627 byJohannes Kepler as part of hisRudolphine Tables. This catalogue was based on Tycho Brahe's observations of 1577–1597, including those on the island ofHven at his observatories ofUraniborg andStjerneborg. The sequence number of Epsilon Eridani in the constellation Eridanus was 10, and it was designatedQuae omnes quatuor antecedit (Latin for 'which precedes all four'); the meaning is the same as Ptolemy's description. Brahe assigned it magnitude 3.[38][41]
Epsilon Eridani'sBayer designation was established in 1603 as part of theUranometria, a star catalogue produced by German celestial cartographerJohann Bayer. His catalogue assigned letters from theGreek alphabet to groups of stars belonging to the same visual magnitude class in each constellation, beginning with alpha (α) for a star in the brightest class. Bayer made no attempt to arrange stars by relative brightness within each class. Thus, although Epsilon is the fifth letter in the Greek alphabet,[42] the star is thetenth-brightest in Eridanus.[43] In addition to the letter ε, Bayer had given it the number 13 (the same as Ptolemy's catalogue number, as were many of Bayer's numbers) and described it asDecima septima (Latin for 'the seventeenth').[b] Bayer assigned Epsilon Eridani magnitude 3.[44]
In 1690 Epsilon Eridani was included in the star catalogue ofJohannes Hevelius. Its sequence number in constellation Eridanus was 14, its designation wasTertia (Latin for 'the third'), and it was assigned magnitude 3 or 4 (sources differ).[38][45] The star catalogue of English astronomerJohn Flamsteed, published in 1712, gave Epsilon Eridani theFlamsteed designation of 18 Eridani, because it was the eighteenth catalogued star in the constellation of Eridanus by order of increasingright ascension.[4] In 1818 Epsilon Eridani was included inFriedrich Bessel's catalogue, based onJames Bradley's observations from 1750–1762, and at magnitude 4.[46] It also appeared inNicolas Louis de Lacaille's catalogue of 398 principal stars, whose 307-star version was published in 1755 in theEphémérides des Mouvemens Célestes, pour dix années, 1755–1765,[47] and whose full version was published in 1757 inAstronomiæ Fundamenta, Paris.[48] In its 1831 edition byFrancis Baily, Epsilon Eridani has the number 50.[49] Lacaille assigned it magnitude 3.[47][48][49]
In 1801 Epsilon Eridani was included inHistoire céleste française,Joseph Jérôme Lefrançois de Lalande's catalogue of about 50,000 stars, based on his observations of 1791–1800, in which observations are arranged in time order. It contains three observations of Epsilon Eridani.[50] In 1847, a new edition of Lalande's catalogue was published by Francis Baily, containing the majority of its observations, in which the stars were numbered in order ofright ascension. Because every observation of each star was numbered and Epsilon Eridani was observed three times, it got three numbers: 6581, 6582 and 6583.[51] (Today numbers from this catalogue are used with the prefix "Lalande", or "Lal".[52]) Lalande assigned Epsilon Eridani magnitude 3.[50][51] Also in 1801 it was included in the catalogue ofJohann Bode, in which about 17,000 stars were grouped into 102 constellations and numbered (Epsilon Eridani got the number 159 in the constellation Eridanus). Bode's catalogue was based on observations of various astronomers, including Bode himself, but mostly on Lalande's and Lacaille's (for the southern sky). Bode assigned Epsilon Eridani magnitude 3.[53] In 1814Giuseppe Piazzi published the second edition of his star catalogue (its first edition was published in 1803), based on observations during 1792–1813, in which more than 7000 stars were grouped into 24 hours (0–23). Epsilon Eridani is number 89 in hour 3. Piazzi assigned it magnitude 4.[54] In 1918 Epsilon Eridani appeared in theHenry Draper Catalogue with the designation HD 22049 and a preliminary spectral classification of K0.[55]
Based on observations between 1800 and 1880, Epsilon Eridani was found to have a largeproper motion across thecelestial sphere, which was estimated at threearcseconds per year (angular velocity).[56] This movement implied it was relatively close to the Sun,[57] making it a star of interest for the purpose ofstellar parallax measurements. This process involves recording the position of Epsilon Eridani as Earth moves around the Sun, which allows a star's distance to be estimated.[56] From 1881 to 1883, American astronomerWilliam L. Elkin used aheliometer at theRoyal Observatory at the Cape of Good Hope, South Africa, to compare the position of Epsilon Eridani with two nearby stars. From these observations, a parallax of0.14 ± 0.02 arcseconds was calculated.[58][59] By 1917, observers had refined their parallax estimate to 0.317 arcseconds.[60] The modern value of 0.3109 arcseconds is equivalent to a distance of about 10.50 light-years (3.22 pc).[1]
Submillimeter wavelength image of a ring of dust particles around Epsilon Eridani (above centre). The brightest areas indicate the regions with the highest concentrations of dust.
Based on apparent changes in the position of Epsilon Eridani between 1938 and 1972,Peter van de Kamp proposed that an unseen companion with an orbital period of 25 years was causing gravitationalperturbations in its position.[61] This claim was refuted in 1993 byWulff-Dieter Heintz and the false detection was blamed on a systematic error in thephotographic plates.[62]
Launched in 1983, thespace telescopeIRAS detectedinfrared emissions from stars near to the Sun,[63] including anexcess infrared emission from Epsilon Eridani.[64] The observations indicated a disk of fine-grainedcosmic dust was orbiting the star;[64] thisdebris disk has since been extensively studied. Evidence for a planetary system was discovered in 1998 by the observation of asymmetries in this dust ring. The clumping in the dust distribution could be explained by gravitational interactions with a planet orbiting just inside the dust ring.[65]
In 1987, the detection of an orbiting planetary object was announced by Bruce Campbell, Gordon Walker and Stephenson Yang.[66][67] From 1980 to 2000, a team of astronomers led byArtie P. Hatzes maderadial velocity observations of Epsilon Eridani, measuring theDoppler shift of the star along the line of sight. They found evidence of a planet orbiting the star with a period of about seven years.[20] Although there is a high level of noise in the radial velocity data due to magnetic activity in itsphotosphere,[68] any periodicity caused by this magnetic activity is expected to show a strong correlation with variations inemission lines of ionized calcium (theCa II H and K lines). Because no such correlation was found, a planetary companion was deemed the most likely cause.[69] This discovery was supported byastrometric measurements of Epsilon Eridani made between 2001 and 2003 with theHubble Space Telescope, which showed evidence forgravitational perturbation of Epsilon Eridani by a planet.[8]
In 1960, physicistsPhilip Morrison andGiuseppe Cocconi proposed thatextraterrestrial civilisations might be using radio signals for communication.[70]Project Ozma, led by astronomerFrank Drake, used theTatel Telescope to search for such signals from the nearbySun-like stars Epsilon Eridani andTau Ceti. The systems were observed at theemission frequency of neutral hydrogen, 1,420 MHz (21 cm). No signals of intelligent extraterrestrial origin were detected.[71] Drake repeated the experiment in 2010, with the same negative result.[70]Despite this lack of success, Epsilon Eridani made its way into science fiction literature and television shows for many years following news of Drake's initial experiment.[72]
InHabitable Planets for Man, a 1964RAND Corporation study by space scientist Stephen H. Dole, the probability of ahabitable planet being in orbit around Epsilon Eridani were estimated at 3.3%. Among the known nearby stars, it was listed with the 14 stars that were thought most likely to have a habitable planet.[73]
William I. McLaughlin proposed a new strategy in the search for extraterrestrial intelligence (SETI) in 1977. He suggested that widely observable events such asnova explosions might be used by intelligent extraterrestrials to synchronise the transmission and reception of their signals. This idea was tested by theNational Radio Astronomy Observatory in 1988, which used outbursts ofNova Cygni 1975 as the timer. Fifteen days of observation showed no anomalous radio signals coming from Epsilon Eridani.[74]
Based on its nearby location, Epsilon Eridani was among the target stars forProject Phoenix, a 1995microwave survey for signals from extraterrestrial intelligence.[77] The project had checked about 800 stars by 2004 but had not yet detected any signals.[78]
Illustration of the relative sizes of Epsilon Eridani (left) and the Sun (right)
At a distance of 10.50 ly (3.22 parsecs), Epsilon Eridani is the 13th-nearest known star (and ninth nearest solitary star orstellar system) to the Sun as of 2014.[9] Its proximity makes it one of the most studied stars of itsspectral type.[79] Epsilon Eridani is located in the northern part of the constellation Eridanus, about 3° east of the slightly brighter starDelta Eridani. With a declination of −9.46°, Epsilon Eridani can be viewed from much of Earth's surface, at suitable times of year. Only to the north oflatitude 80° N is it permanently hidden below the horizon.[80] Theapparent magnitude of 3.73 can make it difficult to observe from an urban area with the unaided eye, because the night skies over cities are obscured bylight pollution.[81]
Epsilon Eridani has an estimated mass of 0.82solar masses[10][11] and a radius of 0.738solar radii.[12] It shines with a luminosity of only 0.34solar luminosities.[82] The estimatedeffective temperature is 5,084 K.[83] With a stellar classification of K2 V, it is the second-nearestK-type main-sequence star (afterAlpha Centauri B).[9] Since 1943 thespectrum of Epsilon Eridani has served as one of the stable anchor points by which other stars are classified.[84] Itsmetallicity, the fraction of elements heavier thanhelium, is slightly lower than the Sun's.[85] In Epsilon Eridani'schromosphere, a region of the outer atmosphere just above the light emittingphotosphere, the abundance of iron is estimated at 74% of the Sun's value.[85] The proportion oflithium in the atmosphere is five times less than that in the Sun.[86]
Epsilon Eridani's K-type classification indicates that the spectrum has relatively weakabsorption lines from absorption by hydrogen (Balmer lines) but strong lines of neutral atoms and singlyionizedcalcium (Ca II). Theluminosity class V (dwarf) is assigned to stars that are undergoingthermonuclear fusion of hydrogen in their core. For a K-type main-sequence star, this fusion is dominated by theproton–proton chain reaction, in which a series of reactions effectively combines four hydrogen nuclei to form a helium nucleus. The energy released by fusion is transported outward from the core throughradiation, which results in no net motion of the surrounding plasma. Outside of this region, in the envelope, energy is carried to the photosphere byplasma convection, where it then radiates into space.[87]
Epsilon Eridani has a higher level ofmagnetic activity than the Sun, and thus the outer parts of its atmosphere (thechromosphere andcorona) are more dynamic. The average magnetic field strength of Epsilon Eridani across the entire surface is(1.65±0.30)×10−2tesla,[88] which is more than forty times greater than the(5–40) × 10−5 T magnetic-field strength in the Sun's photosphere.[89] The magnetic properties can be modelled by assuming that regions with amagnetic flux of about 0.14 T randomly cover approximately 9% of the photosphere, whereas the remainder of the surface is free of magnetic fields.[90] The overall magnetic activity of Epsilon Eridani shows co-existing2.95±0.03 and12.7±0.3 year activity cycles.[86] Assuming that its radius does not change over these intervals, the long-term variation in activity level appears to produce a temperature variation of 15 K, which corresponds to a variation invisual magnitude (V) of 0.014.[91]
The magnetic field on the surface of Epsilon Eridani causes variations in thehydrodynamic behaviour of the photosphere. This results in greaterjitter duringmeasurements of its radial velocity. Variations of15 m s−1 were measured over a 20 year period, which is much higher than themeasurement uncertainty of3 m s−1. This makes interpretation of periodicities in the radial velocity of Epsilon Eridani, such as those caused by an orbiting planet, more difficult.[68]
Alight curve for Epsilon Eridani, showing averages of theb and y band magnitudes between 2014 and 2021.[14] The inset shows the periodic variation over a 12.3-day rotational period.[92]
Epsilon Eridani is classified as aBY Draconis variable because it has regions of higher magnetic activity that move into and out of the line of sight as it rotates.[6] Measurement of thisrotational modulation suggests that its equatorial region rotates with an average period of 11.2 days,[15] which is less than half of the rotation period of the Sun. Observations have shown that Epsilon Eridani varies as much as 0.050 in V magnitude due tostarspots and other short-term magnetic activity.[92]Photometry has also shown that the surface of Epsilon Eridani, like the Sun, is undergoingdifferential rotation i.e. the rotation period at equator differs from that at highlatitude. The measured periods range from 10.8 to 12.3 days.[91][c] Theaxial tilt of Epsilon Eridani toward the line of sight from Earth is highly uncertain: estimates range from 24° to 72°.[15]
The high levels of chromospheric activity, strong magnetic field, and relatively fast rotation rate of Epsilon Eridani are characteristic of a young star.[93] Most estimates of the age of Epsilon Eridani place it in the range from 200 million to 800 million years.[18] The low abundance of heavy elements in the chromosphere of Epsilon Eridani usually indicates an older star, because theinterstellar medium (out of which stars form) is steadily enriched by heavier elements produced by older generations of stars.[94] This anomaly might be caused by adiffusion process that has transported some of the heavier elements out of the photosphere and into a region below Epsilon Eridani'sconvection zone.[95]
TheX-ray luminosity of Epsilon Eridani is about2×1028erg·s–1 (2×1021W). It is more luminous in X-rays than the Sun atpeak activity. The source for this strong X-ray emission is Epsilon Eridani's hot corona.[96][97] Epsilon Eridani's corona appears larger and hotter than the Sun's, with a temperature of3.4×106 K, measured from observation of the corona's ultraviolet and X-ray emission.[98] It displays a cyclical variation in X-ray emission that is consistent with the magnetic activity cycle.[99]
Thestellar wind emitted by Epsilon Eridani expands until it collides with the surroundinginterstellar medium of diffuse gas and dust, resulting in a bubble of heated hydrogen gas (anastrosphere, the equivalent of theheliosphere that surrounds the Sun). Theabsorption spectrum from this gas has been measured with theHubble Space Telescope, allowing the properties of the stellar wind to be estimated.[98] Epsilon Eridani's hot corona results in a mass loss rate in Epsilon Eridani's stellar wind that is 30 times higher than the Sun's. This stellar wind generates the astrosphere that spans about 8,000 au (0.039 pc) and contains abow shock that lies 1,600 au (0.0078 pc) from Epsilon Eridani. At its estimated distance from Earth, this astrosphere spans 42 arcminutes, which is wider than the apparent size of the full Moon.[100]
Epsilon Eridani has a highproper motion, moving −0.976 arcseconds per year inright ascension (the celestial equivalent of longitude) and 0.018 arcseconds per year indeclination (celestial latitude), for a combined total of 0.962 arcseconds per year.[1][d] The star has a radial velocity of +15.5 km/s (35,000 mph) (away from the Sun).[102] Thespace velocity components of Epsilon Eridani in thegalactic co-ordinate system are(U, V, W) =(−3, +7, −20) km/s, which means that it is travelling within theMilky Way at a meangalactocentric distance of 28.7 kly (8.79 kiloparsecs) from the core along an orbit that has aneccentricity of 0.09.[103] Theposition and velocity of Epsilon Eridani indicate that it may be a member of theUrsa Major Moving Group, whose members share a common motion through space. This behaviour suggests that the moving group originated in anopen cluster that has since diffused.[104] The estimated age of this group is500±100 million years,[105] which lies within the range of the age estimates for Epsilon Eridani.
During the past million years, three stars are believed to have come within 7 ly (2.1 pc) of Epsilon Eridani. The most recent and closest of these encounters was withKapteyn's Star, which approached to a distance of about 3 ly (0.92 pc) roughly 12,500 years ago. Two more distant encounters were withSirius andRoss 614. None of these encounters are thought to have been close enough to affect the circumstellar disk orbiting Epsilon Eridani.[106]
Epsilon Eridani made its closest approach to the Sun about 105,000 years ago, when they were separated by 7 ly (2.1 pc).[107] Based upon a simulation of close encounters with nearby stars, the binary star systemLuyten 726-8, which includes thevariable starUV Ceti, will encounter Epsilon Eridani in approximately 31,500 years at a minimum distance of about 0.9 ly (0.29 parsecs). They will be less than 1 ly (0.3 parsecs) apart for about 4,600 years. If Epsilon Eridani has anOort cloud, Luyten 726-8 could gravitationallyperturb some of itscomets with longorbital periods.[108][unreliable source?]
Image of epsilon Eridani's main belt taken by the Atacama Large Millimeter/submillimeter Array (ALMA) at a wavelength of 1.3mm. The star is seen at the centre and two other point sources (one coincident with the belt) are unrelated background galaxies.[24]
An infrared excess around Epsilon Eridani was detected by IRAS[64] indicating the presence of circumstellar dust. Observations with theJames Clerk Maxwell Telescope (JCMT) at awavelength of 850 μm show an extended flux of radiation out to anangular radius of 35 arcseconds around Epsilon Eridani, resolving the debris disc for the first time. Higher resolution images have since been taken with theAtacama Large Millimeter Array, showing that the belt is located 70 au from the star with a width of just 11 au.[112][24] The disc is inclined 33.7° from face-on, making it appear elliptical.
Dust and possibly water ice from this belt migrates inward because of drag from the stellar wind and a process by which stellar radiation causes dust grains to slowly spiral toward Epsilon Eridani, known as thePoynting–Robertson effect.[113] At the same time, these dust particles can be destroyed through mutual collisions. The time scale for all of the dust in the disk to be cleared away by these processes is less than Epsilon Eridani's estimated age. Hence, the current dust disk must have been created by collisions or other effects of larger parent bodies, and the disk represents a late stage in the planet-formation process. It would have required collisions between 11 Earth masses' worth of parent bodies to have maintained the disk in its current state over its estimated age.[109]
Comparison of the planets and debris belts in the Solar System to the Epsilon Eridani system. At the top is the asteroid belt and the inner planets of the Solar System. Second from the top is the proposed inner asteroid belt and planet b of Epsilon Eridani. The lower illustrations show the corresponding features for the two stars' outer systems.
The disk contains an estimated mass of dust equal to a sixth of the mass of the Moon, with individual dust grains exceeding 3.5 μm in size at a temperature of about 55 K. This dust is being generated by the collision of comets, which range up to 10 to 30 km in diameter and have a combined mass of 5 to 9 times that of Earth. This is similar to the estimated 10 Earth masses in the primordial Kuiper belt.[114][115] The disk around Epsilon Eridani contains less than2.2 × 1017 kg ofcarbon monoxide. This low level suggests a paucity of volatile-bearing comets and icyplanetesimals compared to the Kuiper belt.[116]
The JCMT images show signs of clumpy structure in the belt that may be explained by gravitational perturbation from a planet, dubbed Epsilon Eridani c. The clumps in the dust are theorised to occur at orbits that have an integer resonance with the orbit of the suspected planet. For example, the region of the disk that completes two orbits for every three orbits of a planet is in a 3:2orbital resonance.[117] The planet proposed to cause these perturbations is predicted to have a semimajor axis of between 40 and 50 au.[118][119][24] However, the brightest clumps have since been identified as background sources and the existence of the remaining clumps remains debated.[120]
Dust is also present closer to the star. Observations from NASA'sSpitzer Space Telescope suggest that Epsilon Eridani actually has two asteroid belts and a cloud ofexozodiacal dust. The latter is an analogue of thezodiacal dust that occupies the plane of theSolar System. One belt sits at approximately the same position as the one in the Solar System, orbiting at a distance of3.00 ± 0.75 au from Epsilon Eridani, and consists ofsilicate grains with a diameter of 3 μm and a combined mass of about 1018 kg. If the planet Epsilon Eridani b exists then this belt is unlikely to have had a source outside the orbit of the planet, so the dust may have been created by fragmentation and cratering of larger bodies such asasteroids.[121] The second, denser belt, most likely also populated by asteroids, lies between the first belt and the outer comet disk. The structure of the belts and the dust disk suggests that more than two planets in the Epsilon Eridani system are needed to maintain this configuration.[109][122]
In an alternative scenario, the exozodiacal dust may be generated in the outer belt. This dust is then transported inward past the orbit of Epsilon Eridani b. When collisions between the dust grains are taken into account, the dust will reproduce the observed infrared spectrum and brightness. Outside the radius of icesublimation, located beyond 10 au from Epsilon Eridani where the temperatures fall below 100 K, the best fit to the observations occurs when a mix of ice andsilicate dust is assumed. Inside this radius, the dust must consist of silicate grains that lackvolatiles.[113]
The inner region around Epsilon Eridani, from a radius of 2.5 AU inward, appears to be clear of dust down to the detection limit of the 6.5 mMMT telescope. Grains of dust in this region are efficiently removed by drag from the stellar wind, while the presence of a planetary system may also help keep this area clear of debris. Still, this does not preclude the possibility that an inner asteroid belt may be present with a combined mass no greater than the asteroid belt in the Solar System.[123]
Artist's impression, showing two asteroid belts and a planet orbiting Epsilon Eridani
As one of the nearest Sun-like stars, Epsilon Eridani has been the target of many attempts to search for planetary companions.[20][18] Its chromospheric activity and variability mean that finding planets with theradial velocity method is difficult, because the stellar activity may create signals that mimic the presence of planets.[124] Searches for exoplanets around Epsilon Eridani withdirect imaging have been unsuccessful.[69][125]
Infrared observation has shown there are no bodies of three or moreJupiter masses in this system, out to at least a distance of 500 au from the host star.[18] Planets with similar masses and temperatures as Jupiter should be detectable by Spitzer at distances beyond 80 au. One roughly Jupiter-sized long-period planet has been detected and characterized by both the radial velocity and astrometry methods.[126] Planets more than 150% as massive as Jupiter can be ruled out at the inner edge of the debris disk at 30–35 au.[16] Imaging with theJames Webb Space Telescope rules out the presence of any planets more massive than Saturn orbiting at over 16 au.[127]
Referred to asEpsilon Eridani b, this planet was announced in 2000, but the discovery remained controversial over roughly the next two decades. A comprehensive study in 2008 called the detection "tentative" and described the proposed planet as "long suspected but still unconfirmed".[109] Many astronomers believed the evidence is sufficiently compelling that they regard the discovery as confirmed.[18][113][121][125] The discovery was questioned in 2013 because a search program atLa Silla Observatory did not confirm it exists.[128] Further studies since 2018 have gradually reaffirmed the planet's existence through a combination of radial velocity and astrometry.[129][130][131][132][126]
Artist's impression of Epsilon Eridani b orbiting within a zone that has been cleared of dust. Around the planet are conjectured rings and moons.
Published sources remain in disagreement as to the planet's basic parameters. Recent values for its orbital period range from 7.3 to 7.6 years,[126] estimates of the size of its elliptical orbit—thesemimajor axis—range from 3.38 au to 3.53 au,[133][134] and approximations of itsorbital eccentricity range from 0.055 to 0.26.[126]
Initially, the planet's mass was unknown, but a lower limit could be estimated based on the orbital displacement of Epsilon Eridani. Only the component of the displacement along the line of sight to Earth was known, which yields a value for the formulam sin i, wherem is the mass of the planet andi is theorbital inclination. Estimates for the value ofm sini ranged from 0.60Jupiter masses to 1.06 Jupiter masses,[133][134] which sets the lower limit for the mass of the planet (because thesine function has a maximum value of 1). Takingm sini in the middle of that range at 0.78, and estimating the inclination at 30° as was suggested byHubble astrometry, this yields a value of1.55 ± 0.24 Jupiter masses for the planet's mass.[8] More recent astrometric studies have found lower masses, ranging from 0.63 to 0.78 Jupiter masses.[126]
Of all the measured parameters for this planet, the value for orbital eccentricity is the most uncertain. The eccentricity of 0.7 suggested by some older studies[8] is inconsistent with the presence of the proposed asteroid belt at a distance of 3 au. If the eccentricity was this high, the planet would pass through the asteroid belt and clear it out within about ten thousand years. If the belt has existed for longer than this period, which appears likely, it imposes an upper limit on Epsilon Eridani b's eccentricity of about 0.10–0.15.[121][122] If the dust disk is instead being generated from the outer debris disk, rather than from collisions in an asteroid belt, then no constraints on the planet's orbital eccentricity are needed to explain the dust distribution.[113]
Epsilon Eridani is a target for planet finding programs because it has properties that allow an Earth-like planet to form. Although this system was not chosen as a primary candidate for the now-canceledTerrestrial Planet Finder, it was a target star for NASA's proposedSpace Interferometry Mission to search for Earth-sized planets.[135] The proximity, Sun-like properties and suspected planets of Epsilon Eridani have also made it the subject of multiple studies on whether aninterstellar probe can be sent to Epsilon Eridani.[75][76][136]
The orbital radius at which the stellar flux from Epsilon Eridani matches thesolar constant—where the emission matches the Sun's output at the orbital distance of the Earth—is 0.61 au.[137] That is within the maximumhabitable zone of a conjectured Earth-like planet orbiting Epsilon Eridani, which currently stretches from about 0.5 to 1.0 au. As Epsilon Eridani ages over a period of 20 billion years, the net luminosity will increase, causing this zone to slowly expand outward to about 0.6–1.4 au.[138] The presence of a large planet with a highlyelliptical orbit in proximity to Epsilon Eridani's habitable zone reduces the likelihood of aterrestrial planet having a stable orbit within the habitable zone.[139]
A young star such as Epsilon Eridani can produce large amounts ofultraviolet radiation that may be harmful to life, but on the other hand it is a cooler star than the Sun and so produces less ultraviolet radiation to start with.[21][140] The orbital radius where the UV flux matches that on the early Earth lies at just under 0.5 au.[21] Because that is actually slightly closer to the star than the habitable zone, this has led some researchers to conclude there is not enough energy from ultraviolet radiation reaching into the habitable zone for life to ever get started around the young Epsilon Eridani.[140]
^From Epsilon Eridani, the Sun would appear on the diametrically opposite side of the sky at the coordinates RA=15h 32m 55.84496s, Dec=+09° 27′ 29.7312″, which is located nearAlpha Serpentis. The absolute magnitude of the Sun is 4.83,[27] so, at a distance of 3.212 parsecs, the Sun would have an apparent magnitude:,[28] assuming negligibleextinction (AV) for a nearby star.
^This is because Bayer designated 21 stars in the northern part of Eridanus by preceding along the 'river' from east to west, starting from β (Supra pedem Orionis in flumine, prima, meaningabovethe foot ofOrion in the river, the first) to the twenty-first, σ (Vigesima prima, that isthe twenty-first). Epsilon Eridani was the seventeenth in this sequence. These 21 stars are: β, λ, ψ, b, ω, μ, c, ν, ξ, ο (two stars), d, A, γ, π, δ, ε, ζ, ρ, η, σ.[44]
^abcCousins, A. W. J. (1984), "Standardization of Broadband Photometry of Equatorial Standards",South African Astronomical Observatory Circulars,8: 59,Bibcode:1984SAAOC...8...59C.
^abSoubiran, C.; Creevey, O. L.; Lagarde, N.; Brouillet, N.; Jofré, P.; Casamiquela, L.; Heiter, U.; Aguilera-Gómez, C.; Vitali, S.; Worley, C.; de Brito Silva, D. (February 1, 2024). "Gaia FGK benchmark stars: Fundamental Teff and log g of the third version".Astronomy and Astrophysics.682: A145.arXiv:2310.11302.Bibcode:2024A&A...682A.145S.doi:10.1051/0004-6361/202347136.ISSN0004-6361.
^abRoettenbacher, Rachael M.; Cabot, Samuel H. C.; Fischer, Debra A.; Monnier, John D.; Henry, Gregory W.; Harmon, Robert O.; Korhonen, Heidi; Brewer, John M.; Llama, Joe; Petersburg, Ryan R.; Zhao, Lily L.; Kraus, Stefan; Le Bouquin, Jean-Baptiste; Anugu, Narsireddy; Davies, Claire L.; Gardner, Tyler; Lanthermann, Cyprien; Schaefer, Gail; Setterholm, Benjamin; Clark, Catherine A.; Jorstad, Svetlana G.; Kuehn, Kyler; Levine, Stephen (January 2022), "EXPRES. III. Revealing the Stellar Activity Radial Velocity Signature of ϵ Eridani with Photometry and Interferometry",The Astronomical Journal,163 (1): 19,arXiv:2110.10643,Bibcode:2022AJ....163...19R,doi:10.3847/1538-3881/ac3235,S2CID239049996.
^Di Folco, E.; et al. (November 2004), "VLTI near-IR interferometric observations of Vega-like stars. Radius and age of α PsA, β Leo, β Pic, ε Eri and τ Cet",Astronomy and Astrophysics,426 (2):601–617,Bibcode:2004A&A...426..601D,doi:10.1051/0004-6361:20047189.
^abcBuccino, A. P.; Mauas, P. J. D.; Lemarchand, G. A. (June 2003), R. Norris; F. Stootman (eds.), "UV Radiation in Different Stellar Systems",Bioastronomy 2002: Life Among the Stars, Proceedings of IAU Symposium No. 213, vol. 213, San Francisco: Astronomical Society of the Pacific, p. 97,Bibcode:2004IAUS..213...97B.
^abSu, Kate Y. L.; et al. (2017), "The Inner 25 au Debris Distribution in the ϵ Eri System",The Astronomical Journal,153 (5): 226,arXiv:1703.10330,Bibcode:2017AJ....153..226S,doi:10.3847/1538-3881/aa696b,We found that the 24 and 35 μm emission is consistent with the in situ dust distribution produced either by one planetesimal belt at 3–21 au (e.g., Greaves et al. 2014) or by two planetesimal belts at 1.5–2 au (or 3–4 au) and 8–20 au (e.g., a slightly modified form of the proposal in Backman et al. 2009) ... Any planetesimal belt in the inner region of the epsilon Eri system must be located inside 2 au and/or outside 5 au to be dynamically stable with the assumed epsilon Eri b.
^"The Exoworlds",NameExoWorlds, International Astronomical Union, archived fromthe original on December 31, 2016, retrievedSeptember 5, 2015.
^"The Process",NameExoWorlds, International Astronomical Union, August 7, 2015, archived fromthe original on August 15, 2015, retrievedSeptember 5, 2015.
^abcdBaily, Francis (1843), "The Catalogues of Ptolemy, Ulugh Beigh, Tycho Brahe, Halley, Hevelius, Deduced from the Best Authorities. With Various Notes and Corrections, and a Preface to Each Catalogue. To Which is Added the Synonym of each Star, in the Catalogues of Flamsteed of Lacaille, as far as the same can be ascertained",Memoirs of the Royal Astronomical Society,13: 1,Bibcode:1843MmRAS..13....1B.(Epsilon Eridani: for Ptolemy's catalogue see page 60, for Ulugh Beg's – page 109, for Tycho Brahe's – page 156, for Hevelius' – page 209).
^Bessel, Friedrich Wilhelm (1818). "Fundamenta astronomiae pro anno MDCCLV deducta ex observationibus viri incomparabilis James Bradley in specula astronomica Grenovicensi per annos 1750–1762 institutis". Frid. Nicolovius.Google Books id:UHRYAAAAcAAJ. Page with Epsilon Eridani:158.
^abLacaille, Nicolas Louis de. (1755). "Ephemerides des mouvemens celestes, pour dix années, depuis 1755 jusqu'en 1765, et pour le meridien de la ville de Paris". Paris.Google Books id:CGHtdxdcc5UC.(Epsilon Eridani: see page LV of the "Introduction").
^Bode, Johann Elert (1801). "Algemaine Beschreibung u. Nachweisung der gestine nebst Verzeichniss der gerarden Aufsteigung u. Abweichung von 17240 Sternen Doppelsternen Nobelflocken u. Sternhaufen". Berlin: Beym Verfasser.Bibcode:1801abun.book.....B.Google Books id:NUlRAAAAcAAJ.(List of observers and description of the catalogue: see page 32 of the "Introduction". List of constellations: see page 96).(Epsilon Eridani: see page 71).
^Piazzi, Giuseppe. (1814). "Praecipuaram stellarum inerranthium positiones mediae ineunte saeculo 19. EX observationibus habilis in specula panormitana AB anno 1792 AD annum 1813". Palermo: Tip. Militare.Bibcode:1814psip.book.....P.Google Books id:c40RAAAAYAAJ.(Epsilon Eridani: see page 22).
^Heintz, W. D. (March 1992), "Photographic astrometry of binary and proper-motion stars. VII",The Astronomical Journal,105 (3):1188–1195,Bibcode:1993AJ....105.1188H,doi:10.1086/116503. See the note for BD −9°697 on page 1192.
^James E., Hesser (December 1987), "Dominion Astrophysical Observatory, Victoria, British Columbia",Quarterly Journal of the Royal Astronomical Society,28: 510,Bibcode:1987QJRAS..28..510..
^Campbell, Bruce; Walker, G. A. H.; Yang, S. (August 15, 1988), "A search for substellar companions to solar-type stars",Astrophysical Journal, Part 1,331:902–921,Bibcode:1988ApJ...331..902C,doi:10.1086/166608.
^abMarcy, Geoffrey W.; et al. (August 7–11, 2000), A. Penny (ed.), "Planetary Messages in the Doppler Residuals (Invited Review)",Planetary Systems in the Universe, Proceedings of IAU Symposium No. 202, vol. 202, Manchester, United Kingdom, pp. 20–28,Bibcode:2004IAUS..202...20M.
^Henry, T.; et al. (August 16–20, 1993), "The current state of target selection for NASA's high resolution microwave survey",Progress in the Search for Extraterrestrial Life, Astronomical Society of the Pacific Conference Series, vol. 74, Santa Cruz, California:Astronomical Society of the Pacific, pp. 207–218,Bibcode:1995ASPC...74..207H.
^Karttunen, Hannu; Oja, H. (2007),Fundamental astronomy (5th ed.), Heidelberg, Germany: Springer, pp. 209–213,247–249,ISBN978-3-540-34143-7.
^Rüedi, I.; Solanki, S. K.; Mathys, G.; Saar, S. H. (February 1997), "Magnetic field measurements on moderately active cool dwarfs",Astronomy and Astrophysics,318:429–442,Bibcode:1997A&A...318..429R.
^Wang, Y.-M.; Sheeley, N. R. Jr. (July 2003), "Modeling the Sun's Large-Scale Magnetic Field during the Maunder Minimum",The Astrophysical Journal,591 (2):1248–1256,Bibcode:2003ApJ...591.1248W,doi:10.1086/375449.
^Valenti, Jeff A.; Marcy, Geoffrey W.; Basri, Gibor (February 1995), "Infrared zeeman analysis of Epsilon Eridani",The Astrophysical Journal,439 (2):939–956,Bibcode:1995ApJ...439..939V,doi:10.1086/175231.
^abGray, David F.; Baliunas, Sallie L. (March 1995), "Magnetic activity variations of Epsilon Eridani",The Astrophysical Journal,441 (1):436–442,Bibcode:1995ApJ...441..436G,doi:10.1086/175368.
^abFrey, Gary J.; et al. (November 1991), "The rotation period of Epsilon Eri from photometry of its starspots",The Astrophysical Journal,102 (5):1813–1815,Bibcode:1991AJ....102.1813F,doi:10.1086/116005.
^Drake, Jeremy J.; Smith, Geoffrey (August 1993), "The fundamental parameters of the chromospherically active K2 dwarf Epsilon Eridani",The Astrophysical Journal,412 (2):797–809,Bibcode:1993ApJ...412..797D,doi:10.1086/172962.
^Rocha-Pinto, H. J.; et al. (June 2000), "Chemical enrichment and star formation in the Milky Way disk. I. Sample description and chromospheric age-metallicity relation",Astronomy and Astrophysics,358:850–868,arXiv:astro-ph/0001382,Bibcode:2000A&A...358..850R.
^Schmitt, J. H. M. M.; et al. (February 1996), "The extreme-ultraviolet spectrum of the nearby K Dwarf ε Eridani",Astrophysical Journal,457: 882,Bibcode:1996ApJ...457..882S,doi:10.1086/176783.
^Birney, D. Scott; González, Guillermo; Oesper, David (2006),Observational astronomy (2nd ed.), Cambridge, U.K.: Cambridge University Press, p. 75,ISBN0-521-85370-2.
^Evans, D. S. (June 20–24, 1966), Batten, Alan Henry; Heard, John Frederick (eds.), "The revision of the general catalogue of radial velocities",Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30, vol. 30, University of Toronto:International Astronomical Union, p. 57,Bibcode:1967IAUS...30...57E.
^King, Jeremy R.; et al. (April 2003), "Stellar kinematic groups. II. A reexamination of the membership, activity, and age of the Ursa Major group",The Astronomical Journal,125 (4):1980–2017,Bibcode:2003AJ....125.1980K,doi:10.1086/368241.
^Deltorn, J.-M.; Greene, P. (May 16, 2001), "Search for nemesis encounters with Vega, epsilon Eridani, and Fomalhaut", in Jayawardhana, Ray; Greene, Thoas (eds.),Young Stars Near Earth: Progress and Prospects, Astronomical Society of the Pacific Conference Series, vol. 244, San Francisco, CA: Astronomical Society of the Pacific, pp. 227–232,arXiv:astro-ph/0105284,Bibcode:2001ASPC..244..227D,ISBN1-58381-082-X.
^Booth, Mark; Dent, William R. F.; Jordán, Andrés; Lestrade, Jean-François; Hales, Antonio S.; Wyatt, Mark C.; Casassus, Simon; Ertel, Steve; Greaves, Jane S.; Kennedy, Grant M.; Matrà, Luca; Augereau, Jean-Charles; Villard, Eric (May 4, 2017)."The Northern arc of ε Eridani's Debris Ring as seen by ALMA".Monthly Notices of the Royal Astronomical Society.469 (3). Oxford University Press (OUP):3200–3212.doi:10.1093/mnras/stx1072.hdl:10150/625481.ISSN0035-8711.
^Liu, Wilson M.; et al. (March 2009), "Observations of Main-Sequence Stars and Limits on Exozodical Dust with Nulling Interferometry",The Astrophysical Journal,693 (2):1500–1507,Bibcode:2009ApJ...693.1500L,doi:10.1088/0004-637X/693/2/1500.
^Makarov, Valeri V.; Zacharias, Norbert; Finch, Charles T. (2021), "Looking for Astrometric Signals below 20 m s−1: A Jupiter-mass Planet Signature in ε Eri",Research Notes of the AAS,5 (6): 155,arXiv:2107.01090,Bibcode:2021RNAAS...5..155M,doi:10.3847/2515-5172/ac0f59,We conclude that the newest astrometric results confirm the existence of a long-period exoplanet orbiting ε Eri....The results are consistent with the previously reported planet epsEri-b of approximately Jupiter mass and a period of several years.
^Kitzmann, D.; et al. (February 2010), "Clouds in the atmospheres of extrasolar planets. I. Climatic effects of multi-layered clouds for Earth-like planets and implications for habitable zones",Astronomy and Astrophysics,511: 511A66.1–511A66.14,arXiv:1002.2927,Bibcode:2010A&A...511A..66K,doi:10.1051/0004-6361/200913491,S2CID56345031. See table 3.
^Jones, Barrie W.; Underwood, David R.; Sleep, P. Nick (April 22–25, 2003), "The stability of the orbits of Earth-mass planets in and near the habitable zones of known exoplanetary systems",Proceedings of the Conference on Towards Other Earths: DARWIN/TPF and the Search for Extrasolar Terrestrial Planets,539, Heidelberg, Germany: Dordrecht,D. Reidel Publishing Co:625–630,arXiv:astro-ph/0305500,Bibcode:2003ESASP.539..625J,ISBN92-9092-849-2.{{citation}}: CS1 maint: work parameter with ISBN (link)
^abBuccino, A. P.; Lemarchand, G. A.; Mauas, P. J. D. (2006), "Ultraviolet radiation constraints around the circumstellar habitable zones",Icarus,183 (2):491–503,arXiv:astro-ph/0512291,Bibcode:2006Icar..183..491B,doi:10.1016/j.icarus.2006.03.007,ISSN0019-1035,S2CID2241081,In near the 41% stars of the sample: HD19994, 70 Vir, 14 Her, 55 Cnc, 47 UMa, ε Eri and HD3651, there is no coincidence at all between the UV region and the HZ...the traditional HZ would not be habitable following the UV criteria exposed in this work.
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