Epsilon Eridani (Latinized from ε Eridani), proper name Ran, is a star in the southern constellation of Eridanus. At a declination of −9.46°, it is visible from most of Earth's surface. Located at a distance of from the Sun, it has an apparent magnitude of 3.73, making it the third-closest individual star (or star system) visible to the naked eye.
The star is estimated to be about a billion years old. This relative youth gives Epsilon Eridani a higher level of magnetic activity than the Sun, with a stellar wind 30 times as strong. The star's rotation period is 11.2 days at the equator. Epsilon Eridani is smaller and less massive than the Sun, and has a lower level of elements heavier than helium. It is a main-sequence star of spectral class K2, with an effective temperature of about , giving it an orange hue. It is a candidate member of the Ursa Major moving group of stars, which share a similar motion through the Milky Way, implying these stars shared a common origin in an open cluster.
Periodic changes in Epsilon Eridani's radial velocity have yielded evidence of a giant planet orbiting it, designated Epsilon Eridani b. The discovery of the planet was initially controversial, but most astronomers now regard the planet as confirmed. In 2015 the planet was given the proper name AEgir. The Epsilon Eridani planetary system also includes a debris disc consisting of a Kuiper belt analogue at 70 au from the star and warm dust between about 3 au and 20 au from the star. The gap in the debris disc between 20 and 70 au implies the likely existence of outer planets in the system.
As one of the nearest Sun-like stars, Epsilon Eridani has been the target of several observations in the search for extraterrestrial intelligence. Epsilon Eridani appears in science fiction stories and has been suggested as a destination for interstellar travel. From Epsilon Eridani, the Sun would appear as a star in Serpens, with an apparent magnitude of 2.4.
Nomenclature ε Eridani, Latinised to Epsilon Eridani, is the star's Bayer designation. Despite being a relatively bright star, it was not given a proper name by early astronomers. It has several other catalogue designations. Upon its discovery, the planet was designated Epsilon Eridani b, following the usual designation system for extrasolar planets.
The planet and its host star were selected by the International Astronomical Union (IAU) as part of the NameExoWorlds competition for giving proper names to exoplanets and their host stars, for some systems that did not already have proper names. The process involved nominations by educational groups and public voting for the proposed names. In December 2015, the IAU announced the winning names were Ran for the star and AEgir for the planet. Those names had been submitted by the pupils of the 8th grade at Mountainside Middle School in Colbert, Washington, United States. Both names derive from Norse mythology: Rán is the goddess of the sea and Ægir, her husband, is the god of the ocean.
In 2016, the IAU organised a Working Group on Star Names (WGSN) to catalogue and standardise proper names for stars. In its first bulletin of July 2016, 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. Professional astronomers have mostly continued to refer to the star as Epsilon Eridani.
In Chinese, (), meaning Celestial Meadows, refers to an asterism consisting of ε Eridani, γ Eridani, δ Eridani, π Eridani, ζ Eridani, η Eridani, π Ceti, τ1 Eridani, τ2 Eridani, τ3 Eridani, τ4 Eridani, τ5 Eridani, τ6 Eridani, τ7 Eridani, τ8 Eridani and τ9 Eridani. 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, . Consequently, the Chinese name for ε Eridani itself is (, the Fourth [Star] of Celestial Meadows).
Observational history
Cataloguing Epsilon Eridani has been known to astronomers since at least the 2nd century AD, when Claudius Ptolemy (a Greek astronomer from Alexandria, Egypt) included it in his catalogue of more than a thousand stars. The catalogue was published as part of his astronomical treatise the Almagest. The constellation Eridanus was named by Ptolemy—, and Epsilon Eridani was listed as its thirteenth star. Ptolemy called Epsilon Eridani (here 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's magnitude as 3.
Epsilon Eridani was included in several star catalogues of medieval Islamic astronomical treatises, which were based on Ptolemy's catalogue: in Al-Sufi's Book of Fixed Stars, published in 964, Al-Biruni's Mas'ud Canon, published in 1030, and Ulugh Beg's Zij-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. Ulugh Beg carried out new measurements of Epsilon Eridani's coordinates in his observatory at Samarkand, 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).
In 1598 Epsilon Eridani was included in Tycho Brahe's star catalogue, republished in 1627 by Johannes Kepler as part of his Rudolphine Tables. This catalogue was based on Tycho Brahe's observations of 1577–1597, including those on the island of Hven at his observatories of Uraniborg and Stjerneborg. The sequence number of Epsilon Eridani in the constellation Eridanus was 10, and it was designated ; the meaning is the same as Ptolemy's description. Brahe assigned it magnitude 3.
Epsilon Eridani's Bayer designation was established in 1603 as part of the Uranometria, a star catalogue produced by German celestial cartographer Johann Bayer. His catalogue assigned letters from the Greek 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, the star is the tenth brightest in Eridanus. 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 as . Bayer assigned Epsilon Eridani magnitude 3.
In 1690 Epsilon Eridani was included in the star catalogue of Johannes Hevelius. Its sequence number in constellation Eridanus was 14, its designation was , and it was assigned magnitude 3 or 4 (sources differ). The star catalogue of English astronomer John Flamsteed, published in 1712, gave Epsilon Eridani the Flamsteed designation of 18 Eridani, because it was the eighteenth catalogued star in the constellation of Eridanus by order of increasing right ascension. In 1818 Epsilon Eridani was included in Friedrich Bessel's catalogue, based on James Bradley's observations from 1750 to 1762, and at magnitude 4. It also appeared in Nicolas Louis de Lacaille's catalogue of 398 principal stars, whose 307-star version was published in 1755 in the , and whose full version was published in 1757 in , Paris. In its 1831 edition by Francis Baily, Epsilon Eridani has the number 50. Lacaille assigned it magnitude 3.
In 1801 Epsilon Eridani was included in , 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. 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 of right ascension. Because every observation of each star was numbered and Epsilon Eridani was observed three times, it got three numbers: 6581, 6582 and 6583. (Today numbers from this catalogue are used with the prefix "Lalande", or "Lal".) Lalande assigned Epsilon Eridani magnitude 3. Also in 1801 it was included in the catalogue of Johann 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. In 1814 Giuseppe 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 7,000 stars were grouped into 24 hours (0–23). Epsilon Eridani is number 89 in hour 3. Piazzi assigned it magnitude 4. In 1918 Epsilon Eridani appeared in the Henry Draper Catalogue with the designation HD 22049 and a preliminary spectral classification of K0.
Detection of proximity Based on observations between 1800 and 1880, Epsilon Eridani was found to have a large proper motion across the celestial sphere, which was estimated at three arcseconds per year (angular velocity). This movement implied it was relatively close to the Sun, making it a star of interest for the purpose of stellar 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. From 1881 to 1883, American astronomer William L. Elkin used a heliometer at the Royal 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 of was calculated. By 1917, observers had refined their parallax estimate to 0.317 arcseconds. The modern value of 0.3109 arcseconds is equivalent to a distance of about .
Circumstellar discoveries
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 gravitational perturbations in its position. This claim was refuted in 1993 by Wulff-Dieter Heintz and the false detection was blamed on a systematic error in the photographic plates.
Launched in 1983, the space telescope IRAS detected infrared emissions from stars near to the Sun, including an excess infrared emission from Epsilon Eridani. The observations indicated a disk of fine-grained cosmic dust was orbiting the star; this debris 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.