Epsilon Eridani (ε Eri / ε Eridani) is a main-sequence K2 class star. It is the closest star in the constellation Eridanus, as well as the third closest star visible to the naked eye. This star has an estimated age of less than a billion years. Because of its relative youth, this star has a higher level of magnetic activity than the Sun and its stellar wind is an estimated 30 times as strong. The rotation period is a relatively rapid 11.1 days, although this varies by latitude. Epsilon Eridani is both smaller and less massive than the Sun, with a lower level of metallicity, or elements with a higher atomic number than helium.
In 2006, a planet was confirmed in orbit around this star. This planet completes an orbit every 2502 days at a mean distance of 3.4 Astronomical Units (505 million kilometers) from the star. As of 2008, Epsilon Eridani is the nearest star to the Sun that is known to have a planet. The star also has an orbiting disk of dust, and perturbations in this material may be caused by an unconfirmed second planet.
This star is located in the northern part of the constellation Eridanus, about 3° east of the slightly brighter star Delta Eridani. With a declination of −9.45°, Epsilon Eridani can be viewed from much of the Earth's surface. Only to the north of latitude 80° N is it permanently hidden below the horizon. The apparent magnitude of 3.73 can make this star difficult to observe from an urban area with the unaided eye, as the night skies over cities are illuminated by light pollution.
The Bayer designation for this star was established in 1603 as part of the Uranometria, a star catalog produced by Johann Bayer. Epsilon is the fifth letter in the Greek alphabet, and it was assigned to the fifth brightest star in the constellation of Eridanus. The preliminary version of the star catalog by John Flamsteed, published in 1712, gave this star the Flamsteed designation 18 Eridani. In 1918 this star appeared in the Henry Draper Catalogue with the designation HD 22049 and a preliminary spectral classification of K0.
Based on observations between 1800 and 1880, Epsilon Eridani was found to have a large proper motion, which at the time was estimated at three arcseconds annually. This implied a relatively close proximity to the Sun, making it a star of interest for the purpose of trigonometric parallax measurements. From 1881–3, William L. Elkin made a series of heliometric measurements from the Royal Observatory at the Cape of Good Hope, South Africa. As a result of these observations, a preliminary parallax of 0.14 ± 0.02 arcseconds was computed for Epsilon Eridani. By 1917, observers had refined their parallax estimate to 0.317 arcseconds, which is quite close to the modern value of 0.3107 arcseconds. This parallax is equivalent to a distance of about 10.5 light years, making Epsilon Eridani the 13th nearest known star (and ninth nearest stellar system) to the Sun.
The OZMA Project, headed by Dr. Frank Drake, was intended to search for signals from an extraterrestrial intelligence using a radio telescope at Green Bank, West Virginia. The target stars chosen for this project were the two nearest solitary Sun-like stars, Epsilon Eridani and Tau Ceti. However, no signal of extraterrestrial origin was detected. (A false signal was detected on April 8, 1960 that originated from a high-flying aircraft.) In 1995, based on its location within 7.2 pc, Epsilon Eridani was among the target stars of Project Phoenix, a microwave survey for signals from extraterrestrial intelligence. By 2004 Project Phoenix had checked about 800 stars, but had not yet detected an unimpeachable signal.
Based on perturbations in the position of Epsilon Eridani between 1938 and 1972, it was suspected that the star had an unseen companion with an orbital period of 25 years. However, this claim was refuted in 1993. Radial velocity observations between 1980 and 2000 then provided convincing evidence of a planetary companion orbiting the star with a period of about seven years. The evidence for a planetary system was further strengthened by the discovery of a ring of dust orbiting the star in 1998. This dust showed concentrations that could be explained by interaction with a nearby planet. In 2006, the existence of a planet with a 6.9 year orbit was confirmed using the Hubble Space Telescope.
Epsilon Eridani has an estimated 85% of the Sun's mass and 84% of the Sun's radius, but has only 28% of its luminosity. It is the second nearest spectral class K star after α Centauri B. Compared to the Sun, this star is considered slightly low in the abundance of elements with atomic numbers higher than helium. Epsilon Eridani has only about 74% of the Sun's abundance of iron in its chromosphere.
The chromosphere of Epsilon Eridani is more magnetically active than the Sun. Approximately 9% of the deep photosphere is found to have a magnetic field with a strength about 0.14 Tesla. The overall magnetic activity level of this star is irregular but it may vary with a five year period. Assuming that the radius of the star does not change over this interval, then the variation in activity level appears to produce a temperature variation of 15 K, which corresponds to a magnitude variation of 0.014.
Rotational modulation of the magnetic activity shows that the equator of the star rotates with a 11.10 ± 0.03 day period, or more than double the rotation period of the Sun. Stars that vary in magnitude because of magnetic activity coupled with rotation are classified as BY Draconis variables. Photometry has also shown that the surface of Epsilon Eridani, like the Sun, is undergoing differential rotation. That is, the rotation period at the surface varies by latitude, with the measured periods ranging from 10.8 to 12.3 days. The axial tilt of this star remains uncertain, with estimates ranging from a low of 24° up to 72°.
The high level of chromospheric activity, strong magnetic field and the relatively high rotation rate indicate that this is a young star. Computer models give an estimated age of 700–850 million years, although the actual age may be a low as 500 million or as high as a billion years. However, the somewhat low abundance of heavy elements is characteristic of a much older star. This anomaly might be caused by a diffusion process that has transported some of the helium and heavier elements out of the photosphere to a region below the star's outer convection zone.
Relative to the Sun, the outer atmosphere of Epsilon Eridani appears both larger and hotter. This is caused by a 30-fold higher mass loss rate from the star's stellar wind. The wind is generating an astrosphere that spans about 8,000 AU and a bow shock that lies 1,600 AU from the star. At the star's estimated distance from the Earth, this astrosphere would span an angle of 42 arcminutes, which is wider than the appearance of a full Moon.
The space velocity components of Epsilon Eridani are U = −3, V = +7 and W = −20 km/s. It is orbiting within the Milky Way at a mean galactocentric distance of 8.8 kpc and an orbital eccentricity of 0.09. During the past million years, three stars are believed to have come within two parsecs of Epsilon Eridani. The most recent encounter was with Kapteyn's Star, which approached within about a parsec about 12,500 years ago. None of these encounters are thought to have affected the circumstellar disk. Epsilon Eridani made its closest approach to the Solar System about 105,000 years ago, when the two stars were separated by seven light years.
As Epsilon Eridani is one of the nearest solar-type stars to our Sun, many attempts to search for orbiting planets have been made. However, the star's high activity and variability means that finding planets with the radial velocity method is difficult, and stellar activity may mimic the presence of planets.
There is one confirmed planet in the system, and one unconfirmed. A 2500 day-period Jupiter-like planet Epsilon Eridani b orbits at 3.39 AU in one of the most eccentric orbit of any extrasolar planets — 0.7. A possible 280 year-period low-mass planet Epsilon Eridani c orbits at 40 AU in a less eccentric orbit — 0.3.
No bodies of 3 or more Jupiter masses exist in this system.
In the 1964 RAND Corporation study, Habitable Planets for Man by Stephen R. Dole, the odds of a habitable planet in orbit around Epsilon Eridani were estimated as 3.3%. Among the known stars within 22 light years, it was listed with the 14 stars that were thought most likely to have a habitable planet. The maximum habitable zone for Epsilon Eridani currently stretches from about 0.5–1.0 Astronomical Units (A.U.), where an A.U. is equal to the average distance between Earth and the Sun. As the star ages over a period of 20 billion years, this zone will slowly expand outward to about 0.6–1.4 A.U. However, the presence of a large planet with a highly elliptical orbit in proximity to the habitable zone of the star reduces the likelihood of a terrestrial planet having a stable orbit within the habitable zone.
The presence of an outer planet in orbit around Epsilon Eridani would have a perturbing effect on cometary bodies within the dust ring. Some of these bodies would fall toward the inner part of the system, and could cross any planetary orbits within 1 AU of the star. Thus, a terrestrial planet would be subject to bombardment similar to what happened to the Earth during its first 600 million years with the Late Heavy Bombardment.
Epsilon Eridani is a target for planet finding programs because it has the properties to allow an Earthlike planet to form. Although this system was not chosen as a primary candidate for the now-cancelled Terrestrial Planet Finder, it is a target star for NASA's proposed Space Interferometry Mission that will search for Earth-sized planets.
The asymmetrical structure of the dust belt may be explained as the gravitational perturbation by a planet. The clumps in the dust occur at orbits that have an integer resonance with the orbit of the suspected planet. Thus, for example, the region of the disk that completes two orbits for every three orbits of a planet are in 3:2 resonance. With computer simulations, the ring morphology can be reproduced by the capture of dust particles in 5:3 and 3:2 orbital resonances with a planet that has an orbital eccentricity of about 0.3.
The dust disk contains approximately 1000 times more dust than is present in the inner system around our Sun, which may mean it has about 1000 times as much cometary material as our solar system. The dust has an estimated mass equal to a sixth of mass of the Moon. 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 the mass of the Earth. This is similar to the estimated 10 Earth masses in the Kuiper Belt.
Within 35 AU of the star the dust is depleted, which may mean that the system has formed planets which have cleared out the dust in this region. This is consistent with currently accepted models of the inner solar system, and so there may be terrestrial planets around the star.