Epsilon Eridani

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ε Eridani

Location of ε Eridani (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Eridanus
Right ascension	03h 32m 55.84496s
Declination	−09° 27′ 29.7312″
Apparent magnitude (V)	3.736
Characteristics
Spectral type	K2V
Apparent magnitude (B)	4.61
Apparent magnitude (V)	3.73
Apparent magnitude (J)	2.228 ±0.298
Apparent magnitude (H)	1.880 ± 0.276
Apparent magnitude (K)	1.776 ± 0.286
U−B colour index	+0.571
B−V colour index	+0.887
Variable type	BY Dra
Astrometry
Radial velocity (Rv)	+15.5 ± 0.9 km/s
Proper motion (μ)	RA: −975.17 mas/yr Dec.: 19.49 mas/yr
Parallax (π)	311.37 ± 0.1 mas
Distance	10.475 ± 0.003 ly (3.212 ± 0.001 pc)
Absolute magnitude (MV)	6.19
Details
Mass	0.82 ± 0.02 M☉
Radius	0.735 ± 0.005 R☉
Luminosity	0.34 L☉
Surface gravity (log g)	4.30 ± 0.08 cgs
Temperature	5,084 ± 5.9 K
Metallicity [Fe/H]	−0.13 ± 0.04 dex
Rotation	11.2 days
Rotational velocity (v sin i)	2.4 ± 0.5 km/s
Age	400–800 Myr
Other designations
Ran, 18 Eridani, BD -09°697, GCTP 742.00, GJ 144, HD 22049, HIP 16537, HR 1084, LHS 1557, SAO 130564, WDS 03330-0928.

Epsilon Eridani (ε Eridani, abbreviated Epsilon Eri, ε Eri), also named Ran (/ˈrɑːn/), is a star in the southern constellation of Eridanus, at a declination of 9.46° south of the celestial equator. This allows it to be visible from most of Earth's surface. At a distance of 10.5 light-years (3.2 parsecs) from the Sun, it has an apparent magnitude of 3.73. It is the third-closest individual star or star system visible to the unaided eye.

The star is estimated to be less than a billion years old. Because of its relative youth, Epsilon Eridani has a higher level of magnetic activity than the present-day Sun, with a stellar wind 30 times as strong. Its rotation period is 11.2 days at the equator. Epsilon Eridani is smaller and less massive than the Sun, and has a comparatively lower level of elements heavier than helium. It is a main-sequence star of spectral class K2, which means that energy generated at the core through nuclear fusion of hydrogen is emitted from the surface at a temperature of about 5,000 K (8,500 °F), giving it an orange hue.

The Bayer designation ε Eridani (Latinised as Epsilon Eridani) was established in 1603 by Johann Bayer. It may be a member of the Ursa Major Moving Group of stars that share a similar motion through the Milky Way, implying these stars shared a common origin in an open cluster. Its nearest neighbour, the binary star system Luyten 726-8, will have a close encounter with Epsilon Eridani in approximately 31,500 years when they will be separated by about 0.93 ly (0.29 pc).

The motion of Epsilon Eridani along the line of sight to Earth, known as the radial velocity, has been regularly observed for more than twenty years. Periodic changes in its value yielded evidence of a giant planet orbiting the star, making it one of the closest star systems with a candidate exoplanet. The discovery of the planet has been controversial because of the amount of background noise in the radial velocity data, particularly in the early observation, but many astronomers now regard the planet as confirmed. In 2016 it was given the alternative name AEgir.

The Epsilon Eridani system also includes two belts of rocky asteroids: at about 3 AU and 20 AU from the star. The orbital structure could be maintained by a hypothetical second planet, which if confirmed would be Epsilon Eridani c. Epsilon Eridani hosts an extensive outer debris disk of remnant planetesimals left over from the system's formation.

As one of the nearest Sun-like stars with a planet, 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 2.4-magnitude star in Serpens.

Nomenclature

ε Eridani (Latinised to Epsilon Eridani) is the system'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 a 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.

The names at that point remained unofficial, but 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. It is not yet clear whether professional astronomers will generally use the new name, or continue to refer to the star as Epsilon Eridani; both are now equally valid. 

In Chinese, 天苑 (Tiān Yuàn), 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. Consequently, ε Eridani itself is known as 天苑四 (Tiān Yuàn sì, the Fourth [Star] of Celestial Meadows.)

Observational history

Above, the northern section of the Eridanus constellation is delineated in green, while Orion 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.

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 (Ancient Greek: Ποταμού, River), and Epsilon Eridani was listed as its thirteenth star. Ptolemy called Epsilon Eridani ό τών δ προηγούμενος, Greek for a foregoing of the four (here δ 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 Quae omnes quatuor antecedit, Latin for which precedes all four; 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 Decima septima, Latin for the seventeenth. 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 Tertia (the third), 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 1750–1762, 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 Ephémérides des Mouvemens Célestes, pour dix années, 1755–1765, and whose full version was published in 1757 in Astronomiæ Fundamenta, 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 Histoire 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. 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 7000 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 0.14 ± 0.02 arcseconds 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 10.50 light-years (3.22 pc).

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.

In 1987, the detection of an orbiting planetary object was announced by Bruce Campbell, Gordon Walker and Stephenson Yang. From 1980 to 2000, a team of astronomers led by Artie P. Hatzes made radial velocity observations of Epsilon Eridani, measuring the Doppler 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. Although there is a high level of noise in the radial velocity data due to magnetic activity in its photosphere, any periodicity caused by this magnetic activity is expected to show a strong correlation with variations in emission lines of ionized calcium (the Ca II H and K lines). Because no such correlation was found, a planetary companion was deemed the most likely cause. This discovery was supported by astrometric measurements of Epsilon Eridani made between 2001 and 2003 with the Hubble Space Telescope, which showed evidence for gravitational perturbation of Epsilon Eridani by a planet.

Astrophysicist Alice C. Quillen and her student Stephen Thorndike performed computer simulations of the structure of the dust disk around Epsilon Eridani. Their model suggested that the clumping of the dust particles could be explained by the presence of a second planet in an eccentric orbit, which they announced in 2002.

SETI and proposed exploration

In 1960, physicists Philip Morrison and Giuseppe Cocconi proposed that extraterrestrial civilisations might be using radio signals for communication. Project Ozma, led by astronomer Frank Drake, used the Tatel Telescope to search for such signals from the nearby Sun-like stars Epsilon Eridani and Tau Ceti. The systems were observed at the emission frequency of neutral hydrogen, 1,420 MHz (21 cm). No signals of intelligent extraterrestrial origin were detected. Drake repeated the experiment in 2010, with the same negative result. 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.

In Habitable Planets for Man, a 1964 RAND Corporation study by space scientist Stephen H. Dole, the probability of a habitable 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.

William I. McLaughlin proposed a new strategy in the search for extraterrestrial intelligence (SETI) in 1977. He suggested that widely observable events such as nova explosions might be used by intelligent extraterrestrials to synchronise the transmission and reception of their signals. This idea was tested by the National Radio Astronomy Observatory in 1988, which used outbursts of Nova Cygni 1975 as the timer. Fifteen days of observation showed no anomalous radio signals coming from Epsilon Eridani.

Because of the proximity and Sun-like properties of Epsilon Eridani, in 1985 physicist and author Robert L. Forward considered the system as a plausible target for interstellar travel. The following year, the British Interplanetary Society suggested Epsilon Eridani as one of the targets in its Project Daedalus study. The system has continued to be among the targets of such proposals, such as Project Icarus in 2011.

Based on its nearby location, Epsilon Eridani was among the target stars for Project Phoenix, a 1995 microwave survey for signals from extraterrestrial intelligence. The project had checked about 800 stars by 2004 but had not yet detected any signals.

Properties

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 or stellar system) to the Sun as of 2014. Its proximity makes it one of the most studied stars of its spectral type. Epsilon Eridani is located in the northern part of the constellation Eridanus, about 3° east of the slightly brighter star Delta Eridani. With a declination of −9.46°, Epsilon Eridani can be viewed from much of Earth's surface, at suitable times of year. Only to the north of latitude 80° N is it permanently hidden below the horizon. The apparent 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 by light pollution.

Epsilon Eridani has an estimated mass of 0.82 solar masses and a radius of 0.74 solar radii. It shines with a luminosity of only 0.34 solar luminosities. The estimated effective temperature is 5,084 K. With a stellar classification of K2 V, it is the second-nearest K-type main-sequence star (after Alpha Centauri B). Since 1943 the spectrum of Epsilon Eridani has served as one of the stable anchor points by which other stars are classified. Its metallicity, the fraction of elements heavier than helium, is slightly lower than the Sun's. In Epsilon Eridani's chromosphere, a region of the outer atmosphere just above the light emitting photosphere, the abundance of iron is estimated at 74% of the Sun's value. The proportion of lithium in the atmosphere is five times less than that in the Sun.

Epsilon Eridani's K-type classification indicates that the spectrum has relatively weak absorption lines from absorption by hydrogen (Balmer lines) but strong lines of neutral atoms and singly ionized calcium (Ca II). The luminosity class V (dwarf) is assigned to stars that are undergoing thermonuclear fusion of hydrogen in their core. For a K-type main-sequence star, this fusion is dominated by the proton–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 through radiation, which results in no net motion of the surrounding plasma. Outside of this region, in the envelope, energy is carried to the photosphere by plasma convection, where it then radiates into space.

Magnetic activity

Epsilon Eridani has a higher level of magnetic activity than the Sun, and thus the outer parts of its atmosphere (the chromosphere and corona) are more dynamic. The average magnetic field strength of Epsilon Eridani across the entire surface is (1.65 ± 0.30) × 10⁻² tesla, which is more than forty times greater than the (5–40) × 10⁻⁵ T magnetic-field strength in the Sun's photosphere. The magnetic properties can be modeled by assuming that regions with a magnetic flux of about 0.14 T randomly cover approximately 9% of the photosphere, whereas the remainder of the surface is free of magnetic fields. The overall magnetic activity of Epsilon Eridani shows co-existing 2.95±0.03 and 12.7±0.3 year activity cycles. 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 in visual magnitude (V) of 0.014.

The magnetic field on the surface of Epsilon Eridani causes variations in the hydrodynamic behaviour of the photosphere. This results in greater jitter during measurements of its radial velocity. Variations of 15 m s⁻¹ were measured over a 20 year period, which is much higher than the measurement uncertainty of 3 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.

Epsilon Eridani is classified as a BY Draconis variable because it has regions of higher magnetic activity that move into and out of the line of sight as it rotates. Measurement of this rotational modulation suggests that its equatorial region rotates with an average period of 11.2 days, 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 to starspots and other short-term magnetic activity. Photometry has also shown that the surface of Epsilon Eridani, like the Sun, is undergoing differential rotation i.e. the rotation period at equator differs from that at high latitude. The measured periods range from 10.8 to 12.3 days. The axial tilt of Epsilon Eridani toward the line of sight from Earth is highly uncertain: estimates range from 24° to 72°.

The high levels of chromospheric activity, strong magnetic field, and relatively fast rotation rate of Epsilon Eridani are characteristic of a young star. Most estimates of the age of Epsilon Eridani place it in the range from 200 million to 800 million years. The low abundance of heavy elements in the chromosphere of Epsilon Eridani usually indicates an older star, because the interstellar medium (out of which stars form) is steadily enriched by heavier elements produced by older generations of stars. This anomaly might be caused by a diffusion process that has transported some of the heavier elements out of the photosphere and into a region below Epsilon Eridani's convection zone.

The X-ray luminosity of Epsilon Eridani is about 2 × 10²⁸ ergs/s (2 × 10²¹ W). It is more luminous in X-rays than the Sun at peak activity. The source for this strong X-ray emission is Epsilon Eridani's hot corona. Epsilon Eridani's corona appears larger and hotter than the Sun's, with a temperature of 3.4 × 10⁶ K, measured from observation of the corona's ultraviolet and X-ray emission.

The stellar wind emitted by Epsilon Eridani expands until it collides with the surrounding interstellar medium of diffuse gas and dust, resulting in a bubble of heated hydrogen gas (an astrosphere, the equivalent of the heliosphere that surrounds the Sun). The absorption spectrum from this gas has been measured with the Hubble Space Telescope, allowing the properties of the stellar wind to be estimated. 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 a bow 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.

Kinematics

Epsilon Eridani has a high proper motion, moving −0.976 arcseconds per year in right ascension (the celestial equivalent of longitude) and 0.018 arcseconds per year in declination (celestial latitude), for a combined total of 0.962 arcseconds per year. The star has a radial velocity of +15.5 km/s (35,000 mph) (away from the Sun). The space velocity components of Epsilon Eridani in the galactic co-ordinate system are (U, V, W) = (−3, +7, −20) km/s, which means that it is travelling within the Milky Way at a mean galactocentric distance of 28.7 kly (8.79 kiloparsecs) from the core along an orbit that has an eccentricity of 0.09. The velocity and heading of Epsilon Eridani indicate that it may be a member of the Ursa Major Moving Group, whose members share a common motion through space. This behaviour suggests that the moving group originated in an open cluster that has since diffused. The estimated age of this group is 500±100 million years, 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 with Kapteyn's Star, which approached to a distance of about 3 ly (0.92 pc) roughly 12,500 years ago. Two more distant encounters were with Sirius and Ross 614. None of these encounters are thought to have been close enough to affect the circumstellar disk orbiting Epsilon Eridani.

Epsilon Eridani made its closest approach to the Sun about 105,000 years ago, when they were separated by 7 ly (2.1 pc). Based upon a simulation of close encounters with nearby stars, the binary star system Luyten 726-8, which includes the variable star UV 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 an Oort cloud, Luyten 726-8 could gravitationally perturb some of its comets with long orbital periods.

Planetary system

The Epsilon Eridani planetary system

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
Asteroid belt	3 AU				—	—
b (AEgir) (unconfirmed)	1.55 ± 0.24 MJ	3.38–3.50	2,502–2,630	0.25–0.702	—	—
Asteroid belt	20 AU				—	—
c (unconfirmed)	0.1 MJ	40?	102,270	0.3	—	—
Dust disk	35–100 AU				—	—

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.

 

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.

Dust disk

Observations with the James Clerk Maxwell Telescope at a wavelength of 850 μm show an extended flux of radiation out to an angular radius of 35 arcseconds around Epsilon Eridani. The peak emission occurs at an angular radius of 18 arcseconds, which corresponds to a radius of about 60 AU. The highest level of emission occurs over the radius 35–75 AU from Epsilon Eridani and is substantially reduced inside 30 AU. This emission is interpreted as coming from a young analogue of the Solar System's Kuiper belt: a compact dusty disk structure surrounding Epsilon Eridani. From Earth, this belt is viewed at an inclination of roughly 25° to the line of sight.

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 the Poynting–Robertson effect. 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.

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. The disk around Epsilon Eridani contains less than 2.2 × 10¹⁷ kg of carbon monoxide. This low level suggests a paucity of volatile-bearing comets and icy planetesimals compared to the Kuiper belt.

The clumpy structure of the dust belt may be explained by gravitational perturbation from a planet, dubbed Epsilon Eridani b. The clumps in the dust 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 3:2 orbital resonance. In 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. Alternatively, the clumpiness may have been caused by collisions between minor planets known as plutinos.

Observations from NASA's Spitzer Space Telescope suggest that Epsilon Eridani actually has two asteroid belts and a cloud of exozodiacal dust. The latter is an analogue of the zodiacal dust that occupies the plane of the Solar System. One belt sits at approximately the same position as the one in the Solar System, orbiting at a distance of 3.00 ± 0.75 AU from Epsilon Eridani, and consists of silicate 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 as asteroids. 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.

In an alternative scenario, the exozodiacal dust may be generated in an outer belt that is orbiting between 55 and 90 AU from Epsilon Eridani and has an assumed mass of 10⁻³ times the mass of Earth. 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 ice sublimation, 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 and silicate dust is assumed. Inside this radius, the dust must consist of silicate grains that lack volatiles.

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 m MMT 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.

Possible planets

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. Its chromospheric activity and variability mean that finding planets with the radial velocity method is difficult, because the stellar activity may create signals that mimic the presence of planets. Attempts at direct imaging of potential exoplanets have proven unsuccessful to date.

Infrared observation has shown there are no bodies of three or more Jupiter masses in this system, out to at least a distance of 500 AU from the host star. Planets with similar masses and temperatures as Jupiter should be detectable by Spitzer at distances beyond 80 AU, but none has been discovered in this range. Planets more than 150% as massive as Jupiter can be ruled out at the inner edge of the debris disk at 30–35 AU.

Planet b

Referred to as Epsilon Eridani b, this planet was announced in 2000, but the discovery has remained controversial. A comprehensive study in 2008 called the detection "tentative" and described the proposed planet as "long suspected but still unconfirmed". Many astronomers believed the evidence is sufficiently compelling that they regard the discovery as confirmed. As of 2013, the discovery remains in doubt because a search program at La Silla Observatory did not confirm it exists.

Artist's impression of the proposed planet Epsilon Eridani b orbiting within a zone that has been cleared of dust. Near the bottom centre is a conjectured moon.

Published sources remain in disagreement as to the proposed planet's basic parameters. Values for its orbital period range from 6.85 to 7.2 years. Estimates of the size of its elliptical orbit—the semimajor axis—range from 3.38 AU to 3.50 AU and approximations of its orbital eccentricity range from 0.25 ± 0.23 to 0.702 ± 0.039.

The true mass of this planet remains unknown, but it can be estimated based on the displacement effect of the planet's gravity on Epsilon Eridani. Only the component of the displacement along the line of sight to Earth is known, which yields a value for the formula m sin i, where m is the mass of the planet and i is the orbital inclination. Estimates for the value of m sin i range from 0.60 Jupiter masses to 1.06 Jupiter masses, which sets the lower limit for the mass of the planet (because the sine function has a maximum value of 1). Taking m sin i in the middle of that range at 0.78, and estimating the inclination at 30°, this yields the frequently cited value of 1.55 ± 0.24 Jupiter masses for the planet's mass.

Of all the measured parameters for this planet, the value for orbital eccentricity is the most uncertain. The frequently cited value of 0.7 for Epsilon Eridani b's eccentricity is inconsistent with the presence of the proposed asteroid belt at a distance of 3 AU. If the eccentricity was actually 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. 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.

Planet c

Artist's impression of the unconfirmed second planet as seen from a hypothetical moon. The distant Epsilon Eridani is visible on the left, surrounded by a faint disk of dust particles.

Computer simulations of the dusty disk orbiting Epsilon Eridani suggest that the shape of the disk may be explained by the presence of a second planet, tentatively dubbed Epsilon Eridani c. Clumping in the dust disk may occur because dust particles are being trapped in orbits that have resonant orbital periods with a planet in an eccentric orbit. The postulated Epsilon Eridani c would orbit at a distance of 40 AU, with an eccentricity of 0.3 and a period of 280 years. The inner cavity of the disk may be explained by the presence of additional planets. Current models of planet formation cannot easily explain how a planet could have been created at this distance from Epsilon Eridani. The disk is expected to have dissipated long before a giant planet could have formed. Instead, the planet may have formed at an orbital distance of about 10 AU, then migrated outward because of gravitational interaction with the disc or with other planets in the system.

Potential habitability

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-canceled Terrestrial Planet Finder, it was a target star for NASA's proposed Space Interferometry Mission to search for Earth-sized planets. The proximity, Sun-like properties and suspected planets of Epsilon Eridani have also made it the subject of multiple studies on whether an interstellar probe can be sent to Epsilon Eridani.

The orbital radius at which the stellar flux from Epsilon Eridani matches the solar constant—where the emission matches the Sun's output at the orbital distance of the Earth—is 0.61 astronomical units (AU). That is within the maximum habitable 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. The presence of a large planet with a highly elliptical orbit in proximity to Epsilon Eridani's habitable zone reduces the likelihood of a terrestrial planet having a stable orbit within the habitable zone.

A young star such as Epsilon Eridani can produce large amounts of ultraviolet radiation that may be harmful to life. The orbital radius where the UV flux matches that on the early Earth lies at just under 0.5 AU. Epsilon Eridani's proximity, Sun-like properties and suspected planets have made it a destination for interstellar travel in science fiction stories.

From Epsilon Eridani, the Sun would appear as a 2.4-magnitude star in Serpens.

Tau Ceti

From Wikipedia, the free encyclopedia

Tau Ceti

Tau Ceti (circled) in the south of the constellation Cetus.
Observation data Epoch J2000      Equinox J2000
Constellation	Cetus
Pronunciation	/ˌtaʊ ˈsiːtaɪ/
Right ascension	01h 44m 04.08338s
Declination	−15° 56′ 14.9262″
Apparent magnitude (V)	3.50 ± 0.01
Characteristics
Spectral type	G8 V
U−B color index	+0.21
B−V color index	+0.72
Astrometry
Radial velocity (Rv)	−16.68±0.05 km/s
Proper motion (μ)	RA: −1721.05 mas/yr Dec.: +854.16 mas/yr
Parallax (π)	273.96 ± 0.17 mas
Distance	11.905 ± 0.007 ly (3.650 ± 0.002 pc)
Absolute magnitude (MV)	5.69±0.01
Details
Mass	0.783±0.012 M☉
Radius	0.793±0.004 R☉
Luminosity	0.52±0.03 L☉
Luminosity (visual, LV)	0.45 L☉
Surface gravity (log g)	4.4 cgs
Temperature	5,344±50 K
Metallicity	28±3% Sun
Metallicity [Fe/H]	−0.55±0.05 dex
Rotation	34 days
Age	5.8 Gyr
Other designations
52 Cet, BD−16° 295, FK5 59, GJ 71, HD 10700, HIP 8102, HR 509, SAO 147986, LFT 159, LHS 146, LTT 935

Tau Ceti, Latinized from τ Ceti, is a single star in the constellation Cetus that is spectrally similar to the Sun, although it has only about 78% of the Sun's mass. At a distance of just under 12 light-years (3.7 parsecs) from the Solar System, it is a relatively nearby star and the closest solitary G-class star. The star appears stable, with little stellar variation, and is metal-deficient.

Observations have detected more than ten times as much dust surrounding Tau Ceti as is present in the Solar System. Since December 2012, there has been evidence of possibly five planets orbiting Tau Ceti, with two of these being potentially in the habitable zone. Because of its debris disk, any planet orbiting Tau Ceti would face far more impact events than Earth. Despite this hurdle to habitability, its solar analog (Sun-like) characteristics have led to widespread interest in the star. Given its stability, similarity and relative proximity to the Sun, Tau Ceti is consistently listed as a target for the Search for Extra-Terrestrial Intelligence (SETI) and appears in some science fiction literature.

It can be seen with the unaided eye as a third-magnitude star. As seen from Tau Ceti, the Sun would be a third-magnitude star in the northern hemisphere constellation Boötes.

Name

The name "Tau Ceti" is the Bayer designation for this star, established in 1603 as part of German celestial cartographer Johann Bayer's Uranometria star catalogue: it is "number T" in Bayer's sequence of constellation Cetus. In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, written at Cairo about 1650, this star was designated Thālith al Naʽāmāt (ثالث النعامات - thālith al-naʽāmāt), which was translated into Latin as Tertia Struthionum, meaning the third of the ostriches. This star, along with η Cet (Deneb Algenubi), θ Cet (Thanih Al Naamat), ζ Cet (Baten Kaitos), and υ Cet, were Al Naʽāmāt (النعامات), the Hen Ostriches.

In Chinese, the "Square Celestial Granary" (Chinese: 天倉; pinyin: Tiān Cāng) refers to an asterism consisting of τ Ceti, ι Ceti, η Ceti, ζ Ceti, θ Ceti and 57 Ceti. Consequently, τ Ceti itself is known as the "Fifth Star of Square Celestial Granary" (Chinese: 天倉五; pinyin: Tiān Cāng wǔ).

Motion

The proper motion of a star is its rate of movement across the celestial sphere, determined by comparing its position relative to more distant background objects. Tau Ceti is considered to be a high-proper-motion star, although it only has an annual traverse of just under 2 arc seconds. Thus it will require about 2000 years before the location of this star shifts by more than a degree. A high proper motion is an indicator of closeness to the Sun. Nearby stars can traverse an angle of arc across the sky more rapidly than the distant background stars and are good candidates for parallax studies. In the case of Tau Ceti, the parallax measurements indicate a distance of 11.9 ly. This makes it one of the closest star systems to the Sun and the next-closest spectral class-G star after Alpha Centauri A.

The radial velocity of a star is the component of its motion that is toward or away from the Sun. Unlike proper motion, a star's radial velocity cannot be directly observed, but can be determined by measuring its spectrum. Due to the Doppler shift, the absorption lines in the spectrum of a star will be shifted slightly toward the red (or longer wavelengths) if the star is moving away from the observer, or toward blue (or shorter wavelengths) when it moves toward the observer. In the case of Tau Ceti, the radial velocity is about −17 km/s, with the negative value indicating that it is moving toward the Sun. The star will make its closest approach to the Sun in about 43,000 years, when it comes to within 10.6 ly (3.25 pc).

The distance to Tau Ceti, along with its proper motion and radial velocity, together give the motion of the star through space. The space velocity relative to the Sun is 37.2 km/s. This result can then be used to compute an orbital path of Tau Ceti through the Milky Way. It has a mean galacto-centric distance of 9.7 kiloparsec (32000 ly) and an orbital eccentricity of 0.22.

Physical properties

The Sun (left) is both larger and somewhat hotter than the less active Tau Ceti (right).

The Tau Ceti system is believed to have only one stellar component. A dim optical companion has also been observed with magnitude 13.1. As of 2000, it was 137 arcseconds distant from the primary. It may be gravitationally bound, but it is considered more likely to be a line-of-sight coincidence.

Most of what is known about the physical properties of Tau Ceti and its system has been determined through spectroscopic measurements. By comparing the spectrum to computed models of stellar evolution, the age, mass, radius and luminosity of Tau Ceti can be estimated. However, using an astronomical interferometer, measurements of the radius of the star can be made directly to an accuracy of 0.5%. Through such means, the radius of Tau Ceti has been measured to be 79.3%±0.4% of the solar radius. This is about the size that is expected for a star with somewhat lower mass than the Sun.

Rotation

The rotation period for Tau Ceti was measured by periodic variations in the classic H and K absorption lines of singly ionized calcium (Ca II). These lines are closely associated with surface magnetic activity, so the period of variation measures the time required for the activity sites to complete a full rotation about the star. By this means the rotation period for Tau Ceti is estimated to be 34 d. Due to the Doppler effect, the rotation rate of a star affects the width of the absorption lines in the spectrum (light from the side of the star moving away from the observer will be shifted to a longer wavelength; light from the side moving towards the observer will be shifted toward a shorter wavelength). By analyzing the width of these lines, the rotational velocity of a star can be estimated. The projected rotation velocity for Tau Ceti is

v_eq · sin i ≈ 1 km/s,

where v_eq is the velocity at the equator, and i is the inclination angle of the rotation axis to the line of sight. For a typical G8 star, the rotation velocity is about 2.5 km/s. The relatively low rotational velocity measurements may indicate that Tau Ceti is being viewed from nearly the direction of its pole.

Metallicity

The chemical composition of a star provides important clues to its evolutionary history, including the age at which it formed. The interstellar medium of dust and gas from which stars form is primarily composed of hydrogen and helium with trace amounts of heavier elements. As nearby stars continually evolve and die, they seed the interstellar medium with an increasing portion of heavier elements. Thus younger stars tend to have a higher portion of heavy elements in their atmospheres than do the older stars. These heavy elements are termed "metals" by astronomers, and the portion of heavy elements is the metallicity. The amount of metallicity in a star is given in terms of the ratio of iron (Fe), an easily observed heavy element, to hydrogen. A logarithm of the relative iron abundance is compared to the Sun. In the case of Tau Ceti, the atmospheric metallicity is

${\displaystyle \left[{\frac {\text{Fe}}{\text{H}}}\right]\approx -0.50}$ dex,

equivalent to about a third the solar abundance. Past measurements have varied from −0.13 to −0.60.

This lower abundance of iron indicates that Tau Ceti is almost certainly older than the Sun. Its age had previously been estimated to be about 10 Gyr, but is now thought to be around half that, at 5.8 Gyr. This compares with 4.57 Gyr for the Sun. However, computed age estimates for Tau Ceti can range from 4.4 to 12 Gyr, depending on the model adopted.

Besides rotation, another factor that can widen the absorption features in the spectrum of a star is pressure broadening. The presence of nearby particles affects the radiation emitted by an individual particle. So the line width is dependent on the surface pressure of the star, which in turn is determined by the temperature and surface gravity. This technique was used to determine the surface gravity of Tau Ceti. The log g, or logarithm of the star's surface gravity, is about 4.4, very close to the log g = 4.44 for the Sun.

Luminosity and variability

The luminosity of Tau Ceti is equal to only 55% of the Sun's luminosity. A terrestrial planet would need to orbit this star at a distance of about 0.7 AU in order to match the solar insolation level of Earth. This is approximately the same as the average distance between Venus and the Sun. 

The chromosphere of Tau Ceti—the portion of a star's atmosphere just above the light-emitting photosphere—currently displays little or no magnetic activity, indicating a stable star. One 9-year study of temperature, granulation, and the chromosphere showed no systematic variations; Ca II emissions around the H and K infrared bands show a possible 11-year cycle, but this is weak relative to the Sun. Alternatively it has been suggested that the star could be in a low-activity state analogous to a Maunder minimum—a historical period, associated with the Little Ice Age in Europe, when sunspots became exceedingly rare on the Sun's surface. Spectral line profiles of Tau Ceti are extremely narrow, indicating low turbulence and observed rotation. The star's oscillations have an amplitude about half that of the Sun and a lower mode lifetime.

Life and planet searches

Principal factors driving research interest in Tau Ceti are its proximity, its Sun-like characteristics and their implications for possible planets and life. For categorization purposes, Hall and Lockwood report that "the terms 'solarlike star', 'solar analog', and 'solar twin' [are] progressively restrictive descriptions". Tau Ceti fits the second category, given its similar mass and low variability, but relative lack of metals. The similarities have inspired popular culture references for decades, as well as scientific examination. 

Tau Ceti has been a target of radial-velocity planetary searches. As of 1988, observations ruled out any periodical variations attributable to massive planets around Tau Ceti inside of Jupiter-like distances. Ever more precise measurements continue to rule out such planets, at least until December 2012. The velocity precision reached is about 11 m/s measured over a 5-year time span. This result excludes the presence of hot Jupiters and probably excludes any planets with minimal mass greater than or equal to Jupiter's mass and with orbital periods less than 15 years. In addition, a survey of nearby stars by the Hubble Space Telescope's Wide Field and Planetary Camera was completed in 1999, including a search for faint companions to Tau Ceti; none were discovered to limits of the telescope's resolving power.

These searches only excluded larger brown dwarf bodies and giant planets, so smaller, Earth-like planets in orbit around the star were not precluded. If "hot Jupiters" did exist in close orbit, they would likely disrupt the star's habitable zone; their exclusion was thus considered positive for the possibility of Earth-like planets. General research has shown a positive correlation between the presence of planets and a relatively high-metallicity parent star, suggesting that stars with lower metallicity such as Tau Ceti have a lower chance of having planets. Primitive life on Tau Ceti's planets might reveal itself through an atmospheric composition unlikely to be abiotic, just as oxygen on Earth is indicative of life.

SETI and HabCat

Tau Ceti could have been a search target for the canceled Terrestrial Planet Finder

The most optimistic search project to date was Project Ozma, which was intended to "search for extraterrestrial intelligence" (SETI) by examining selected stars for indications of artificial radio signals. It was run by the astronomer Frank Drake, who selected Tau Ceti and Epsilon Eridani as the initial targets. Both are located near the Solar System and are physically similar to the Sun. No artificial signals were found despite 200 hours of observations. Subsequent radio searches of this star system have also turned up negative. 

This lack of results has not dampened interest in observing the Tau Ceti system for biosignatures. In 2002, astronomers Margaret Turnbull and Jill Tarter developed the Catalog of Nearby Habitable Systems (HabCat) under the auspices of Project Phoenix, another SETI endeavour. The list contained more than 17000 theoretically habitable systems, approximately 10% of the original sample. The next year, Turnbull would further refine the list to the 30 most promising systems out of 5000 within 100 light-years from the Sun, including Tau Ceti; this will form part of the basis of radio searches with the Allen Telescope Array. She also chose Tau Ceti for a final shortlist of just five stars suitable for searches by the (indefinitely postponed) Terrestrial Planet Finder telescope system, commenting that "these are places I'd want to live if God were to put our planet around another star".

Planetary system

On December 19, 2012, evidence was presented that suggest a system of five planets orbiting Tau Ceti. The planets' estimated minimal masses are between 2 and 6 Earth masses, and their orbital periods range from 14 to 640 days. One of them, tentatively named Tau Ceti e, appears to orbit about half as far from Tau Ceti as Earth does from the Sun. With Tau Ceti's luminosity of 52% that of the Sun and a distance from the star of 0.552 AU, the planet would receive 1.71 times as much stellar radiation as Earth does, slightly less than Venus with 1.91 times Earth's. Nevertheless, some research places it within the star's habitable zone. The Planetary Habitability Laboratory has estimated that Tau Ceti f, which would receive 28.5% as much starlight as Earth, would be narrowly within the habitable zone of the star as well.

The Tau Ceti candidate planetary system

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	≥2.00 ± 0.80 M⊕	0.105 ± 0.006	13.965 ± 0.024	0.16 ± 0.22	—	—
c	≥3.1 ± 1.40 M⊕	0.195 ± 0.011	35.362 ± 0.106	0.03 ± 0.28	—	—
d	≥3.60 ± 1.7 M⊕	0.374 ± 0.02	94.11 ± 0.7	0.08 ± 0.26	—	—
e	≥4.30 ± 2.1 M⊕	0.552 ± 0.03	168.12 ± 2.32	0.05 ± 0.22	—	—
f	≥6.6 ± 3.5 M⊕	1.35 ± 0.09 AU	642 ± 37	0.03 ± 0.26	—	—
Debris disk	35–50 (or 2+8 −1–55+5 −5) AU				35±10°	—

The team that made the discovery in 2013 went on to refine and improve their methodology, and with updated radial-velocity measurements, published new results in August 2017. They confirmed Tau Ceti e and f as candidates, but failed to detect Tau Ceti b and c (which had appeared to orbit the star at a suspicious 0.4 and 1 times the star's rotation period, which would have suggested that their apparent signal is related to stellar rotation rather than an orbiting planet, although this can be false negative). Instead they found two new planetary candidates, Tau Ceti g and h, with orbits of 20 and 49 days. They did find some evidence for the existence of Tau Ceti d, but, because it did not show up in all data sets, they were unable to confirm it as a candidate planet. 

The Tau Ceti updated candidate planetary system

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
g	≥1.75+0.25 −0.40 M⊕	0.133+0.001 −0.002	20.00+0.02 −0.01	0.06+0.13 −0.06	—	—
h	≥1.83+0.68 −0.26 M⊕	0.243 ± 0.003	49.41+0.08 −0.10	0.23+0.16 −0.15	—	—
e	≥3.93+0.83 −0.64 M⊕	0.538 ± 0.006	162.87+1.08 −0.46	0.18+0.18 −0.14	—	—
f	≥3.93+1.05 −1.37 M⊕	1.334+0.017 −0.044	636.13+11.70 −47.69	0.16+0.07 −0.16	—	—
Debris disk	6.2+9.8 −4.6 – 52+3 −8 AU				35±10°	—

The updated 4-planet model is dynamically packed and potentially stable for billions of years, but with further refinements more planet candidates could still be detected (including b and c). The signals detected from the candidate planets have radial velocities as low as 30 cm/s, and the experimental method used in their detection, as it was applied to HARPS, could in theory have detected down to around 20 cm/s. In order to detect an Earth-sized planet candidate between Tau Ceti e and f, in the system's habitable zone, a radial velocity precision of approximately 10 cm/s would need to be achieved.

The habitable zone for this star, defined as the locations where liquid water could be present on an Earth-sized planet, is at a radius of 0.55–1.16 AU, where 1 AU is the average distance from the Earth to the Sun.

If Tau Ceti is aligned in such a way that it is nearly pole-on to Earth (as its rotation could indicate), that would mean that the planets would be, rather than slightly larger to a few times Earth's mass, they would be on the leagues of tens of Earth masses to even hundreds. For example, if Tau Ceti f's orbit was inclined 70 degrees from being head-on to Earth, its mass would be 11.49+3.07
−4 Earth masses, making it a mini-Neptune or mega-Earth on the middle-to-high end, and a larger super-Earth on the lower end. However, this scenario isn't necessarily true; since Tau Ceti's debris disk has an inclination of 35±10, though, this could mean that the planets' orbits are similarly inclined. This would put f from 5.56+1.48
−1.94 to 9.29+2.49
−3.23 Earth masses, which means it's less likely to be a mini-Neptune even at the higher threshold, if it is assumed that the debris disk ad f's orbits are equal.

Tau Ceti e

Tau Ceti e is a candidate planet orbiting Tau Ceti that was detected by statistical analyses of the data of the star's variations in radial velocity that were obtained using HIRES, AAPS, and HARPS. Its possible properties were refined in 2017: it orbits at a distance of 0.552 AU (between the orbits of Venus and Mercury in the Solar System) with an orbital period of 168 days and has a minimum mass of 3.29 Earth masses. Because the minimum mass of a super-Earth is 5 Earth masses, Tau Ceti e may be Earth-sized. If it possesses an Earth-like atmosphere, the surface temperature would be around 68 °C (154 °F).

Tau Ceti f

Tau Ceti f is a candidate super-Earth orbiting Tau Ceti that was discovered in 2012 by statistical analyses of the star's variations in radial velocity, based on data obtained using HIRES, AAPS, and HARPS. It is of interest because its orbit places it in Tau Ceti's extended habitable zone. However, a 2015 study implies that it has been in the temperate zone for less than one billion years, so there may not be a detectable biosignature.

Few properties of the planet are known other than its orbit and mass. It orbits Tau Ceti at a distance of 1.35 AU (near Mars's orbit in the Solar System) with an orbital period of 642 days and has a minimum mass of 2.66 Earth masses, which means it may be either a super-Earth or terrestrial planet.

Debris disk

In 2004, a team of UK astronomers led by Jane Greaves discovered that Tau Ceti has more than ten times the amount of cometary and asteroidal material orbiting it than does the Sun. This was determined by measuring the disk of cold dust orbiting the star produced by collisions between such small bodies. This result puts a damper on the possibility of complex life in the system, because any planets would suffer from large impact events roughly ten times more frequently than Earth. Greaves noted at the time of her research that "it is likely that [any planets] will experience constant bombardment from asteroids of the kind believed to have wiped out the dinosaurs". Such bombardments would inhibit the development of biodiversity between impacts. However, it is possible that a large Jupiter-sized gas giant could deflect comets and asteroids.

The debris disk was discovered by measuring the amount of radiation emitted by the system in the far infrared portion of the spectrum. The disk forms a symmetric feature that is centered on the star, and its outer radius averages 55 AU. The lack of infrared radiation from the warmer parts of the disk near Tau Ceti imply an inner cut-off at a radius of 10 AU. By comparison, the Solar System's Kuiper belt extends from 30 to 50 AU. To be maintained over a long period of time, this ring of dust must be constantly replenished through collisions by larger bodies. The bulk of the disk appears to be orbiting Tau Ceti at a distance of 35–50 AU, well outside the orbit of the habitable zone. At this distance, the dust belt may be analogous to the Kuiper belt that lies outside the orbit of Neptune in the Solar System.

Tau Ceti shows that stars need not lose large disks as they age, and such a thick belt may not be uncommon among Sun-like stars. Tau Ceti's belt is only 1/20 as dense as the belt around its young neighbor, Epsilon Eridani. The relative lack of debris around the Sun may be the unusual case: one research-team member suggests the Sun may have passed close to another star early in its history and had most of its comets and asteroids stripped away. Stars with large debris disks have changed the way astronomers think about planet formation because debris disk stars, where dust is continually generated by collisions, appear to form planets readily.