Tau Ceti

Tau Ceti (τ Cet) is a yellow main sequence star located in the constellation Cetus. With an apparent magnitude of 3.50, it is visible to the unaided eye. At a distance of 11.912 light years, the yellow dwarf is the nearest single G-type star to the Sun and the second nearest G-class star, after Rigil Kentaurus (Alpha Centauri A). It hosts a planetary system, with two candidate planets potentially orbiting in the star’s habitable zone. The presence of the planets is still unconfirmed.

Star type

Tau Ceti is a main sequence star of the spectral type G8V. It has a mass of 0.69 solar masses and a radius of 0.793 solar radii. With an effective temperature of 5,320 K, it shines with 48.8% of the Sun’s luminosity.

The star’s rotation period is difficult to determine because Tau Ceti appears nearly pole-on from our point of view. It appears to have a projected rotational velocity of around 0.1 km/s, taking about 46 days to complete a rotation. The pole-on orientation also makes it difficult to detect any surface features like starspots.

Tau Ceti is much older than the Sun, with an estimated age of 8 – 10 billion years. It has a metallicity around 28% that of the Sun. Because of its lower mass, it will spend more time on the main sequence than our parent star.

Tau Ceti, image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

Tau Ceti is considered a stable star. Its chromosphere shows little or no magnetic activity. It does not display any systematic variations in the temperature, chromosphere and granulation (the pattern of convection cells in the stellar photosphere).

Tau Ceti is believed to be a single star. It has a visual companion at a separation of 137 arcseconds, but the star is thought to just appear along the same line of sight. With an apparent magnitude of 13.1, it is invisible to the unaided eye. It the two stars are gravitationally bound, that would make the companion a red dwarf at a separation of at least 325 astronomical units (average Earth – Sun distances) from the primary. The star system would have an orbital period of at least 6,000 years.

Tau Ceti is surrounded by ten times as much dust as is found in the solar system. Astronomers attribute this to dust particles produced in collisions of planetesimals in the Tau Ceti system.

The star has a debris disk, first detected as an infrared excess by the Infrared Astronomical Satellite (IRAS) in 1985.

In 2004, a team led by Jane Greaves of the UK Astronomy Technology Centre, Royal Observatory in Edinburgh, Scotland, marginally resolved the disk with the James Clerk Maxwell Telescope (JCMT). They found that it extended out to 55 astronomical units, corresponding to an angular radius of 15 arcseconds. This was the first disk found around a Sun-like star to be confirmed by imaging.

Tau Ceti’s debris disk is 10 – 50 astronomical units across and has a dust mass of around 1.2 Earth masses. The largest portion of the disk orbits the star at a distance of 35 – 50 AU, which is well outside the zone where liquid water may be sustained on a hypothetical planet’s surface.

The disk may be similar to the Kuiper belt, which lies outside Neptune’s orbit in our solar system. It has only 1/20 of the density of the disk found around Epsilon Eridani.

In 2016, a team of astronomers led by Meredith A. MacGregor analysed the data obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) to gain new insights about the debris disk. The team’s models indicated an inner belt edge of around 6.2 astronomical units.

Because of the debris disk, any planets in the Tau Ceti system would be affected by more impact events than the planets in the Sol system. This would have profound effects on the habitability of planets orbiting the star. However, any Jupiter-sized gas giants may be able to deflect any asteroids and comets.

Tau Ceti debris (infrared) by the James Clerk Maxwell Telescope. The James Clerk Maxwell Telescope is operated by The Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the United Kingdom, the Netherlands Organisation for Scientific Research, and the National Research Council of Canada. (GNU FDL 1.2)

Planets

Tau Ceti is one of the top targets for the search for extraterrestrial intelligence (SETI). The star’s Sun-like characteristics, proximity and stability make it a frequent target for planet searching studies.

As many as eight extrasolar planets have been reported orbiting the Sun-like star, but the existence and possible nature of the planets are either disputed or unconfirmed.

On December 19, 2012, a team of astronomers led by Mikko Tuomi of the University of Hertfordshire, Centre for Astrophysics Research, Science and Technology Research Institute, Hatfield, UK, reported the detection of five planets in the Tau Ceti system. The planets were discovered by analysing data on the star’s radial velocity variations.

The data was obtained with the High Resolution Echelle Spectrometer (HIRES) at the W. M. Keck Observatory on Mauna Kea in Hawaii, the High Accuracy Radial Velocity Planet Searcher (HARPS) at La Silla Observatory in Chile, and using the Anglo-Australian Telescope (AAT) in the Anglo Australian Planet Search (AAPS).

The planets were designated Tau Ceti b, c, d, e, and f. Astronomers derived masses of 2.0, 3.1, 3.6, 4.3, and 6.6 Earth masses for the planets and orbital periods of 13.965, 35.362, 94.1, 168.12, and 642 days.

Some sources have placed the planet Tau Ceti e within the habitable zone of Tau Ceti. The planet was found to orbit the star at about half the distance as that from the Earth to Sun, which meant that it would receive 1.71 as much stellar radiation as Earth does from the Sun.

The habitable zone of Tau Ceti is believed to be at a distance of 0.55 to 1.16 astronomical units.

Tau Ceti is only about half as luminous as the Sun. For a hypothetical terrestrial planet to receive the same amount of solar insolation as Earth, it would need to orbit the host star at a distance of about 0.7 astronomical units, or 70% of the distance from the Earth to the Sun (comparable to the average distance between Venus and the Sun).

In 2015, Arizona State University researchers conducted a study of the chemical composition of Tau Ceti and the possible implications on any terrestrial planets orbiting the star. They found that the star is 70 percent more magnesium rich than the Sun. It has a ratio of magnesium to silicon of 1.78, which is much greater than that of Earth (around 1.2).

While stellar abundances may not be reflected in the same ratio in the abundances of planets, they are believed to be reflected nonetheless. For instance, the Mg/Si ratio in the Earth’s upper mantle is over 20 percent higher than that in the Sun’s photosphere. Scientists attribute this to processes that occur during planet formation and evolution.

The high magnesium abundance means that the mineralogical makeup of planets orbiting Tau Ceti may be very different than that of our planet. The elevated magnesium to silicon ratio may have a significant impact on mantle dynamics and surface tectonics.

Because of the highly unusual composition, the study authors placed Tau Ceti low on the list of potentially habitable worlds.

The study was a collaboration between astrobiologist and astrophysicist Michael Pagano, astrophysicist Amanda Truitt, theoretical astrophysicist Patrick A. Young, and mineral physicist Sang-Heon Shim at the School of Earth and Space Exploration, Arizona State University. It was published in The Astrophysical Journal.

In August 2017, astronomers confirmed the existence of Tau Ceti e and Tau Ceti f, but were unable to do the same for the planets Tau Ceti b, c, and d. Tau Ceti b is suspected to be a false negative, while the signal of Tau Ceti c was associated with stellar rotation, and Tau Ceti d did not consistently show up in all the data sets.

If Tau Ceti e has an Earth-like atmosphere, it would have a surface temperature of about 68° C (154° F). The amount of sunlight it receives may place the planet outside the habitable zone. If so, Tau Ceti e could be a Venus-like world.

The astronomers refined the properties of Tau Ceti e and Tau Ceti f. The planet e has a mass of at least 3.93 Earth masses and orbits Tau Ceti with a period of 162.87 days at a distance of 0.538 astronomical units. The planet f also has a minimum mass of 3.93 Earth masses. It orbits the parent star with a period of 636.13 days at a distance of 1.334 astronomical units.

The astronomers also reported the discovery of two new candidate planets, Tau Ceti g and Tau Ceti h, with orbital periods of 20 and 49 days.

Additional planets have been reported more recently. In 2019, astronomers suggested that the star may host a Jupiter or super-Jupiter with a mass of up to 5 Jupiter masses at an orbital distance between 3 and 20 AU. The study was published in Astronomy & Astrophysics.

In 2020, astronomers explored the orbits in which the presence of additional planets was the most likely based on the orbital stability of the planetary system. They predicted three candidates at orbits that correspond to the candidate planets Tau Ceti b, c, and d, and at least one more planet orbiting between the planets e and f, within the goldilocks zone. The study was published in The Astronomical Journal, Volume 161.

In 2021, a team of astronomers led by M. Cretignier at the Astronomy Department of the University of Geneva, Switzerland, developed YARARA, a post-processing pipeline that cleans up any instrumental and atmospheric contamination in the high-resolution spectra of stars. The astronomers ran YARARA on the radial velocity data sets of Tau Ceti and were unable to confirm the planets.

In 2023, a study led by Maria Korolik, Department of Astronomy, Yale University, derived a rotation period of 46 ± 4 days and a nearly pole-on inclination of 7° ± 7° for Tau Ceti. Based on observations with the Center for High Angular Resolution Astronomy (CHARA) Array, the study proposed that Tau Ceti’s planets also have a pole-on inclination, which would make their orbits unstable within only 10 million years. This would make it highly unlikely for the planets orbiting Tau Ceti to have survived over the course of the star’s life. The astronomers also found that there is a misalignment between Tau Ceti’s axis of rotation and the rotation axis of the star’s debris disk.

Facts

With an apparent visual magnitude of 3.50, Tau Ceti is the fifth brightest star in the constellation Cetus, after Diphda (Beta Ceti), Menkar (Alpha Ceti), Eta Ceti, and Kaffaljidhma (Gamma Ceti). Even though it is by far the closest of these stars to the Sun, Tau Ceti is not as intrinsically luminous as the giants Diphda, Menkar and Eta Ceti, and the hotter A-type main sequence star Kaffaljidhma. To the unaided eye, Tau Ceti appears just as bright as Eta and Gamma Ceti. All these stars are occasionally outshone by the variable Mira, a pulsating star that can become as bright as magnitude 2.0.

Tau Ceti is the fifth brightest star within 12 light-years of the Sun, after Sirius in Canis Major, Procyon in Canis Minor, and Rigil Kentaurus and Toliman (the brighter components of the Alpha Centauri system) in Centaurus.

Like all stars in the solar neighbourhood, Tau Ceti is a high proper motion star. It moves across the sky by just under 2 seconds of arc per year, taking about 2 millennia to shift by a degree.

Tau Ceti is moving toward the Sun with a radial velocity of about -17 km/s. The star will make its closest approach to the solar system in around 43,000 years, when it comes within 10.6 light-years of the Sun.

Seen from a hypothetical planet in the Tau Ceti system, the Sun would appear as a magnitude 2.6 star near Arcturus and Muphrid in the constellation Boötes (the Herdsman).

Sun view from Tau Ceti, rendered by Celestia, image credit: Wikimedia Commons/Kirk 39 (GPL 3.0)

Tau Ceti and the orange dwarf Epsilon Eridani were the targets of Project Ozma, the first modern SETI experiment. Project Ozma was started by the American astronomer Frank Drake at the National Radio Astronomy Observatory (NRAO) at Green Bank, West Virginia, in 1960. Drake aimed to search for signs of extraterrestrial life using radio waves. The project was named after Princess Ozma from the Land of Oz book series. After 150 hours of observation over the course of four months, no detectable biosignatures were found.

In 2002, Tau Ceti was included in the Catalog of Nearby Habitable Systems (HabCat), developed by American astronomers Margaret Turnbull and Jill Tarter. The catalogue included over 17,000 habitable systems. In 2003, Turnbull refined the list to 30 of the most promising star systems within 100 light-years and again included Tau Ceti. She eventually created a shortlist of five most promising systems, which included Tau Ceti, Epsilon Indi A, Ran (Epsilon Eridani), Keid (Omicron² Eridani), and Toliman (Alpha Centauri B).

Like many other nearby stars, Tau Ceti has often been used and referenced in works of science fiction. It was mentioned in the Star Trek films and television series as the home of the Traveller and the Kobayashi Maru spacecraft. It was also memorably used in Doctor Who (1978) and Barbarella (1968).

Notable uses in literature include Robert Heinlein’s novel Time for the Stars (1956), Frank Herbert’s Destination: Void (1965), Samuel R. Delany’s novella Empire Star (1966), Larry Niven’s novel A Gift from Earth (1968), Norman Spinrad’s The Iron Dream (1972), Ursula K. Le Guin’s The Disposessed (1974), Joan Slonczewski’s Still Forms on Foxfield (1980), C. J. Cherryh’s Downbellow Station (1981), L. Sprague de Camp’s Viagens Interplanetarias series, L. Sprague de Camp and Catherine Crook de Camp’s novel The Bones of Zora (1983), Larry Niven, Jerry Pournelle and Steven Barnes’ The Legacy of Heorot (1987), Dan Simmons’ Hyperion (1989) and The Fall of Hyperion (1990), Arthur C. Clarke and Gentry Lee’s Rama Revealed (1993), Michael McCollum’s The Sails of Tau Ceti (1993), Harry Turtledove’s Homeward Bound (2004), Walter Jon Williams’ Implied Spaces (2008), and Kim Stanley Robinson’s Aurora (2015),

Name

Tau Ceti (pronunciation: /ˌtaʊ ˈsiːtaɪ/) does not have a proper name formally approved by the International Astronomical Union (IAU). The star is commonly referred to by its Bayer designation, Tau Ceti. The designation comes from German astronomer Johann Bayer’s 1603 star atlas Uranometria.

In Arabic astronomy, Tau Ceti was called Thālith al Naʽāmāt, “the third of the ostriches.” The name appeared in the 17th century Egyptian astronomer Al Achsasi al Mouakket’s Calendarium. It was translated into Latin as Tertia Struthionum. The name refers to an asterism Al Naʽāmāt (the Hen Ostriches), formed by Tau Ceti with Eta Ceti, Theta Ceti, Baten Kaitos (Zeta Ceti), and Upsilon Ceti.

In traditional Chinese astronomy, Tau Ceti was called 天倉五 (Tiān Cāng wǔ), the Fifth Star of Square Celestial Granary. The Square Celestial Granary was an asterism formed by Tau Ceti with Iota Ceti, Eta Ceti, Theta Ceti, Baten Kaitos (Zeta Ceti), and 57 Ceti. The asterism was part of the larger Bond mansion, which represented the body of the White Tiger of the West. The White Tiger was one of the four symbols of the Chinese constellations.

Location

Tau Ceti appears near Diphda in the sky. Diphda can be found along the imaginary line extended from Alpheratz through Algenib in the Great Square of Pegasus. Tau Ceti lies along the line drawn from Diphda in the direction of Orion’s Belt.

Constellation

Tau Ceti is located in the constellation Cetus. It appears in the lower body of the Sea Monster, along with Theta Ceti and Baten Kaitos (Zeta Ceti).

Cetus is one of the Greek constellations, catalogued by the Greco-Roman astronomer Ptolemy of Alexandria in his Almagest in the 2nd century CE. In Greek mythology, it is associated with the sea monster from the myth of Andromeda and her vain mother Cassiopeia.

Cetus is the fourth largest constellation in the sky. It stretches across 1,231 square degrees of the sky on the celestial equator. Like all equatorial constellations, it is visible from virtually any location on Earth.

Even though it is one of the largest constellations, Cetus is not particularly prominent. It hosts only two stars brighter than magnitude 3.0, the giants Diphda (Beta Ceti) and Menkar (Alpha Ceti). Diphda, the constellation’s brightest star, shines at magnitude 2.02 and appears in the Sea Monster’s tail. The more distant Menkar appears in its head.

Cetus constellation map by IAU and Sky&Telescope magazine (Roger Sinnott & Rick Fienberg) (CC BY 3.0)

Cetus is home to many notable stars. These include the pulsating variable red giant Mira (Omicron Ceti), Earendel, the most distant star discovered, the flare stars UV Ceti and BL Ceti, the binary system Zeta Ceti (Baten Kaitos), the triple star system Gamma Ceti (Kaffaljidhma), and the orange giants Eta Ceti, Iota Ceti, and Theta Ceti.

Notable deep sky objects in Cetus include the planetary nebula NGC 246 (the Skull Nebula), the barred spiral galaxy Messier 77 (the Squid Galaxy), the spiral galaxies NGC 1042, NGC 247, NGC 1055, and NGC 1035, and the irregular dwarf galaxy IC 1613.

The best time of the year to observe the stars and deep sky objects in Cetus is during the month of November, when the constellation appears higher in the sky in the early evening. The entire constellation is visible from locations between the latitudes 70° N and 90° S.

The 10 brightest stars in Cetus are Diphda (Beta Ceti, mag. 2.02), Menkar (Alpha Ceti, mag. 2.53), Eta Ceti (mag. 3.446), Kaffaljidhma (Gamma Ceti, mag. 3.47), Tau Ceti (mag. 3.50), Iota Ceti (mag. 3.562), Theta Ceti (mag. 3.60), Baten Kaitos (Zeta Ceti, mag. 3.742), Upsilon Ceti (mag. 3.95), and Delta Ceti (mag. 4.06). The pulsating variable star Mira (Omicron Ceti, mag. 2.0 – 10.1) is sometimes among the brightest stars in the constellation and at other times it is invisible to the unaided eye.

Tau Ceti