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Tau Ceti

Coordinates:Sky map01h44m04.0829s,−15° 56′ 14.928″
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Not to be confused withT Ceti.For the video game, seeTau Ceti (video game).

Tau Ceti
Location of τ Ceti (circled)
Observation data
EpochJ2000EquinoxJ2000
Constellation Cetus
Pronunciation /ˌtaʊˈsiːtaɪ/
Right ascension 01h44m04.083s[1]
Declination −15° 56′ 14.93″[1]
Apparent magnitude(V) 3.50±0.01[2]
Characteristics
Evolutionary stage Main sequence
Spectral type G8V[3]
U−Bcolor index +0.21[4]
B−Vcolor index +0.72[4]
Astrometry
Radial velocity(Rv)−16.68±0.05[5]km/s
Proper motion(μ)RA:−1721.728mas/yr[1]
Dec.:+854.963mas/yr[1]
Parallax(π)273.8097 ± 0.1701mas[1]
Distance11.912 ± 0.007ly
(3.652 ± 0.002pc)
Absolute magnitude(MV)5.69±0.01[2]
Absolute bolometric
magnitude(Mbol)		5.52±0.02[2]
Details
Mass0.783±0.012[2]M☉
Radius0.796±0.004[6]R☉
Luminosity0.488±0.010[2]L☉
Luminosity (visual, LV)0.45[nb 1]L☉
Surface gravity(logg)4.48±0.05[7]cgs
Temperature5,320±40[7]K
Metallicity28±3% Sun
Metallicity[Fe/H]−0.55±0.05[8]dex
Rotation34d,[9]46±4d[7]
Rotational velocity(vsini)0.1±0.1[7]km/s
Age8–10[10]Gyr
Other designations
52 Cet,BD−16° 295,FK559,GJ71,HD10700,HIP8102,HR509,SAO147986,LFT159,LHS146,LTT935[4]
Database references
SIMBADdata
Exoplanet Archivedata
ARICNSdata

Tau Ceti,Latinizedfromτ Ceti,is a singlestarin theconstellationCetusthat isspectrallysimilar to theSun,although it has only about 78% of theSun's mass.At a distance of just under 12light-years(3.7parsecs) from theSolar System,it is arelatively nearby starand the closest solitaryG-classstar. The star appears stable, with littlestellar variation,and ismetal-deficient(low in elements other than hydrogen and helium) relative to the Sun.

It can be seen with the unaided eye with anapparent magnitudeof 3.5.[2]As seen from Tau Ceti, the Sun would be in the northern hemisphere constellationBoöteswith an apparent magnitude of about 2.6.[nb 2][11]

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 at least four planets—all likelysuper-Earths—orbiting Tau Ceti, and two of these are potentially in thehabitable zone.[12][13][14]There is evidence of up to an additional four unconfirmed planets, one of which would be a Jovian planet between 3 and 20AUfrom the star.[15]Because of itsdebris disk,any planet orbiting Tau Ceti would face far moreimpact eventsthan Earth. Note that those planetary candidates have been contested recently[16]and recent discoveries about the stellar inclination cast doubt about the terrestrial nature of these worlds.[7]Despite this hurdle tohabitability,itssolar 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 thesearch for extraterrestrial intelligence(SETI).[17]

Name[edit]

The name "Tau Ceti" is theBayer designationfor this star, established in 1603 as part of German celestial cartographerJohann Bayer'sUranometriastar catalogue: it is "number T" in Bayer's sequence of constellation Cetus. In the catalogue of stars in theCalendariumofAl Achsasi al Mouakket,written atCairoabout 1650, this star was designatedThālith al Naʽāmāt(ثالث النعامات -thālith al-naʽāmāt), which was translated intoLatinasTertia Struthionum,meaningthe third of the ostriches.[18]This star, along withη Cet(Deneb Algenubi),θ Cet(Thanih Al Naamat),ζ Cet(Baten Kaitos), andυ Cet,wereAl Naʽāmāt(النعامات), the Hen Ostriches.[19][20]

InChinese astronomy,the "Square Celestial Granary"(Chinese:Thiên thương;pinyin:Tiān Cāng) refers to anasterismconsisting of τ Ceti,ι Ceti,η Ceti,ζ Ceti,θ Cetiand57 Ceti.[21]Consequently, theChinese namefor τ Ceti itself is "the Fifth Star of Square Celestial Granary" (Chinese:Thiên thương ngũ;pinyin:Tiān Cāng wǔ).[22]

Motion[edit]

Theproper motionof a star is its rate of movement across thecelestial 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 2arc seconds.[nb 3]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.[23]Nearby stars can traverse an angle of arc across the sky more rapidly than the distant background stars and are good candidates forparallaxstudies. In the case of Tau Ceti, the parallax measurements indicate a distance of11.9ly.This makes itone of the closeststar systems to the Sun and the next-closestspectral class-G starafterAlpha Centauri A.[24]

Theradial velocityof 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 itsspectrum.Due to theDoppler shift,theabsorption linesin 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.[25]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).[26]

The distance to Tau Ceti, along with its proper motion and radial velocity, together give the motion of the star through space. Thespace velocityrelative to the Sun is37.2 km/s.[27]This result can then be used to compute an orbital path of Tau Ceti through theMilky Way.It has a mean galacto-centric distance of9.7 kiloparsecs(32000ly) and anorbital eccentricityof 0.22.[28]

Physical properties[edit]

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 been observed with magnitude 13.1. As of 2000, it was137arcsecondsdistant from the primary. It may be gravitationally bound, but it is considered more likely to be a line-of-sight coincidence.[29][30][31]

Most of what is known about the physical properties of Tau Ceti and its system has been determined throughspectroscopicmeasurements. By comparing the spectrum to computed models ofstellar evolution,the age, mass, radius and luminosity of Tau Ceti can be estimated. However, using anastronomical interferometer,measurements of the radius of the star can be made directly to an accuracy of 0.5%.[2]Through such means, the radius of Tau Ceti has been measured to be79.3%±0.4%of thesolar radius.[2]This is about the size that is expected for a star with somewhat lower mass than the Sun.[32]

Rotation[edit]

Therotation periodfor Tau Ceti was measured by periodic variations in the classic H and K absorption lines of singlyionizedcalcium(Ca II). These lines are closely associated with surfacemagneticactivity,[33]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 be34 d.[9]Due to theDoppler effect,therotation rate of a staraffects 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

veq· sini≈ 1 km/s,

whereveqis the velocity at theequator,andiis theinclinationangle of therotation axisto theline of sight.For a typical G8 star, the rotation velocity is about2.5 km/s.The relatively low rotational velocity measurements may indicate that Tau Ceti is being viewed from nearly the direction of its pole.[34][35]

More recently, a 2023 study has estimated a rotation period of46±4 dand aveqsiniof0.1±0.1 km/s,corresponding to a pole-on inclination of7°±7°.[7]

Metallicity[edit]

The chemical composition of a star provides important clues to its evolutionary history, including the age at which it formed. Theinterstellar mediumof dust and gas from which stars form is primarily composed ofhydrogenandheliumwith 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 themetallicity.[36]The amount of metallicity in a star is given in terms of the ratio ofiron(Fe), an easily observed heavy element, to hydrogen. Alogarithmof the relative iron abundance is compared to the Sun. In the case of Tau Ceti, the atmospheric metallicity is

dex,

equivalent to about a third the solar abundance. Past measurements have varied from −0.13 to −0.60.[37][38]

This lower abundance of iron indicates that Tau Ceti is almost certainly older than the Sun. Its age had previously been estimated to be5.8Gyr,but is now thought to be around9 Gyr.[10]This compares with4.57 Gyrfor the Sun. However, age estimates for Tau Ceti can range from 4.4 to12 Gyr,depending on the model adopted.[32]

Besides rotation, another factor that can widen the absorption features in the spectrum of a star ispressure 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. Thelogg,or logarithm of the star's surface gravity, is about 4.4, very close to thelogg= 4.44for the Sun.[37]

Luminosity and variability[edit]

Theluminosityof Tau Ceti is equal to only 55% of theSun's luminosity.[28]Aterrestrial planetwould need to orbit this star at a distance of about0.7AUto match the solarinsolationlevel of Earth. This is approximately the same as the average distance betweenVenusand the Sun.

Thechromosphereof Tau Ceti—the portion of a star's atmosphere just above the light-emittingphotosphere—currently displays little or no magnetic activity, indicating a stable star.[39]One 9-year study of temperature,granulation,and the chromosphere showed no systematic variations; Ca II emissions around the H and Kinfrared bandsshow a possible 11-year cycle, but this is weak relative to the Sun.[34]Alternatively it has been suggested that the star could be in a low-activity state analogous to aMaunder Minimum—a historical period, associated with theLittle Ice Agein Europe, whensunspotsbecame exceedingly rare on the Sun's surface.[40][41]Spectral lineprofiles of Tau Ceti are extremely narrow, indicating low turbulence and observed rotation.[42]The star's asteroseismological oscillations have an amplitude about half that of the Sun and a lower mode lifetime.[2]

Planetary system[edit]

The Tau Ceti planetary system[8][15][43][44][45]
Companion
(in order from star) 		Mass Semimajor axis
(AU) 		Orbital period
(days) 		Eccentricity Inclination Radius
b(unconfirmed) ≥2.0 ± 0.8M🜨 0.105+0.005
−0.006 		13.965+0.017
−0.024 		0.16 ± 0.22 — —
g(unconfirmed) ≥1.75+0.25
−0.40M🜨 		0.133+0.001
−0.002 		20.00+0.02
−0.01 		0.06 ± 0.13 — —
c(unconfirmed) ≥3.1+1.4
−1.1M🜨 		0.195+0.009
−0.011 		35.362+0.088
−0.106 		0.03 ± 0.28 — —
h(unconfirmed) ≥1.83+0.68
−0.26M🜨 		0.243 ± 0.003 49.41+0.08
−0.10 		0.23+0.16
−0.15 		— —
d(unconfirmed) ≥3.6 ± 1.7M🜨 0.374+0.017
−0.020 		94.11+0.70
−0.63 		0.08 ± 0.26 — —
e(unconfirmed) ≥3.93+0.83
−0.64M🜨 		0.538 ± 0.006 162.87+1.08
−0.46 		0.18+0.18
−0.14 		— —
f(unconfirmed) ≥3.93+1.05
−1.37M🜨 		1.334+0.017
−0.044 		636.13+11.70
−47.69 		0.16+0.07
−0.16 		— —
i(unconfirmed) ≤5MJ 3–20 — — — —
Debris disk 6.2+9.8
−4.6–52+3
−8AU 		35±10° —

Principal factors driving research interest in Tau Ceti are its proximity, its Sun-like characteristics, and the implications for possible life on its planets. For categorization purposes, Hall and Lockwood report that "the terms 'solarlike star', 'solar analog', and 'solar twin' [are] progressively restrictive descriptions ".[46]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. In 1988, radial-velocity observations ruled out any periodical variations attributable to massive planets around Tau Ceti inside of Jupiter-like distances.[47][48]Ever more precise measurements continue to rule out such planets, at least until December 2012.[48]The velocity precision reached is about 11 m/s measured over a 5-year time span.[49]This result excludeshot Jupitersand probably excludes any planets with minimal mass greater than or equal to Jupiter's mass and with orbital periods less than 15 years.[50]In addition, a survey of nearby stars by theHubble Space Telescope'sWide Field and Planetary Camerawas completed in 1999, including a search for faint companions to Tau Ceti; none were discovered to limits of the telescope's resolving power.[51]

However, these searches only excluded largerbrown dwarfbodies and closer orbiting giant planets, so smaller, Earth-like planets in orbit around the star, like those discovered in 2012, were not precluded.[51]If hot Jupiters were to exist in close orbit, they would likely disrupt the star'shabitable zone;their exclusion was thus considered positive for the possibility of Earth-like planets.[47][52]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.[53]

Discovery[edit]

On December 19, 2012, evidence was presented that suggested a system of five planets orbiting Tau Ceti.[8]The planets' estimatedminimum masseswere between 2 and 6Earth masses,with orbital periods ranging from 14 to 640 days. One of them, 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.[12][13]The Planetary Habitability Laboratory has estimated that Tau Ceti f, which receives 28.5% as much starlight as Earth, would be within the star's habitable zone, albeit narrowly.[14]

New results were published in August 2017.[44]They confirmed Tau Ceti e and f as candidates but failed to consistently detect planets b (which may be afalse negative), c (whose weakly defined apparent signal was correlated to stellar rotation), and d (which did not show up in all data sets). Instead, they found two new planetary candidates, g and h, with orbits of 20 and 49 days. 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.[44]The updated 4-planet model is dynamically packed and potentially stable for billions of years.

However, with further refinements, even more candidate planets have been detected. In 2019, a paper published inAstronomy & Astrophysicssuggested that Tau Ceti could have a Jupiter or super-Jupiter based on a tangential astrometric velocity of around 11.3 m/s. The exact size and position of this conjectured object have not been determined, though it is at most 5 Jupiter masses if it orbits between 3 and 20 AU.[15][nb 4]A 2020Astronomical Journalstudy by astronomers Jamie Dietrich andDaniel Apaianalyzed the orbital stability of the known planets and, considering statistical patterns identified from hundreds of other planetary systems, explored the orbits in which the presence of additional, yet-undetected planets are most likely. This analysis predicted three planet candidates at orbits coinciding with planet candidates b, c, and d.[55]The close match between the independently predicted planet periods and the periods of the three planet candidates previously identified in radial velocity data supports the genuine planetary nature of candidates b, c, and d. Furthermore, the study also predicts at least one yet-undetected planet between planets e and f, i.e., within the habitable zone.[55]This predicted exoplanet is identified as PxP-4.[nb 5]

Since Tau Ceti is likely aligned in such a way that it is nearly pole-on to Earth (as indicated by its rotation),[7]if its planets share this alignment and have nearly face-on orbits, they would be less similar to Earth's mass and more toNeptune,Saturn,orJupiter.For example, were Tau Ceti f's orbit inclined 70 degrees from beingface-onto Earth, its mass would be4.18+1.12
−1.46Earth masses, making it a middle-to-low end super-Earth. However, these scenarios aren't necessarily true; since Tau Ceti's debris disk has an inclination of35±10,the planets' orbits could be similarly inclined. If the debris disk and f's orbits were assumed to be equal, f would be between5.56+1.48
−1.94and9.30+2.48
−3.24Earth masses, making it slightly more likely to be amini-Neptune.On top of that, the lower the inclination of the planetary orbits the less stable they tend to be over a given time period, as the planets would have greater masses and therefore more gravitational pull which would in turn disturb the orbital stability of neighbouring planets. So, for example, if as estimated in the Korolik et al 2023 study Tau Ceti has a pole-on inclination of around 7 degrees, and the postulated planets do as well, then those planets' orbits would be verging on instability within just a 10 million year timeframe, and therefore it is extremely unlikely they would have survived for the billions of years that make up the lifetime of the star system.[7]

Tau Ceti e[edit]

Main article:Tau Ceti e

Tau Ceti eis a candidate[44]planetorbiting Tau Ceti that was first proposed in 2012 by statistical analyses of the data of the star's variations in radial velocity that were obtained usingHIRES,AAPS,andHARPS.[8][56]Its possible properties were refined in 2017:[44]if confirmed, it would orbit at a distance of 0.552 AU (between the orbits of Venus andMercuryin theSolar System) with an orbital period of 168 days and has aminimum massof 3.93 Earth masses. If Tau Ceti e possessed an Earth-like atmosphere, the surface temperature would be around 68 °C (154 °F).[57]Based upon the incident flux upon the planet, a study by Güdel et al. (2014) speculated that the planet may lie outside the habitable zone and closer to a Venus-like world.[58]

Tau Ceti f[edit]

Main article:Tau Ceti f

Tau Ceti fis a candidate[44]planet orbiting Tau Ceti that was proposed in 2012 by statistical analyses of the star's variations in radial velocity, and also recovered by further analysis in 2017.[8]It is of interest because its orbit places it in Tau Ceti's extended habitable zone.[59]However, a 2015 study implies that it would have been in the temperate zone for less than one billion years, so there may not be a detectablebiosignature.[60]

Few properties of the planet are known other than its orbit and mass. It orbits Tau Ceti at a distance of 1.35 AU (nearMars's orbit in the Solar System) with an orbital period of 642 days and has a minimum mass of 3.93 Earth masses.[44]

However, a reanalysis of the data in 2021 provided an in-depth study of the HARPS spectrograph systematics, showing that the 600-day signal was likely a spurious combination of instrumental systematics with a potential 1000-day yet unknown signal.[16]

Debris disk[edit]

In 2004, a team of UKastronomersled byJane Greavesdiscovered that Tau Ceti has more than ten times the amount ofcometaryandasteroidalmaterial 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.[61]This result puts a damper on the possibility of complex life in the system, because any planets would suffer from largeimpact eventsroughly 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 thedinosaurs".[62]Such bombardments would inhibit the development ofbiodiversitybetween impacts.[63]However, it is possible that a largeJupiter-sizedgas giant(such as the proposed planet "i" ) could deflect comets and asteroids.[61]

The debris disk was discovered by measuring the amount of radiation emitted by the system in thefar infraredportion of thespectrum.The disk forms a symmetric feature that is centered on the star, and its outer radius averages55 AU.The lack of infrared radiation from the warmer parts of the disk near Tau Ceti implies an inner cut-off at a radius of10 AU.By comparison, the Solar System'sKuiper beltextends from 30 to50 AU.To be maintained over a long period of time, this ring of dust must be constantly replenished through collisions by larger bodies.[61]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 ofNeptunein the Solar System.[61]

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.[64]Tau Ceti's belt is only 1/20 as dense as the belt around its young neighbor, Epsilon Eridani.[61]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.[62]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.[64]

Habitability[edit]

Tau Ceti'shabitable zone—the locations where liquid water could be present on an Earth-sized planet—spans a radius of 0.55–1.16AU,where 1 AU is the average distance from the Earth to the Sun.[65]Primitive life on Tau Ceti's planets may reveal itself through an analysis of atmospheric composition via spectroscopy, if the composition is unlikely to be abiotic, just as oxygen on Earth is indicative of life.[66]

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

The most optimistic search project to date wasProject Ozma,which was intended to "search forextraterrestrial intelligence"(SETI) by examining selected stars for indications of artificial radio signals. It was run by the astronomerFrank Drake,who selected Tau Ceti andEpsilon Eridanias 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.[67]Subsequent radio searches of this star system have turned up negative.

This lack of results has not dampened interest in observing the Tau Ceti system for biosignatures. In 2002, astronomersMargaret TurnbullandJill Tarterdeveloped theCatalog of Nearby Habitable Systems(HabCat) under the auspices ofProject Phoenix,another SETI endeavour. The list contained more than17000theoretically habitable systems, approximately 10% of the original sample.[68]The next year, Turnbull would further refine the list to the 30 most promising systems out of5000within 100 light-years from the Sun, including Tau Ceti; this will form part of the basis of radio searches with theAllen Telescope Array.[69]She chose Tau Ceti for a final shortlist of just five stars suitable for searches by the (now cancelled)[70]Terrestrial Planet Findertelescope system, commenting that "these are places I'd want to live if God were to put our planet around another star".[71]

See also[edit]

Notes[edit]

  1. ^From knowing the absolute visual magnitude of Tau Ceti,,and the absolute visual magnitude of the Sun,,the visual luminosity of Tau Ceti can therefore be calculated:.
  2. ^From Tau Ceti the Sun would appear on the diametrically opposite side of the sky at the coordinates RA =13h44m04s,Dec = 15° 56′ 14″, which is located nearTau Boötis.The absolute magnitude of the Sun is 4.8, so, at a distance of3.65pc,the Sun would have an apparent magnitude.
  3. ^The net proper motion is given by,whereμαandμδare the components of proper motion in the RA and declination respectively, andδis the declination. See:Majewski, Steven R. (2006)."Stellar Motions".University of Virginia. Archived fromthe originalon 2012-01-25.Retrieved2007-09-27.
  4. ^If a planet is confirmed to be the cause of this signal, as of August 2020 it would be designated as Tau Ceti i in accordance with IAU exoplanet naming policies.[54]
  5. ^If a planet corresponding to this predicted candidate is confirmed, as of August 2020 it would be designated as Tau Ceti i in accordance with IAU exoplanet naming policies,[54]or Tau Ceti j were the candidate Jovian planet confirmed first.

References[edit]

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