Tau Ceti

Tau Ceti

Location of τ Ceti (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Cetus
Pronunciation	/ˌtaʊ ˈsiːtaɪ/
Right ascension	01h 44m 04.083s[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−B color index	+0.21[4]
B−V color index	+0.72[4]
Distance		11.912 ± 0.007 ly (3.652 ± 0.002 pc)
Absolute magnitude (MV)	5.69±0.01[2]
Absolute bolometric magnitude (Mbol)	5.52±0.02[2]
Details
Gyr
LTT 935[4]
Database references
Exoplanet Archive	data
ARICNS	data

Tau Ceti,

It can be seen with the unaided eye with an apparent magnitude of 3.5.^[2] As seen from Tau Ceti, the Sun would be in the northern hemisphere constellation Boötes with an apparent magnitude of about 2.6.^{[nb 2][10]}

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 likely super-Earths—orbiting Tau Ceti, and two of these are potentially in the habitable zone.^[11][12][13] There is evidence of up to an additional four unconfirmed planets, one of which would be a Jovian planet between 3 and 20 AU from the star.^[14] Because of its debris disk, any planet orbiting Tau Ceti would face far more impact events than Earth. Note that those planetary candidates have been contested recently^[15] and recent discoveries about the stellar inclination cast doubt about the terrestrial nature of these worlds.^[6] 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 extraterrestrial intelligence (SETI) and appears in some science fiction literature.^[16]

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.^[17] 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.^[18][19]

In

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.^[26] 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 kiloparsecs (32000 ly) and an orbital eccentricity of 0.22.^[27]

Physical properties

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 was 137 

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%.^[2] Through such means, the radius of Tau Ceti has been measured to be 79.3%±0.4% of the solar radius.^[2] This is about the size that is expected for a star with somewhat lower mass than the Sun.^[31]

Rotation

The

More recently, a 2023 study has estimated a rotation period of 46±4 d and a v_eq sin i of 0.1±0.1 km/s, corresponding to a pole-on inclination of 7°±7°.[6]

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.^[35] 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.^[36][37]

This lower abundance of iron indicates that Tau Ceti is almost certainly older than the Sun. Its age had previously been estimated to be 5.8 

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".^[45] 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.^[46][47] Ever more precise measurements continue to rule out such planets, at least until December 2012.^[47] The velocity precision reached is about 11 m/s measured over a 5-year time span.^[48] This result excludes 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.^[49] 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.^[50]

However, these searches only excluded larger brown dwarf bodies and closer orbiting giant planets, so smaller, Earth-like planets in orbit around the star, like those discovered in 2012, were not precluded.^[50] If hot Jupiters were to 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.^[46][51] 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.^[52]

Discovery

On December 19, 2012, evidence was presented that suggested a system of five planets orbiting Tau Ceti.^[7] The planets' estimated minimum masses were between 2 and 6 Earth 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.^[11][12] 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.^[13]

New results were published in August 2017.

However, with further refinements, even more candidate planets have been detected. In 2019, a paper published in Astronomy & Astrophysics suggested 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.[14]^{[nb 4]} A 2020 Astronomical Journal study by astronomers Jeremy Dietrich and Daniel Apai analyzed 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.^[54] 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.^[54] 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),^[6] if its planets share this alignment and have nearly face-on orbits, they would be less similar to Earth's mass and more to Neptune, Saturn, or Jupiter. For example, were Tau Ceti f's orbit inclined 70 degrees from being face-on to Earth, its mass would be 4.18+1.12
−1.46 Earth 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 of 35±10, the planets' orbits could be similarly inclined. If the debris disk and f's orbits were assumed to be equal, f would be between 5.56+1.48
−1.94 and 9.30+2.48
−3.24 Earth masses, making it slightly more likely to be a mini-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.^[6]

Tau Ceti e

Tau Ceti f is a candidate^[43] 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.^[7] It is of interest because its orbit places it in Tau Ceti's extended habitable zone.^[58] 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 detectable biosignature.^[59]

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 3.93 Earth masses.^[43]

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.^[15]

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.^[60] 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".^[61] Such bombardments would inhibit the development of biodiversity between impacts.^[62] However, it is possible that a large Jupiter-sized gas giant (such as the proposed planet "i") could deflect comets and asteroids.^[60]

The debris disk was discovered by measuring the amount of radiation emitted by the system in the

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.[63] Tau Ceti's belt is only 1/20 as dense as the belt around its young neighbor, Epsilon Eridani.^[60] 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.^[61] 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.^[63]

Habitability

Tau Ceti's

The most optimistic search project to date was

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.^[67] 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.^[68] She chose Tau Ceti for a final shortlist of just five stars suitable for searches by the (now cancelled)^[69] 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".^[70]

In fiction

See also

• Epsilon Eridani
• List of nearest stars and brown dwarfs
• List of potentially habitable exoplanets
• List of nearest terrestrial exoplanet candidates

Notes

References

Further reading

• Pagano, Michael; et al. (April 2015), "The Chemical Composition of τ Ceti and Possible Effects on Terrestrial Planets", The Astrophysical Journal, 803 (2): 90,
  S2CID 119103881
  , 90.
• Teixeira, T. C.; et al. (January 2009), "Solar-like oscillations in the G8 V star τ Ceti", Astronomy and Astrophysics, 494 (1): 237–242,
  S2CID 59353134
  .
• di Folco, E.; et al. (November 2007), "A near-infrared interferometric survey of debris disk stars. I. Probing the hot dust content around ɛ Eridani and τ Ceti with CHARA/FLUOR", Astronomy and Astrophysics, 475 (1): 243–250,
  S2CID 9594917
  .
• Rammacher, W.; Cuntz, M. (September 2003), "Acoustic Heating Models for the Basal Flux Star τ Ceti Including Time-dependent Ionization: Results for Ca II and Mg II Emission", The Astrophysical Journal, 594 (1): L51–L54,
  S2CID 120328068
  .
• Gray, David F.; Baliunas, Sallie L. (June 1994), "The Activity Cycle of tau Ceti", Astrophysical Journal, 427: 1042,
  doi:10.1086/174210
  .
• Arribas, S.; Martinez-Roger, C. (1988), Cayrel de Strobel, G.; Spite, Monique (eds.), "Iron Lines and Surface Gravity Determination for τ Ceti", The Impact of Very High S/N Spectroscopy on Stellar Physics: Proceedings of the 132nd Symposium of the International Astronomical Union held in Paris, France, June 29-July 3, 1987. International Astronomical Union. Symposium no. 132, vol. 132, Dordrecht: Kluwer Academic Publishers, p. 445,
  Bibcode:1988IAUS..132..445A
  .

External links

Wikimedia Commons has media related to Tau Ceti.

• Near Star Catalog
• Tau Ceti at Jim Kaler's STARS site
• Tau Ceti: Life Amidst Catastrophe? at Centauri Dreams