Tau Ceti,
Latinized from τ Ceti, is a single
star
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
in the
constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The origins of the e ...
Cetus
Cetus () is a constellation, sometimes called 'the whale' in English. The Cetus (mythology), Cetus was a sea monster in Greek mythology which both Perseus and Heracles needed to slay. Cetus is in the region of the sky that contains other water- ...
that is
spectrally similar to the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, although it has only about 78% of the
Sun's mass
The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass of ...
. At a distance of just under from the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
, it is a
relatively nearby star and the closest solitary
G-class star. The star appears stable, with little
stellar variation
A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as e ...
, and is
metal-deficient relative to the Sun.
It can be seen with the unaided eye with an
apparent magnitude
Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
of 3.5.
As seen from Tau Ceti, the Sun would be in the northern hemisphere constellation
Boötes with an apparent magnitude of about 2.6.
[From Tau Ceti the Sun would appear on the diametrically opposite side of the sky at the coordinates RA = , Dec = , which is located near ]Tau Boötis
Tau Boötis, Latinised from τ Boötis, is an F-type main-sequence star approximately 51 light-years away in the constellation of Boötes. It is a binary star system, with the secondary star being a red dwarf. As of 1999, an extrasol ...
. The absolute magnitude of the Sun is 4.8, so, at a distance of , the Sun would have an apparent magnitude .
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 confirmed as
super-Earth
A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively.
The term "super-Earth" refers only to ...
s—orbiting Tau Ceti, with two of these being potentially in the
habitable zone
In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kas ...
.
There are an additional four unconfirmed planets, one of which is a Jovian planet between 3 and 20
AU from the star.
Because of its
debris disk
A debris disk (American English), or debris disc (Commonwealth English), is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris di ...
, any planet orbiting Tau Ceti would face far more
impact event
An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...
s than Earth. Despite this hurdle to
habitability
Habitability refers to the adequacy of an environment for human living. Where housing is concerned, there are generally local ordinances which define habitability. If a residence complies with those laws it is said to be habitable. In extreme e ...
, its
solar analog
Solar-type star, solar analogs (also analogues), and solar twins are stars that are particularly similar to the Sun. The stellar classification is a hierarchy with solar twin being most like the Sun followed by solar analog and then solar-type ...
(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
The search for extraterrestrial intelligence (SETI) is a collective term for scientific searches for intelligent extraterrestrial life, for example, monitoring electromagnetic radiation for signs of transmissions from civilizations on other p ...
(SETI) and appears in some
science fiction
Science fiction (sometimes shortened to Sci-Fi or SF) is a genre of speculative fiction which typically deals with imaginative and futuristic concepts such as advanced science and technology, space exploration, time travel, parallel unive ...
literature.
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
''Uranometria'' is a star atlas produced by Johann Bayer. It was published in Augsburg in 1603 by Christoph Mangle (''Christophorus Mangus'') under the full title ''Uranometria: omnium asterismorum continens schemata, nova methodo delineata, a ...
'' 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
Muḥammad al-Akhṣāṣī al-Muwaqqit ( ar, محمد الاخصاصي الموقت) was an Egyptian astronomer whose and catalogue of stars, ('Pearls of brilliance upon the solar operations'), was written at Cairo about 1650.
Al-Akhsasi was a s ...
, written at
Cairo
Cairo ( ; ar, القاهرة, al-Qāhirah, ) is the capital of Egypt and its largest city, home to 10 million people. It is also part of the largest urban agglomeration in Africa, the Arab world and the Middle East: The Greater Cairo metro ...
about 1650, this star was designated ''Thālith al Naʽāmāt'' (ثالث النعامات - ''thālith al-naʽāmāt''), which was translated into
Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
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 astronomy
Astronomy in China has a long history stretching from the Shang dynasty, being refined over a period of more than 3,000 years. The ancient Chinese people have identified stars from 1300 BCE, as Chinese star names later categorized in the twe ...
, the "
Square Celestial Granary" () refers to an
asterism consisting of τ Ceti,
ι Ceti,
η Ceti,
ζ Ceti,
θ Ceti and
57 Ceti. Consequently, the
Chinese name
Chinese names or Chinese personal names are names used by individuals from Greater China and other parts of the Chinese-speaking world throughout East and Southeast Asia (ESEA). In addition, many names used in Japan, Korea and Vietnam are often a ...
for τ Ceti itself is "the Fifth Star of Square Celestial Granary" ().
Motion
The
proper motion
Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more dista ...
of a star is its rate of movement across the
celestial sphere
In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of 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 second
A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The n ...
s.
[The net proper motion is given by , where ''μα'' and ''μδ'' are the components of proper motion in the RA and declination respectively, and ''δ'' is the seclination. See: ] 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
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...
studies. In the case of Tau Ceti, the parallax measurements indicate a distance of . This makes it
one of the closest star systems to the Sun and the next-closest
spectral class-G star after
Alpha Centauri A
Alpha Centauri (Latinisation of names, Latinized from α Centauri and often abbreviated Alpha Cen or α Cen) is a Star System, triple star system in the constellation of Centaurus (constellation), Centaurus. It consists of 3 stars: ...
.
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
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
. 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 .
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 .
This result can then be used to compute an orbital path of Tau Ceti through the
Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...
. It has a mean galacto-centric distance of () and an
orbital eccentricity
In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values betwee ...
of 0.22.
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 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
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
measurements. By comparing the spectrum to computed models of
stellar evolution
Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is cons ...
, 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 of the
solar radius
Solar radius is a unit of distance used to express the size of stars in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3:
:1\,R_ = 6.957\tim ...
.
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
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
ized
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
(Ca II). These lines are closely associated with surface
magnetic
Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particle ...
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 .
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
:
where ''v''
eq is the velocity at the
equator
The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...
, and ''i'' is the
inclination
Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...
angle of the
rotation axis
Rotation around a fixed axis is a special case of rotational motion. The fixed-axis hypothesis excludes the possibility of an axis changing its orientation and cannot describe such phenomena as wobbling or precession. According to Euler's rot ...
to the
line of sight
The line of sight, also known as visual axis or sightline (also sight line), is an imaginary line between a viewer/observer/ spectator's eye(s) and a subject of interest, or their relative direction. The subject may be any definable object taken ...
. For a typical G8 star, the rotation velocity is about . 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
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
of dust and gas from which stars form is primarily composed of
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
and
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
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
In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...
. The amount of metallicity in a star is given in terms of the ratio of
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
(Fe), an easily observed heavy element, to hydrogen. A
logarithm
In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number to the base is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 o ...
of 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.
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 , but is now thought to be around half that, at .
This compares with for the Sun. However, computed age estimates for Tau Ceti can range from 4.4 to , depending on the model adopted.
Besides rotation, another factor that can widen the absorption features in the spectrum of a star is
pressure broadening
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission (electromagnetic radiation), emission or absorption (electromagnetic radiation), absorption of light in a narrow frequency range, ...
. 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 , or logarithm of the star's surface gravity, is about 4.4, very close to the for the Sun.
Luminosity and variability
The
luminosity
Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...
of Tau Ceti is equal to only 55% of the
Sun's luminosity
The solar luminosity (), is a unit of radiant flux (power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun.
One nominal ...
.
A
terrestrial planet
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, ...
would need to orbit this star at a distance of about to match the solar
insolation level of Earth. This is approximately the same as the average distance between
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
and the Sun.
The
chromosphere of Tau Ceti—the portion of a star's atmosphere just above the light-emitting
photosphere
The photosphere is a star's outer shell from which light is radiated.
The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
—currently displays little or no magnetic activity, indicating a stable star. One 9-year study of temperature,
granulation
Granulation is the process of forming grains or granules from a powdery or solid substance, producing a granular material. It is applied in several technological processes in the chemical and pharmaceutical industries. Typically, granulation invo ...
, 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
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...
profiles of Tau Ceti are extremely narrow, indicating low turbulence and observed rotation. The star's asteroseismological oscillations have an amplitude about half that of the Sun and a lower mode lifetime.
Planetary system
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
Solar-type star, solar analogs (also analogues), and solar twins are stars that are particularly similar to the Sun. The stellar classification is a hierarchy with solar twin being most like the Sun followed by solar analog and then solar-type ...
', and 'solar twin'
reprogressively 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](https://upload.wikimedia.org/wikipedia/commons/5/59/Tau_ceti.png)
In 1988, radial-velocity 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 hot Jupiter
Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere tem ...
s and probably excludes any planets with minimal mass greater than or equal to Jupiter's mass and with orbital periods less than 15 years.[ However, this does not exclude the possibility of a large planet with a mass greater than Jupiter's and an ]orbital plane
The orbital plane of a revolving body is the geometric plane in which its orbit lies. Three non-collinear points in space suffice to determine an orbital plane. A common example would be the positions of the centers of a massive body (host) an ...
that is nearly perpendicular to the line of sight. In addition, a survey of nearby stars by the Hubble Space Telescope
The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
's Wide Field and Planetary Camera
The Wide Field/Planetary Camera (WFPC) (pronounced as wiffpick (Operators of the WFPC1 were known as "whiff-pickers")) was a camera installed on the Hubble Space Telescope launched in April 1990 and operated until December 1993. It was one of ...
was completed in 1999, including a search for faint companions to Tau Ceti; none were discovered to limits of the telescope's resolving power.
However, these searches only excluded larger brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
bodies and closer orbiting giant planets, so smaller, Earth-like planets in orbit around the star, like those discovered in 2012, were not precluded.[ If hot Jupiters were to exist in close orbit, they would likely disrupt the star's ]habitable zone
In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kas ...
; 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.
Discovery
On December 19, 2012, evidence was presented that suggested a system of five planets orbiting Tau Ceti. The planets' estimated minimal 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.[ 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.]
The discovery team refined their methodology, improved their radial-velocity measurements, and published their new results in August 2017. They confirmed Tau Ceti e and f as candidates but failed to consistently detect planets b (which may be a false negative
A false positive is an error in binary classification in which a test result incorrectly indicates the presence of a condition (such as a disease when the disease is not present), while a false negative is the opposite error, where the test resul ...
), 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. 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 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. A 2020 ''Astronomical Journal'' study by astronomers Jeremy Dietrich and Daniel Apai
Daniel Apai (born 1977) is a professor and astrophysicist at The University of Arizona in Tucson, Arizona. He is known for his studies of astrobiology, extrasolar planets, and the formation of planetary systems. He is the principal investigator ...
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. The close match between the independently predicted planet periods and the periods of the three planet candidates previously identified in radial velocity data strongly supports the genuine planet 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. This predicted exoplanet is identified as PxP-4.
If Tau Ceti is aligned in such a way that it is nearly pole-on to Earth (as its rotation could indicate), its planets would be less similar to Earth's mass and more to Neptune
Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
, Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
, or Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
. For example, were Tau Ceti f's orbit inclined 70 degrees from being face-on to Earth, its mass would be 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 , the planets' orbits could be similarly inclined. If the debris disk and f's orbits were assumed to be equal, f would be between and Earth masses, making it slightly more likely to be a mini-Neptune
A Mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen–helium atmosphere, probably with deep layers of ice, rock or liquid oceans (mad ...
.
Tau Ceti e
Tau Ceti e
Tau Ceti e, also called 52 Ceti e, is an exoplanet 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 wer ...
is a confirmed planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
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 Hires may refer to:
* High Resolution Fly's Eye, ultra-high-energy cosmic ray observatory
* High Resolution Echelle Spectrometer or High Resolution Echelle Spectrograph, W. M. Keck Observatory's spectrometer
* Hires Big H, restaurant chain headquar ...
, AAPS, and HARPS
The High Accuracy Radial Velocity Planet Searcher (HARPS) is a high-precision echelle planet-finding spectrograph installed in 2002 on the ESO's 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003. ...
. 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
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
) with an orbital period of 168 days and has a minimum mass
In astronomy, minimum mass is the lower-bound calculated mass of observed objects such as planets, stars and binary systems, nebulae, and black holes.
Minimum mass is a widely cited statistic for extrasolar planets detected by the radial veloc ...
of 3.93 Earth masses. If Tau Ceti e possessed an Earth-like atmosphere, the surface temperature would be around . 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.
Tau Ceti f
Tau Ceti f is a confirmed super-Earth
A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively.
The term "super-Earth" refers only to ...
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
A biosignature (sometimes called chemical fossil or molecular fossil) is any substance – such as an element, isotope, or molecule – or phenomenon that provides scientific evidence of past or present life. Measurable attribute ...
.
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
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
's orbit in the Solar System) with an orbital period of 642 days and has a minimum mass of 3.93 Earth masses.
Debris disk
In 2004, a team of UK astronomer
An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, g ...
s led by Jane Greaves discovered that Tau Ceti has more than ten times the amount of comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...
ary and asteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
al 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 event
An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...
s roughly ten times more frequently than Earth. Greaves noted at the time of her research that "it is likely that ny planetswill experience constant bombardment from asteroids of the kind believed to have wiped out the dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s". Such bombardments would inhibit the development of biodiversity
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
between impacts. However, it is possible that a large Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
-sized gas giant
A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
(such as the proposed planet "i") could deflect comets and asteroids.
The debris disk was discovered by measuring the amount of radiation emitted by the system in the far infrared
Far infrared (FIR) is a region in the infrared spectrum of electromagnetic radiation. Far infrared is often defined as any radiation with a wavelength of 15 micrometers (μm) to 1 mm (corresponding to a range of about 20 THz to ...
portion of the spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
. The disk forms a symmetric feature that is centered on the star, and its outer radius averages . The lack of infrared radiation from the warmer parts of the disk near Tau Ceti implies an inner cut-off at a radius of . By comparison, the Solar System's Kuiper belt
The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
extends from 30 to . 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–, 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
Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
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.][
]
Habitability
Tau Ceti's habitable zone
In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kas ...
—the locations where liquid water could be present on an Earth-sized planet—spans a radius of 0.55–1.16 AU, where 1 AU is the average distance from the Earth to the Sun. 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.
The most optimistic search project to date was Project Ozma
Project Ozma was a search for extraterrestrial intelligence (SETI) experiment started in 1960 by Cornell University astronomer Frank Drake, at the National Radio Astronomy Observatory, Green Bank at Green Bank, West Virginia. The object of the e ...
, which was intended to "search for extraterrestrial intelligence
Extraterrestrial intelligence (often abbreviated ETI) refers to hypothetical intelligent extraterrestrial life. The question of whether other inhabited worlds might exist has been debated since ancient times. The modern form of the concept emerged ...
" (SETI
The search for extraterrestrial intelligence (SETI) is a collective term for scientific searches for intelligent extraterrestrial life, for example, monitoring electromagnetic radiation for signs of transmissions from civilizations on other p ...
) by examining selected stars for indications of artificial radio signals. It was run by the astronomer Frank Drake
Frank Donald Drake (May 28, 1930 – September 2, 2022) was an American astrophysicist and astrobiologist.
He began his career as a radio astronomer, studying the planets of the Solar System and later pulsars. Drake expanded his interests ...
, who selected Tau Ceti and Epsilon Eridani
Epsilon Eridani ( Latinized from ε Eridani), formally named Ran, 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 surf ...
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 turned up negative.
This lack of results has not dampened interest in observing the Tau Ceti system for biosignatures. In 2002, astronomers Margaret Turnbull
Margaret Carol "Maggie" Turnbull (born 1975) is an American astronomer and astrobiologist. She received her PhD in Astronomy from the University of Arizona in 2004. Turnbull is an authority on star systems which may have habitable planets, so ...
and Jill Tarter
Jill Cornell Tarter (born January 16, 1944) is an American astronomer best known for her work on the search for extraterrestrial intelligence (SETI). Tarter is the former director of the Center for SETI Research, holding the Bernard M. Oliver Cha ...
developed the Catalog of Nearby Habitable Systems
The Catalog of Nearby Habitable Systems (HabCat) is a catalogue of star systems which conceivably have habitable planets. The list was developed by scientists Jill Tarter and Margaret Turnbull under the auspices of Project Phoenix, a part of SET ...
(HabCat) under the auspices of Project Phoenix, another SETI endeavour. The list contained more than 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 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 chose Tau Ceti for a final shortlist of just five stars suitable for searches by the (now cancelled) Terrestrial Planet Finder
The Terrestrial Planet Finder (TPF) was a proposed project by NASA to construct a system of space telescopes for detecting extrasolar terrestrial planets. TPF was postponed several times and finally cancelled in 2011. There were two telescope ...
telescope system, commenting that "these are places I'd want to live if God were to put our planet around another star".
See also
* Epsilon Eridani
Epsilon Eridani ( Latinized from ε Eridani), formally named Ran, 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 surf ...
* List of nearest stars and brown dwarfs
This list covers all known stars, brown dwarfs, and sub-brown dwarfs within of the Sun. So far, 131 such objects have been found, of which only 22 are bright enough to be visible without a telescope. The visible light needs to reach or exce ...
* List of potentially habitable exoplanets
This is a list of potentially habitable exoplanets. The list is mostly based on estimates of habitability by the Habitable Exoplanets Catalog (HEC), and data from the NASA Exoplanet Archive. The HEC is maintained by the Planetary Habitability Lab ...
* List of nearest terrestrial exoplanet candidates
* Tau Ceti in fiction
Notes
References
Further reading
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External links
Near Star Catalog
at Jim Kaler's STARS site
Tau Ceti: Life Amidst Catastrophe?
at Centauri Dreams
{{DEFAULTSORT:Tau Ceti
G-type main-sequence stars
Ceti, Tau
Maunder Minimum
Ceti, Tau
Cetus (constellation)
Circumstellar disks
BD-16 0295
Ceti, 52
0071
010700
008102
0509
Thālith al Naʽāmāt
Planetary systems with four confirmed planets