# Is there any habitable exoplanet around Tau Ceti?

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I was wondering… is there any habitable exoplanet around Tau Ceti?

TLDR version: probably not, and claims for the habitability of planets in this system are on shaky ground.

Long version follows.

# Planets

So as of Feng et al. (2017), there are four planet candidates around Tau Ceti:

PlanetMinimum mass ($$M_odot$$)Semimajor axis (AU)
Tau Ceti g$$1.75^{+0.25}_{-0.40}$$$$0.133^{+0.001}_{-0.002}$$
Tau Ceti h$$1.83^{+0.68}_{-0.26}$$$$0.243^{+0.003}_{-0.003}$$
Tau Ceti e$$3.93^{+0.83}_{-0.64}$$$$0.538^{+0.006}_{-0.006}$$
Tau Ceti f$$3.93^{+1.05}_{-1.37}$$$$1.334^{+0.017}_{-0.044}$$

Note that the designations Tau Ceti b, c and d refer to planet candidates that are no longer thought to exist. The error bars refer to the 1% and 99% percentiles. $$M_oplus$$ is the mass of the Earth.

The Feng et al. (2017) paper also notes that the system is dynamically packed, which does not bode well for the prospects for additional planets between the known planet candidates (note that their figure 17 shows the regions where the planets would interfere with each other, not the regions of stability for an additional planet).

# The habitable zone

The conclusion of the paper gives the luminosity of Tau Ceti as 0.52 times solar and the effective temperature as 5344 K. Using these values, the habitable zone limits can be calculated from Kopparapu et al. (2013), which assumes that habitable conditions are maintained by the carbonate-silicate cycle with carbon dioxide as the main (non-condensible) greenhouse gas.

## Inner boundaries

BoundaryDistance (AU)
Recent Venus0.551
Runaway greenhouse0.723
Moist greenhouse0.729

The moist greenhouse limit is the most conservative inner boundary, it occurs where sufficient water vapour enters the upper atmosphere that water loss begins to occur from the planet. In our solar system, Earth is located close to this limit in the inner part of the most conservative habitable zone.

The runaway greenhouse limit occurs where the positive feedback from water vapour overwhelms the stabilising negative feedback from the silicate-carbonate cycle, driving further evaporation of the oceans and higher temperatures. This is thought to have occurred on Venus, leaving the planet in the state it is in today.

The recent Venus limit is based on the possibility that Venus may have retained oceans for several billion years. This is not known for certain as our knowledge of Venus's evolution is rather incomplete and the conditions on the planet's surface are not favourable for driving rovers around investigating the geology.

From this we see that Tau Ceti e is located close to the recent Venus limit and is closer to the star than the runaway greenhouse limit. This suggests that any oceans that may once have existed would likely have boiled off, leaving the planet in a Venus-like state.

Planets g and h are too close to the star.

## Outer boundaries

BoundaryDistance (AU)
Maximum greenhouse1.279
Early Mars1.330

The maximum greenhouse limit is the furthest distance from the star that a cloud-free carbon dioxide atmosphere can support conditions compatible with liquid water. Beyond this, the increased scattering leads to increased reflectivity of the planet and the CO2 would begin to condense, removing it from the atmosphere and leading to runaway cooling. This is the most conservative outer habitable zone boundary. Note that by this point, the planet would require several bars of CO2 which would make it toxic for humans.

The early Mars limit is based on the observation that Mars managed to maintain surface water (e.g. various rivers, and a possible northern ocean) in the early solar system when the Sun was significantly fainter than it is today. Tau Ceti f is located right at this limit.

## Extensions to the habitable zone

None of the planets fall into the most conservative habitable zone, and Tau Ceti e and f are at the boundaries of the most optimistic estimates for the habitable zone boundaries. There are nevertheless options for extending the habitable zone.

At the inner boundary, a runaway greenhouse effect could be avoided on dry planets, where there simply isn't enough water to evaporate to drive the positive feedback, see Zsom et al. (2013). It isn't clear to me that such a planet can be described as habitable, since such planets may lack the hydrothermal systems that could act as the sites for abiogenesis. Their geological evolution would likely be substantially different to Earth's without water to lubricate plate tectonics.

Another possibility is on slowly-rotating planets, where substantial cloud layers can build up on the day side of the planet and increase the reflectivity, as noted by Yang et al. (2014). On the other hand, Scholz et al. (2018) have noted that there appears to be a universal mass-spin relationship that extends from planets to brown dwarfs. This predicts that super-Earths would likely spin too fast for this mechanism to work unless they have been spun down by stellar tides or a large moon.

On the outer boundary, adding additional greenhouse gases such as methane may work to extend the outer habitable zone, see for example Ramirez & Kaltenegger (2018). This has been suggested as the mechanism for allowing surface water on Mars, which would suggest that the "Early Mars" limit is an observed data point within the methane habitable zone. Another possibility is that a dense hydrogen atmosphere could maintain liquid water, e.g. Pierrehumbert & Gaidos (2011) though the pressure of such an atmosphere may well have implications for the geology of the planet and hence the potential for abiogenesis.

Planets whose climates are stabilised by something other than the carbonate-silicate cycle, or have substantially different atmospheric compositions would have different habitable zone boundaries (if subsurface oceans on icy worlds are habitable, there may be interesting prospects for dwarf planets in the outer debris belt), but this is already getting speculative enough, besides there is another possible objection to the habitability of these planets…

# Planetary masses

A limitation of the radial velocity method is that only the minimum masses can be derived. With Tau Ceti, we have a possible means to estimate the true masses: the star is surrounded by a debris disc (this would likely provide a source of impactors onto the planets, how bad the situation is depends on how much material is being perturbed into the inner system). Using Herschel observations, Lawler et al. (2014) give an inclination of 35±10 degrees. Assuming that the planets lie in the same plane as the disc, the true masses would therefore be approximately 1.74 times greater than the minimum masses.

Under this assumption, the true masses of the planets e and f both come out as about 6.85 Earth masses. Taking the 99% lower limit on the minimum mass error bars and a 45° orbital inclination as a low estimate, these would be 4.65 Earth masses for e and 3.62 Earth masses for f.

# The nature of the planets

According to Rogers (2014), the transition between rocky and Neptune-like planets is somewhere in the region of 1.4 to 1.6 Earth radii. Using the mass-radius relationship from Zeng et al. (2016) and their core mass fraction of 0.26 for typical terrestrial planets, these radius limits correspond to terrestrial planets of roughly 3.3 to 5.4 Earth masses.

This suggests that Tau Ceti e and f are fairly likely to be sub-Neptunes rather than rocky planets, although the caveats are that in the optimistic case they can have masses below the rocky/Neptune-like transition, and that there do seem to be a few cases of rocky planets above the transition (most of those are likely to be evaporated cores of Neptune-like planets, which wouldn't apply to Tau Ceti e and f as they have much lower levels of stellar irradiation).

# Conclusion

Given the current state of knowledge, Tau Ceti does not look like a good prospect for habitable planets. Tau Ceti e and f are rather marginal in terms of their location within the habitable zone, and their masses are sufficiently high that there is a good chance that they are sub-Neptunes rather than rocky planets. The dynamical packing of the system makes it unlikely that there can be a smaller, temperate planet in the habitable zone between the known planets.

## Potentially Habitable ‘Another Earth’ Found Orbiting Tau Ceti

Tau Ceti is the closest single star that has the same spectral classification as our Sun. It is located about 12 light-years away in the constellation of Cetus.

The astronomers combined more than six-thousand observations from three different instruments and intensively modeled the data. Using new techniques, they have found a method to detect signals half the size previously thought possible. This greatly improves the sensitivity of searches for small planets and suggests that Tau Ceti is not a lone star but hosts a rich planetary system.

“We pioneered new data modeling techniques by adding artificial signals to the data and testing our recovery of the signals with a variety of different approaches. This significantly improved our noise modeling techniques and increased our ability to find low mass planets,” explained Dr Tuomi, who co-authored a paper to be published in the journal Astronomy & Astrophysics (arXiv.org version).

Tau Ceti’s five planets are estimated to have masses between two and six times the mass of the Earth – making it the lowest-mass planetary system yet detected. One of the planets lies in the habitable zone of the star and has a mass around five times that of Earth, making it the smallest planet found to be orbiting in the habitable zone of any Sun-like star.

This is an artist’s impression of the Tau Ceti system (J. Pinfield / RoPACS Network / University of Hertfordshire)

“We chose Tau Ceti for this noise modeling study because we had thought it contained no signals. And as it is so bright and similar to our Sun it is an ideal benchmark system to test out our methods for the detection of small planets,” said co-author Dr Hugh Jones.

“Tau Ceti is one of our nearest cosmic neighbors and so bright that we may be able to study the atmospheres of these planets in the not too distant future. Planetary systems found around nearby stars close to our Sun indicate that these systems are common in our Milky Way Galaxy,” added Dr James Jenkins of the Universidad de Chile and the University of Hertfordshire.

“This discovery is in keeping with our emerging view that virtually every star has planets, and that the galaxy must have many such potentially habitable Earth-sized planets. They are everywhere, even right next door!” said co-author Dr Steve Vogt of the University of California Santa Cruz.

“We are now beginning to understand that Nature seems to overwhelmingly prefer systems that have a multiple planets with orbits of less than one hundred days. This is quite unlike our own Solar system where there is nothing with an orbit inside that of Mercury. So our Solar system is, in some sense, a bit of a freak and not the most typical kind of system that Nature cooks up.”

“As we stare at the night sky, it is worth contemplating that there may well be more planets out there than there are stars … some fraction of which may well be habitable,” said co-author Dr Chris Tinney of the University of New South Wales.

Bibliographic information: M. Tuomi et al. 2012. Signals embedded in the radial velocity noise. Accepted for publication in the Astronomy and Astrophysics arXiv: 1212.4277

## Five Planets Around Nearby Star Tau Ceti One in Habitable Zone

Look up in the sky tonight towards the southeast in the constellation Cetus. There’s a naked-eye star named Tau Ceti that lies about 12 light-years away from Earth, and astronomers have discovered a system of at least five planets orbiting Tau Ceti, including one in the star’s habitable zone.

While the recent discovery of a Earth-sized planet around the triple star system Alpha Centauri is the closest planet that has been discovered at just 4.3 light years away, this new discovery is the closest single sun-like star that we know of to host of an entire system of planets. The five planets are estimated to have masses between two and six times the mass of the Earth, making it the lowest-mass planetary system yet detected. The planet in the habitable zone of the star has a mass around five times that of Earth, making it the smallest planet found to be orbiting in the habitable zone of any Sun-like star.

“This discovery is in keeping with our emerging view that virtually every star has planets, and that the galaxy must have many such potentially habitable Earth-sized planets,” said astronomer Steve Vogt from UC Santa Cruz, coauthor of the paper describing the discovery. “We are now beginning to understand that nature seems to overwhelmingly prefer systems that have multiple planets with orbits of less than 100 days. This is quite unlike our own solar system, where there is nothing with an orbit inside that of Mercury. So our solar system is, in some sense, a bit of a freak and not the most typical kind of system that Nature cooks up.”

An artist’s impression of the Tau Ceti system. (Image by J. Pinfield for the RoPACS network at the University of Hertfordshire.)

Tau Ceti has long been a target of both detailed astronomical study and hopeful science fiction, since it is among one of the 20 closest stars to Earth. It is also easily visible to the naked eye and can be seen from both the Northern and Southern Hemisphere. During the 1960’s, Project Ozma, led by SETI’s Frank Drake, probed Tau Ceti for signs of life by studying interstellar radio waves with the Green Bank radio telescope. Science fiction authors like Robert Heinlein, Isaac Asimov and Frank Herbert used Tau Ceti as destinations and focal points in their books.

Scientists know this star has a dusty debris disk at least 10 times more massive than our solar system’s Kuiper Belt, and it has been observed long enough that no planets larger than Jupiter have been found.

An international team of astronomers from the United Kingdom, Chile, United States, and Australia, combined more than six-thousand observations from the UCLES spectrograph on the Anglo-Australian Telescope, the HIRES spectrograph on the Keck Telescope, and reanalysis of spectra taken with the HARPS spectrograph available through the European Southern Observatory public archive.

Using new techniques, the team found a method to detect signals half the size of previous observations, greatly improving the sensitivity of searches for small planets.

“We pioneered new data modeling techniques by adding artificial signals to the data and testing our recovery of the signals with a variety of different approaches,” said lead author Mikko Tuomi of the University of Hertfordshire. “This significantly improved our noise modeling techniques and increased our ability to find low-mass planets.”

Tau Ceti e is the planet in the habitable zone, and its year is about half as long as ours. An independent study of the data from the system done by Abel Méndez at the University of Puerto Rico at Arecibo says that the fifth planet, Tau Ceti f, may also be in the habitable zone.

While over 800 planets have been discovered orbiting other worlds, planets in orbit around the nearest Sun-like stars are particularly valuable to study, the team said.

“Tau Ceti is one of our nearest cosmic neighbors and so bright that we may be able to study the atmospheres of these planets in the not-too-distant future. Planetary systems found around nearby stars close to our Sun indicate that these systems are common in our Milky Way galaxy,” said James Jenkins of Universidad de Chile, a visiting fellow at the University of Hertfordshire.

The team’s paper that has been accepted for publication in Astronomy & Astrophysics.

## Contents

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. [18] 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. [19] [20]

In Chinese astronomy, the "Square Celestial Granary" (Chinese: 天倉 pinyin: Tiān Cāng ) refers to an asterism consisting of τ Ceti, ι Ceti, η Ceti, ζ Ceti, θ Ceti and 57 Ceti. [21] Consequently, the Chinese name for τ Ceti itself is "the Fifth Star of Square Celestial Granary" (Chinese: 天倉五 pinyin: Tiān Cāng wǔ ). [22]

The proper motion of a star is its rate of movement across the celestial sphere, determined by comparing its position relative to more distant background objects. Tau Ceti is considered to be a high-proper-motion star, although it only has an annual traverse of just under 2 arc seconds. [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 for parallax studies. In the case of Tau Ceti, the parallax measurements indicate a distance of 11.9 ly . This makes it one of the closest star systems to the Sun and the next-closest spectral class-G star after Alpha Centauri A. [24]

The radial velocity of a star is the component of its motion that is toward or away from the Sun. Unlike proper motion, a star's radial velocity cannot be directly observed, but can be determined by measuring its spectrum. Due to the Doppler shift, the absorption lines in the spectrum of a star will be shifted slightly toward the red (or longer wavelengths) if the star is moving away from the observer, or toward blue (or shorter wavelengths) when it moves toward the observer. In the case of Tau Ceti, the radial velocity is about −17 km/s, with the negative value indicating that it is moving toward the Sun. [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. The space velocity relative to the Sun is 37.2 km/s . [27] This result can then be used to compute an orbital path of Tau Ceti through the Milky Way. It has a mean galacto-centric distance of 9.7 kiloparsec ( 32 000 ly ) and an orbital eccentricity of 0.22. [28]

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 arcseconds distant 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 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. [32]

### Rotation Edit

The rotation period for Tau Ceti was measured by periodic variations in the classic H and K absorption lines of singly ionized calcium (Ca II). These lines are closely associated with surface magnetic activity, [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 be 34 d . [10] 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

veq · sin i ≈ 1 km/s,

where veq is the velocity at the equator, and i is the inclination angle of the rotation axis to the line of sight. For a typical G8 star, the rotation velocity is about 2.5 km/s . The relatively low rotational velocity measurements may indicate that Tau Ceti is being viewed from nearly the direction of its pole. [34] [35]

### Metallicity Edit

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. [36] 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

equivalent to about a third the solar abundance. Past measurements have varied from −0.13 to −0.60. [7] [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 about 10 Gyr , but is now thought to be around half that, at 5.8 Gyr . [11] This compares with 4.57 Gyr for the Sun. However, computed age estimates for Tau Ceti can range from 4.4 to 12 Gyr , depending on the model adopted. [32]

Besides rotation, another factor that can widen the absorption features in the spectrum of a star is pressure broadening. The presence of nearby particles affects the radiation emitted by an individual particle. So the line width is dependent on the surface pressure of the star, which in turn is determined by the temperature and surface gravity. This technique was used to determine the surface gravity of Tau Ceti. The log g , or logarithm of the star's surface gravity, is about 4.4, very close to the log g = 4.44 for the Sun. [7]

### Luminosity and variability Edit

The luminosity of Tau Ceti is equal to only 55% of the Sun's luminosity. [28] A terrestrial planet would need to orbit this star at a distance of about 0.7 AU to match the solar insolation level of Earth. This is approximately the same as the average distance between Venus and the Sun.

The chromosphere of Tau Ceti—the portion of a star's atmosphere just above the light-emitting photosphere—currently displays little or no magnetic activity, indicating a stable star. [38] One 9-year study of temperature, granulation, and the chromosphere showed no systematic variations Ca II emissions around the H and K infrared bands show a possible 11-year cycle, but this is weak relative to the Sun. [34] 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. [39] [40] Spectral line profiles of Tau Ceti are extremely narrow, indicating low turbulence and observed rotation. [41] The star's oscillations have an amplitude about half that of the Sun and a lower mode lifetime. [2]

The Tau Ceti planetary system [9] [15] [42] [43] [44] [45]
Companion
(in order from star)
Mass Semimajor axis
(AU)
Orbital period
(days)
b (unconfirmed) ≥2.00 ± 0.80 M 0.105 ± 0.006 13.965 ± 0.024 0.16 ± 0.22
g ≥ 1.75 +0.25
−0.40 M
0.133 +0.001
−0.002
20.00 +0.02
−0.01
0.06 +0.13
−0.06
c (unconfirmed) ≥3.1 ± 1.40 M 0.195 ± 0.011 35.362 ± 0.106 0.03 ± 0.28
h ≥ 1.83 +0.68
−0.26 M
0.243 ± 0.003 49.41 +0.08
−0.10
0.23 +0.16
−0.15
d (unconfirmed) ≥3.60 ± 1.7 M 0.374 ± 0.02 94.11 ± 0.7 0.08 ± 0.26
e ≥ 3.93 +0.83
−0.64 M
0.538 ± 0.006 162.87 +1.08
−0.46
0.18 +0.18
−0.14
f ≥ 3.93 +1.05
−1.37 M
1.334 +0.017
−0.044
636.13 +11.70
−47.69
0.16 +0.07
−0.16
i (unconfirmed) 1 – 2 MJ 3 – 20
Debris disk 6.2 +9.8
−4.6 – 52 +3
−8 AU
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 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. [50] 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. [51]

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. [51] 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. [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]

On December 19, 2012, evidence was presented that suggested a system of five planets orbiting Tau Ceti. [9] 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. [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]

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), 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 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. [15] [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. [45] 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 c. Furthermore, the study also predicts at least one yet-undetected planet between planets e and f, i.e., within the habitable zone. [45] This predicted exoplanet is identified as PxP-4. [nb 5] 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. [43]

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, 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.

### Tau Ceti e Edit

Tau Ceti e is a confirmed [43] planet orbiting Tau Ceti that was detected by statistical analyses of the data of the star's variations in radial velocity that were obtained using HIRES, AAPS, and HARPS. [9] [55] Its possible properties were refined in 2017: [43] it orbits at a distance of 0.552 AU (between the orbits of Venus and Mercury in the Solar System) with an orbital period of 168 days and has a minimum mass of 3.93 Earth masses. If Tau Ceti e possessed an Earth-like atmosphere, the surface temperature would be around 68 °C (154 °F). [56] 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. [57]

### Tau Ceti f Edit

Tau Ceti f is a confirmed [43] super-Earth orbiting Tau Ceti that was discovered in 2012 by statistical analyses of the star's variations in radial velocity, based on data obtained using HIRES, AAPS, and HARPS. [9] It is of interest because its orbit places it in Tau Ceti's extended habitable zone. [58] 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. [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]

### Debris disk Edit

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 far infrared portion of the spectrum. The disk forms a symmetric feature that is centered on the star, and its outer radius averages 55 AU . The lack of infrared radiation from the warmer parts of the disk near Tau Ceti implies an inner cut-off at a radius of 10 AU . By comparison, the Solar System's Kuiper belt extends from 30 to 50 AU . To be maintained over a long period of time, this ring of dust must be constantly replenished through collisions by larger bodies. [60] The bulk of the disk appears to be orbiting Tau Ceti at a distance of 35– 50 AU , well outside the orbit of the habitable zone. At this distance, the dust belt may be analogous to the Kuiper belt that lies outside the orbit of Neptune in the Solar System. [60]

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 Edit

Tau Ceti's habitable zone—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. [64] 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. [65]

The most optimistic search project to date was Project Ozma, which was intended to "search for extraterrestrial intelligence" (SETI) by examining selected stars for indications of artificial radio signals. It was run by the astronomer Frank Drake, who selected Tau Ceti and Epsilon Eridani as the initial targets. Both are located near the Solar System and are physically similar to the Sun. No artificial signals were found despite 200 hours of observations. [66] 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 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 17 000 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 (indefinitely postponed) [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]

1. ^ From knowing the absolute visual magnitude of Tau Ceti, M V ∗ = 5.69 >=5.69> , and the absolute visual magnitude of the Sun, M V ⊙ = 4.83 >=4.83> , the visual luminosity of Tau Ceti can therefore be calculated: L V ∗ / L V ⊙ = 10 0.4 ( M V ⊙ − M V ∗ ) >/L_>=10^<0.4(M_>-M_>)>> .
2. ^ From Tau Ceti the Sun would appear on the diametrically opposite side of the sky at the coordinates RA = 13 h 44 m 04 s , Dec = 15° 56′ 14″, which is located near Tau Boötis. The absolute magnitude of the Sun is 4.8, so, at a distance of 3.65 pc , the Sun would have an apparent magnitude m = M v + 5 ⋅ ( log 10 ⁡ 3.64 − 1 ) = 2.6 +5cdot (log _<10>3.64-1)=2.6> .
3. ^ The net proper motion is given by μ = μ δ 2 + μ α 2 ⋅ cos 2 ⁡ δ = 1907.79 mas/y ^<2>+mu _^<2>cdot cos ^<2>delta >>=1907.79
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## Habitable Worlds Around Tau Ceti?

Yesterday’s look at NExSS (the Nexus for Exoplanet System Science), NASA’s new ‘virtual institute,’ focused on the multidisciplinary nature of the effort. The work I’m looking at today, an analysis of the planets around Tau Ceti performed at Arizona State University, only emphasizes the same point. To get a read on whether two planets that are possibly in Tau Ceti’s habitable zone are likely to be terrestrial worlds like Earth, the ASU team brought the tools of Earth science into play, in particular the work of Sang-heon Shim.

Shim is a mineral physicist who worked with astrophysicists Michael Pagano, Patrick Young and Amanda Truitt in the Tau Ceti analysis. His perspective was vital because early work had already suggested that Tau Ceti has an unusual balance between the rock-forming minerals magnesium and silicon. In fact, the ratio of magnesium to silicon here is 1.78, about 70% more than we find in the Sun. That casts long-standing views of Tau Ceti as Sol’s twin into doubt, and raises questions about the nature of the planets that formed around it.

There is evidence for five of these, with two — Tau Ceti e and f — thought to be in the habitable zone. That’s an attractive possibility, for Tau Ceti is nearby at 12 light years, a solitary G-class star like the Sun, and a relatively stable one at that. No wonder it figures prominently in science fiction, its very proximity made a significant plot issue by Larry Niven in his 1968 novel A Gift from Earth, which depicts an isolated Tau Ceti colony that can still receive the occasional cargo from Earth. Isaac Asimov made a Tau Ceti planet the home of the first human extrasolar settlement in The Caves of Steel (1954).

Image: The Sun is at the left in this comparison with Tau Ceti. Credit: R.J. Hall via Wikimedia Commons.

In fact, I can think of few stars that have received so much attention from writers. Might some of the planets there really be habitable? The two planets we are looking at are ‘super-Earths,’ with masses of 4.29±2.00 and 6.67±3.50 times that of Earth respectively. The new work makes the prospect of Earth-like conditions unlikely. In fact, Shim’s mineralogical study indicates that the high magnesium/silicon ratio of the parent star could produce planets unlike any we’re familiar with, as the scientist explains:

“With such a high magnesium and silicon ratio it is possible that the mineralogical make-up of planets around Tau Ceti could be significantly different from that of Earth. Tau Ceti’s planets could very well be dominated by the mineral olivine at shallow parts of the mantle and have lower mantles dominated by ferropericlase.”

Ferropericlase is a magnesium/iron oxide that is thought to be a major constituent of the Earth’s lower mantle, along with silicate perovskite, which is a magnesium/iron silicate. Because ferropericlase is viscous, an abundance of it in the mantle would make mantle rock flow more readily, possibly affecting plate tectonics and volcanism at the planetary surface. The resulting world would pose challenges for the development of life, and certainly for its detection. “Faster geochemical cycling,” the paper notes, “could impede the buildup of biologically produced non-equilibrium chemical species in the planet’s atmosphere.”

The paper describes what it calls a detectability index, or DI, that gauges the ability of a planet to house life and to maintain biosignatures of the kind we hope to detect with new space telescope missions. Tau Ceti planets in the habitable zone might, in other words, be habitable, but unlikely to produce detectable life signs in their atmospheres. Life would not necessarily be absent, but detecting it would require a thorough study of planetary evolution.

Another issue is the length of time a planet spends in the habitable zone. Tau Ceti e’s position is deeply problematic. The authors believe the world is reaching the end of its habitable lifetime and is at best on the extreme inner edge of the habitable zone. Tau Ceti f, meanwhile, appears to be near the outer edge of the habitable zone, but evidently moved into it within the past 1.5 billion years, and probably in much less time than this.

Assume even a billion years in the habitable zone and bear in mind that Earth’s biosphere took roughly two billion year to produce biosignatures that would be theoretically detectable. The DI for this world — our ability to find life if it does exist on Tau Ceti f — would be low indeed. A long habitable lifetime may be in this planet’s future, but that doesn’t help us now:

Even in the most pessimistic case, the planet will have about 7 Gy of habitable lifetime until the end of the main sequence, plus additional time while the star traverses the subgiant branch. From a detectability standpoint, however, f is a poor candidate. At best, the planet has been in the HZ for

Comments on this entry are closed.

We should be a bit careful of using Earth as the gold standard for life and assume that different conditions for life’s emergence will be poorer with different conditions. Reality may turn out to be very different.

In our own solar system, we are going to know within the next century whether the lack of bio-signatures except for Earth indicates an absence, or hidden, life. I personally think life in the solar system is confined to Earth, and all other worlds are currently sterile. I would be delighted to be proved wrong, however.

Exoplanet bio-signatures will likely be very rare, but represent the low hanging fruit for discovery. I hope that they stimulate the drive to send fast probes (like the DragonFly Project) to search/characterize for life on the nearer exoplanets.

The masses given for e and f are the minimum masses from radial velocity measurements. If the Tau Ceti planets are aligned with the debris disc which is inclined at 35 degrees from the plane of the sky, the masses are a factor of

1.7 times greater. The resultant true masses are sufficiently high that it seems more likely that they are mini-Neptunes rather than solid planets.

The other question is whether these planets exist: according to the discovery paper the HARPS dataset only contained evidence for the innermost 3 planets b, c and d, while the 5-planet solution arises from the combination of the HARPS data with lower-precision data from HIRES and AAPS.

Nevertheless, the possibilities for planetary composition in the system are definitely worth bearing in mind if an Earth-mass planet is discovered in the system’s habitable zone (e.g. in the five-planet solution there seems to be a stable zone between the orbits of e and f that could perhaps host such a planet).

The increased gravity and resultant far greater mantle density wouldnt increase tectonics, vulcanism or geochemical cycling, though they might prologue it, which is good especially around a G8 star with a long main sequence lifetime . I’m sure Dimitar Sasselov might have a different view if these planets were found to be big terrestrials. Kite et al 2008 describe the extended “volcanism” of large terrestrial planets. What’s a billion years when your star still has 7 left on the main sequence ?

Tau Ceti, Please fight on ! Never mind your large planet bombarding asteroid/Kuiper belt , you can deliver !

@Ashley Baldwin April 24, 2015 at 13:39

‘The increased gravity and resultant far greater mantle density wouldnt increase tectonics, vulcanism or geochemical cycling, though they might prologue it…’

I would tend to agree, magnesium oxide has a higher melting point, higher density and significantly higher thermal conductivity than silicon dioxide and would tend to suppress convection in the upper/lower mantle. How this affects the formation of a magnetic field is unknown as well though as our Earth is thought to generate it’s magnetic field from solidification of the outer core. Heat that can’t get out would tend to form a more liquid core than a solid one I would think.

I wrote in Centauri Dreams three months about Tau Ceti in my review of habitable planet candidates.

Basically, Tau Ceti f doesn’t even make the cut on the PHL’s “Habitable Exoplanet Catalog” (which is usually far too liberal with its definitions of “habitable”). In the case of Taus Ceti e, it is far more likely to be a hot mini-Neptune or maybe, optimistically, a cool super-Venus. Of course, this assumes that these planets even exist in the first place. They remain unconfirmed and there is reason to believe that the RV signatures of the putative planets are the result of surface activity. A similar thing happened last year with the disappearing habitable planets of GJ 581 and 667C.

With the RV range on ‘f’ it could easily be anything from a 3M terrestrial to a 10M ice giant . The dreaded msinp ! It’s this combined with uncertainty over the RV discovery in the first place that confine it to the ” need more information category”.
David Spergel , not content with getting a coronagraph on board is hoping to combine the completed Gaia dataset with WFIRST observations to produce in essence a watered down TPF-I /NEAT. It would have a 15 year baseline. From speaking to Mike Perryman of Hipparchus /Gaia fame and a close colleague of Spergel at Princeton , this is just possible though difficult. It will provide accurate astrometric data on planets down to Earth mass , with less than an 18 year period , out to 10 parsecs. Circa 119 suitable stars apparently . Detailed presentation available from January AAS and DS himself 2013. Meantime the 2016/17 VLT ESPRESSO spectrograph has the sensitivity though whether it can overcome Tau Ceti’s stellar “noise” is unclear.

With all that extra magnesium to react with the available oxygen, there may be less oxygen available to react with the iron. This could give a larger iron core and a mantle depleted in iron oxides.
I believe the masses are high enough that the planet should capture significant hydrogen even if it doesn’t form in the ice zones. It will be hard for any life to generate enough free oxygen to oxidize all the hydrogen and elemental surface iron, a real challenge for any advanced life.

Colonizing habitable planets in other star systems is a science fiction staple. These stories are examples of planetary chauvinism, in my opinion.

A civilization capable of making a generation star ship is also capable of exploiting and settling smaller bodies like the Main Belt asteroids or Kuiper Belt snowballs.

We can only exploit the top few kilometers of a planetary surface. Heat and pressure bar us from burrowing deeper. In contrast the entire *volume* of a small body is accessible. When it comes to accessible real estate and resources, asteroids, KBOs and Oort objects have a lot more to offer than Sol’s planets and large moons.

By the time humankind reaches a level where we can even think of traveling to another star system, I suspect planetary chauvinism will have become ancient history.

I’m looking forward to increased capabilities of observing and measuring exoplanets. We’ve made lots of progress, but there’s still plenty of ambiguity. Getting definite results, and finding a habitable exo-earth would go a long way in increasing interest in space.

Hop David: I’m with you on the small bodies. It seems to me that is really the only way to a true spacefaring civilization. Even if someone did a Mars One and succeeded in setting up self sustaining population, I’m not sure I would truly consider that a spacefaring civilization.

Small body colonies make a lot of sense to me too, although the issues of human adaptation to micro gravity (be they solved technologically or biologically) will need to be adressed.

I wonder if our solar system might actually be sub-optimal for a space faring race – perhaps a system that never formed major planets and instead retained remnants of its protoplanetary disk as massive asteroid belts and comet clouds might be a more attractive home, once we’ve (if we!) become truly at home in space.

In my opinion, terraformed planets are very important as the role of “true biospheres”. Ring worlds are probably beyond truly engineering. Big colonies will not be enough to have a biosphere like a planet. As humen cities will be great, but no as biospheres.

Given the prospect of a world moving out of the habitable zone whilst a world has recently moved into it… how would panspermia affect things? If life originated on the inner habitable planet and was carried through space to colonise the outer one (similar to what has been suggested for Mars and Terra), then life could get a foothold on f much quicker, which would affect detectability.

The Culture of small body colonizers will be substantially different, because the governance will look very different than Western Democracies, consequently the rate of their technological advancement will be different.

Assuming a colonizing attempt was launched at a relatively near-by solar system that has a marginally suitable. Somewhat Like Mars but with enough atmosphere so that water is stable but you cannot be outside unprotected for more than 20 mins (mostly due to solar radiation slightly higher CO2 levels) . Other colonists wish to colonize dwarf planets/large asteroids.

In general the more technological support is needed for basic survival the more likely, a colonizing site will lean towards oligarchy/police state. This a consequence of maintaining the technology to enable human habitation, and to keep the critical systems from damage through internal disturbances (which would kill everyone). Things will be heavily monitored/controlled. Also generation after generation of technologists will have to be assured by directive most likely. So not too many letters and arts majors there. That is contrast to the Planet bound settlers who only need very basic technology to survive, and are more free range, in poultry parlance.
I think the technological race between the Soviets and US in the post war is somewhat illustrative to the effects of differing governance on technology advances vis-à-vis Planets VS Small body colonists.

@RobFlores April 27, 2015 at 11:58

‘Somewhat Like Mars but with enough atmosphere so that water is stable but you cannot be outside unprotected for more than 20 mins (mostly due to solar radiation slightly higher CO2 levels) . Other colonists wish to colonize dwarf planets/large asteroids. ‘

Venus type worlds could also be colonised via cloud cities, CO2 makes a very nice lifting gas. Energy and surface area wise Venus has multiples of liveable space than Earth.

Stan Robinson’s fine new novel has a starship venturing to Tau Ceti, with many ecological problems afoot: AURORA.

@ John If we ever move into the galaxy I definitely think that hot young stars with lots of small bodies will be the gold standard. Of course assuming no FTL our descendants probably won’t wait for one to come around. Assumes of course they plan on moving on in a few million years.

@Zanstel I think your biosphere point is a good one. Of course maybe thats a good reason to make planets biological reserves. I do think spreading the biosphere is a big reason for spreading out. Whether the AIs will think so I don’t know. Also biospheres may come in many sizes, especially if you include advanced human technology as part of them.

@ RobFlores You make a good argument. Of course I’m not sure it applies any more to colonies than it would to most planets, at least not before a great deal of terraforming is completed. On the other hand I wouldn’t lay odds that the necessary tech will be anymore confining to our descendants than oil refineries and air conditioner factories are to us.

RobFlores, a world advanced enough for interstellar travel is one where joe citizen can enrich uranium and make smallpox in his garage. It would have to be a police state everywhere except on colonies that had such low populations that everyone knew and respected everyone (and thus exactly the opposite to your claim). All others would be very suspicious of these micro-colonies, but they could exist if all their interactions with others were very heavily monitored.

Rob Henry, are you so sure about that?
A small population trusting each other?

I draw you to a contra-factual, ALASKA (albeit an imperfect example)
There Is a lot of literature and pop culture pointing to the fact that in the
recent past Alaskans where human habitation was sparse, had a very cloistered mindset. Not a spirit of trusting your fellow man or even caring about them.
Maybe a better comparison is a Polynesian colonial expansion, but note
they were of the same race and creed, will space colonies be multi-cultural?

@Rob Henry MAD on an individual basis. What a terrible thought.

We all should be wary of making assumptions based on theoretical outcomes. Until the said planets are observed directly and in full, we might find out that the theory doesn’t match reality.Time will tell.
I am with Hop David re colonization and spaceships. As I stated before, once you have technology allowing you to travel to other stars to colonize their planets, you no longer have to. You already have technology allowing you to create sustainable artificial environments with far less cost intensive measures and far less time intensive measures.
If a planet has a bioshpere it would be for us too much time consuming and wasteful to adopt either it or our species for colonization. It is much better as living natural laboratory to study.
Exception from this would be natural “habitable dead worlds” where it would be relatively easy to terraform the planets.But I would guess this would be pretty rare.
Gregory Benford-thank you for reminding me of the much anticipated novel by Kim Stanley Robinson. I almost forgot that it will be published in a month or so :)

I am looking forward to similar analyses of other solar type stars with planets in the solar neighborhood, such as 82 Eridani, 61 Virginis, Nu 2 Lupi.
And even more so to the time that we have so many reliable data that we can really do modeling of planetary systems based on stellar type and elemental abundances.

These planets are huge, but what if either of these two words have Mars-sized moons? On Tau Ceti F life could have existed under frozen, cracking ice with thermal heating caused by tidal heating with other moons, Life that could then move to the surface when the habitable zone moved over the planet. In the case of Tau Ceti E, life could have retreated into the rocky layers of the planet or floated in the upper atmosphere (remember that on Venus there is an Earth-pressure environment high in the clouds.)

@Hop David. “reach a level where we can even think about traveling to another star system.” We’ve had the technology and the ability to travel to another star system for over 55 years. Its called a nuclear pulse rocket. The idea is pretty simple, very powerful, passed initial testing and quite practical with today’s technology. We could easily reach another star system with one. Sadly NASA has blocked scientific progress and its monopoly on progress is only now beginning to break. NASA crushed Project Orion, the nuclear pulse rocket project, and chose a chemical rocket program run by former nazi scientists.

In addition there are other crafts that we could make or at least think of today that could reach another star system. Such as a solar sail driven by a laser, some type of ion drive propelled by nuclear reactors, etc. A very promising idea is micro-sized spacecraft propelled by a solar sail. Really, just any craft that is based on an energy other than chemical rockets or batteries which are both very inefficient (though a nuclear battery is certainly better than the others we’ve used, such as the one used in Curiosity rover).

I think the only practical spacecraft that we will be able to use anytime soon though is a nuclear fission based craft. Whether than be by a nuclear reactor or an external shaped nuclear charge explosion. We just have to have the will and the ability to get past the bureaucratic red tape.

## Tau Ceti: Potentially Habitable Planet Found Orbiting Nearby Star, Study Suggests

A sun-like star in our solar system's backyard may host five planets, including one perhaps capable of supporting life as we know it, a new study reports.

Astronomers have detected five possible alien planets circling the star Tau Ceti, which is less than 12 light-years from Earth -- a mere stone's throw in the cosmic scheme of things. One of the newfound worlds appears to orbit in Tau Ceti's habitable zone, a range of distances from a star where liquid water can exist on a planet's surface.

With a minimum mass just 4.3 times that of Earth, this potential planet would be the smallest yet found in the habitable zone of a sun-like star if it's confirmed, researchers said.

"This discovery is in keeping with our emerging view that virtually every star has planets, and that the galaxy must have many such potentially habitable Earth-sized planets," study co-author Steve Vogt, of the University of California, Santa Cruz, said in a statement. "They are everywhere, even right next door." [Gallery: 7 Potentially Habitable Exoplanets]

The five planet candidates are all relatively small, with minimum masses ranging from 2 to 6.6 times that of Earth. The possibly habitable world, which completes one lap around Tau Ceti every 168 days, is unlikely to be a rocky planet like Earth, researchers said.

"It is impossible to tell the composition, but I do not consider this particular planet to be very likely to have a rocky surface," lead author Mikko Tuomi, of the University of Hertfordshire in England, told SPACE.com via email. "It might be a 'water world,' but at the moment it's anybody's guess."

Spotting signals in the noise

Tau Ceti is slightly smaller and less luminous than our sun. It lies 11.9 light-years away in the constellation Cetus (the Whale) and is visible with the naked eye in the night sky. Because of its proximity and sun-like nature, Tau Ceti has featured prominently in science fiction over the years.

Astronomers have searched for exoplanets around Tau Ceti before and turned up nothing. But in the new study, researchers were able to pull five possible planetary signals out from under a mountain of noise.

Tuomi and his team re-analyzed 6,000 observations of Tau Ceti made by three different spectrographs, instruments that allow researchers to detect the tiny gravitational wobbles orbiting planets induce in their parent stars.

The three instruments are the High Accuracy Radial velocity Planet Searcher (HARPS), on the European Southern Observatory's 3.6-meter telescope in La Silla, Chile the University College London Echelle Spectrograph (UCLES) on the Anglo-Australian Telescope in Siding Spring, Australia and the High Resolution Echelle Spectrometer, or HIRES, on the 10-meter Keck telescope atop Mauna Kea in Hawaii.

Using new analysis and modeling techniques, the team spotted the five faint signals, successfully separating them from noise caused by stellar activity and other factors.

"We pioneered new data modeling techniques by adding artificial signals to the data and testing our recovery of the signals with a variety of different approaches," Tuomi said in statement. "This significantly improved our noise modeling techniques and increased our ability to find low-mass planets."

The new analysis methods should aid the search for small planets, allowing more and more of them to be spotted throughout the galaxy, researchers said. [A Galaxy Full of Alien Planets (Infographic)]

A nearby planetary system?

The five planets remain candidates at this point and will not become official discoveries until they're confirmed by further analysis or observations. And that's not a sure thing, researchers said.

"I am very confident that the three shortest periodicities are really there, but I cannot be that sure whether they are of planetary origin or some artifacts of insufficient noise modelling or stellar activity and/or magnetic cycles at this stage," Tuomi said, referring to the potential planets with orbital periods of 14, 35 and 94 days (compared to 168 days for the habitable zone candidate and 640 days for the most distantly orbiting world).

"The situation is even worse for the possible habitable zone candidate, because the very existence of that signal is uncertain, yet according to our detection criteria the signal is there and we cannot rule out the possibility that it indeed is of planetary origin," he added. "But we don't know what else it could be, either."

If the Tau Ceti planets do indeed exist, their proximity would make them prime targets for future instruments to study, researchers said.

"Tau Ceti is one of our nearest cosmic neighbors and so bright that we may be able to study the atmospheres of these planets in the not-too-distant future," James Jenkins, of the Universidad de Chile and the University of Hertfordshire, said in a statement. "Planetary systems found around nearby stars close to our sun indicate that these systems are common in our Milky Way galaxy."

If confirmed, the Tau Ceti planets would not be the closest exoplanets to Earth. That title still goes to Alpha Centauri Bb, a roasting-hot, rocky world recently spotted just 4.3 light-years away, in the closest star system to our own.

The new study has been accepted for publication in the journal Astronomy & Astrophysics.

## Potentially Habitable Planet Detected around Nearby Star

A sun-like star in our solar system's backyard may host five planets, including one perhaps capable of supporting life as we know it, a new study reports.

Astronomers have detected five possible alien planets circling the star Tau Ceti, which is less than 12 light-years from Earth &mdash a mere stone's throw in the cosmic scheme of things. One of the newfound worlds appears to orbit in Tau Ceti's habitable zone, a range of distances from a star where liquid water can exist on a planet's surface.

With a minimum mass just 4.3 times that of Earth, this potential planet would be the smallest yet found in the habitable zone of a sun-like star if it's confirmed, researchers said.

"This discovery is in keeping with our emerging view that virtually every star has planets, and that the galaxy must have many such potentially habitable Earth-sized planets," study co-author Steve Vogt, of the University of California, Santa Cruz, said in a statement. "They are everywhere, even right next door." [Gallery: 7 Potentially Habitable Exoplanets]

The five planet candidates are all relatively small, with minimum masses ranging from 2 to 6.6 times that of Earth. The possibly habitable world, which completes one lap around Tau Ceti every 168 days, is unlikely to be a rocky planet like Earth, researchers said.

"It is impossible to tell the composition, but I do not consider this particular planet to be very likely to have a rocky surface," lead author Mikko Tuomi, of the University of Hertfordshire in England, told SPACE.com via email. "It might be a 'water world,' but at the moment it's anybody's guess."

Spotting signals in the noise

Tau Ceti is slightly smaller and less luminous than our sun. It lies 11.9 light-years away in the constellation Cetus (the Whale) and is visible with the naked eye in the night sky. Because of its proximity and sun-like nature, Tau Ceti has featured prominently in science fiction over the years.

Astronomers have searched for exoplanets around Tau Ceti before and turned up nothing. But in the new study, researchers were able to pull five possible planetary signals out from under a mountain of noise.

Tuomi and his team re-analyzed 6,000 observations of Tau Ceti made by three different spectrographs, instruments that allow researchers to detect the tiny gravitational wobbles orbiting planets induce in their parent stars.

The three instruments are the High Accuracy Radial velocity Planet Searcher (HARPS), on the European Southern Observatory's 3.6-meter telescope in La Silla, Chile the University College London Echelle Spectrograph (UCLES) on the Anglo-Australian Telescope in Siding Spring, Australia and the High Resolution Echelle Spectrometer, or HIRES, on the 10-meter Keck telescope atop Mauna Kea in Hawaii.

Using new analysis and modeling techniques, the team spotted the five faint signals, successfully separating them from noise caused by stellar activity and other factors.

"We pioneered new data modeling techniques by adding artificial signals to the data and testing our recovery of the signals with a variety of different approaches," Tuomi said in statement. "This significantly improved our noise modeling techniques and increased our ability to find low-mass planets."

The new analysis methods should aid the search for small planets, allowing more and more of them to be spotted throughout the galaxy, researchers said. [A Galaxy Full of Alien Planets (Infographic)]

A nearby planetary system?

The five planets remain candidates at this point and will not become official discoveries until they're confirmed by further analysis or observations. And that's not a sure thing, researchers said.

"I am very confident that the three shortest periodicities are really there, but I cannot be that sure whether they are of planetary origin or some artifacts of insufficient noise modelling or stellar activity and/or magnetic cycles at this stage," Tuomi said, referring to the potential planets with orbital periods of 14, 35 and 94 days (compared to 168 days for the habitable zone candidate and 640 days for the most distantly orbiting world).

"The situation is even worse for the possible habitable zone candidate, because the very existence of that signal is uncertain, yet according to our detection criteria the signal is there and we cannot rule out the possibility that it indeed is of planetary origin," he added. "But we don't know what else it could be, either."

If the Tau Ceti planets do indeed exist, their proximity would make them prime targets for future instruments to study, researchers said.

"Tau Ceti is one of our nearest cosmic neighbors and so bright that we may be able to study the atmospheres of these planets in the not-too-distant future," James Jenkins, of the Universidad de Chile and the University of Hertfordshire, said in a statement. "Planetary systems found around nearby stars close to our sun indicate that these systems are common in our Milky Way galaxy."

If confirmed, the Tau Ceti planets would not be the closest exoplanets to Earth. That title still goes to Alpha Centauri Bb, a roasting-hot, rocky world recently spotted just 4.3 light-years away, in the closest star system to our own.

The new study has been accepted for publication in the journal Astronomy & Astrophysics.

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018 illustrated by Karl Tate), a book about the search for alien life.

## Planets Around Tau Ceti? Not So Fast.

By: Camille M. Carlisle December 20, 2012 5

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Be skeptical about the hubbub over Tau Ceti and its five planets.

Astronomers might have found hints of five planets orbiting the solar-type star Tau Ceti, but the evidence is still shaky.

University of Hertfordshire

solar-type star Tau Ceti (HD 10700), my colleagues and I were skeptical. Astronomers have studied this nearby star for years — at a distance of 11.9 light-years, it’s one of the closest star systems to us — and there’s been no hint of a planet. To go from zero to five sounded rather like conjuring from the ether.

So we went to the source: the actual study and the astronomers who wrote it. The authors are more cautious in their paper. Yes, they announce the detection of five itsy-bitsy signals that might be planets, but they don’t trumpet the planet conclusion from the highest hills. As lead author Mikko Tuomi (University of Hertfordshire, England, and University of Turku, Finland) told me, “I believe the signals are there beyond reasonable doubt. However, their planetary origin is more uncertain, and an independent confirmation is necessary to conclude that we have indeed found planets.”

Let’s step back and look at what the team did.

Tuomi and his colleagues wanted to test a new analysis method for uncovering periodic signals in starlight, created by stars’ tiny wobbles along our line of sight as one or more planets orbits the stars. Such radial velocity measurements are the most successful planet-hunting technique thus far (unless you add in Kepler’s unconfirmed planets, which I don’t).

The astronomers used more than 4,000 archival observations of Tau Ceti by the HARPS spectrograph in Chile, one of the world’s premier instruments for finding exoplanets. To see if their techniques could find small signals hidden in data, they added fake signals to the Tau Ceti observations — Tau Ceti is a quiet star with no known planets, so they figured that using it as a source of “noise” was reasonable.

Cetus the Whale, which the star Tau Ceti forms a part of. A recent report of potential planets around Tau Ceti has people abuzz.

University of Hertfordshire

planet around Alpha Centauri showed, new tools could reveal planets where none were detected before. There might well be one or more planets around Tau Ceti. But we shouldn’t jump on the exoplanet bandwagon just because it rolls by.

Speaking of which, you shouldn’t take my word on this stuff: read the paper yourself:

Reference: M. Tuomi et al. “Signals embedded in the radial velocity noise.” Accepted to Astronomy & Astrophysics.

## Spotting signals in the noise

Tau Ceti is slightly smaller and less luminous than our sun. It lies 11.9 light-years away in the constellation Cetus (the Whale) and is visible with the naked eye in the night sky. Because of its proximity and sun-like nature, Tau Ceti has featured prominently in science fiction over the years.

Astronomers have searched for exoplanets around Tau Ceti before and turned up nothing. But in the new study, researchers were able to pull five possible planetary signals out from under a mountain of noise.

Tuomi and his team re-analyzed 6,000 observations of Tau Ceti made by three different spectrographs, instruments that allow researchers to detect the tiny gravitational wobbles orbiting planets induce in their parent stars.

The three instruments are the High Accuracy Radial velocity Planet Searcher (HARPS), on the European Southern Observatory's 3.6-meter telescope in La Silla, Chile the University College London Echelle Spectrograph (UCLES) on the Anglo-Australian Telescope in Siding Spring, Australia and the High Resolution Echelle Spectrometer, or HIRES, on the 10-meter Keck telescope atop Mauna Kea in Hawaii.

Using new analysis and modeling techniques, the team spotted the five faint signals, successfully separating them from noise caused by stellar activity and other factors.

"We pioneered new data modeling techniques by adding artificial signals to the data and testing our recovery of the signals with a variety of different approaches," Tuomi said in statement. "This significantly improved our noise modeling techniques and increased our ability to find low-mass planets."

The new analysis methods should aid the search for small planets, allowing more and more of them to be spotted throughout the galaxy, researchers said. [A Galaxy Full of Alien Planets (Infographic)]

## Tau Ceti's planets nearest around single, Sun-like star

Tau Ceti's planetary quintet - reported in an online paper that will appear in Astronomy and Astrophysics - was found in existing planet-hunting data.

The study's refined methods of sifting through data should help find even more far-flung worlds.

The star now joins Alpha Centauri B as a nearby star known to host planets.

Tau Ceti lies 12 light-years distant Alpha Centauri B, just four. In both cases, the planets were found not by spying them through a telescope but rather by measuring the subtle effects they have on their host stars' light.

In the gravitational dance of a planet around a star, the planet does most of the moving. But the star too is tugged slightly to and fro as the planet orbits, and these subtle movements of the star show up as subtle shifts in the colour of the star's light we see from Earth.

This "radial velocity" measurement is a tricky one stars' light changes also for a range of other reasons, and requires picking out the specifically planetary component from all this "noise".

Now, Hugh Jones of the University of Hertfordshire and colleagues have refined their "noise modelling" in order to subtract it, and thereby see the smallest signals hiding in the data - starting with Tau Ceti.

"It's a star on which we have a lot of data - an order of magnitude more data than we have for pretty much any other star," Prof Jones told BBC News.

"It's a good test case for how low can we go, what size of signals can we pick up."

The team started with data from three planet-hunting missions: Harps, AAPS, and HiRes, all of which had data on Tau Ceti.

The trick to honing the technique was to put in "fake planets" - to add signals into the messy data that planets should add - and find ways to reduce the noise until the fake planets became more and more visible in the data.

"Putting all that together, we optimised a noise-modelling strategy which allows us to recover our fake signals - but in the process of doing that, we actually saw that we were finding signals as well," Prof Jones said - actual planets.

The quintet includes planets between two and six times the Earth's mass, with periods ranging from 14 to 640 days.

One of them, dubbed HD 10700e, lies about half as far from Tau Ceti as the Earth is from the Sun - and because Tau Ceti is slightly smaller and dimmer than our Sun, that puts the planet in the so-called habitable zone.

It is increasingly clear that in existing data from radial velocity measurements there may be evidence of many more planets.

On Monday, Philip Gregory at the University of British Columbia in Canada posted an as-yet unpublished paper to the arXiv repository, claiming to have seen three planets in the habitable zone of Gliese 667C, one of three stars in a triple-star system, 22 light-years away.

It is also clear that in almost every direction we look and in every way that we look, there are planets around stars near and far.