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Tau Ceti system is the planetary system around Tau Ceti, located approximately 11.9 light-years away in the constellation Cetus in the caelregio Hippocampus. There are 13 planets, five were discovered on December 19, 2012.


Tau Ceti, like our Sun, is a G-type main sequence star, but it is fainter, smaller, cooler than our Sun. It is also metal-deficient relative to our Sun as the star contains just 28% as much metals as our Sun based on iron. Tau Ceti is over a billion years older than our Sun, 5.8 Gyr vs. 4.6 Gyr.

Tau Ceti's high proper motion is typical of nearby stars. Tau Ceti would still require 2000 years to transverse about a degree and about 1000 years to transverse the diameter of the full moon.

Planetary system[]

Planet Class Semimajor axis (AU) Eccentricity Orbital period (yr) Radius Mass Density (g/cm3) Gravity (g) Rotation period (d) Surface temp (K)
Penthus (b) LEaV 0.1033 0.15761 0.0375 2.0966 M 790
g BEbT 0.133 0.06 0.9013 1.75 M
Zeus (c) BeaV 0.1944 0.03412 0.0968 3.2433 M 655
h BEbT 0.8601 0.23 0.9013 1.83 M
Hades (d) DEaH 0.3732 0.07714 0.2577 3.6557 M 347
Aita (e) SDBEaC 0.5495 0.18 0.4603 4.4839 M 334
PxP-4 (j) G 0.6-1.2?
Poseidon (f) SiOEaM 1.3429 0.16 1.7585 6.9204 M 315
i GJbJ 3-20 18.3776 1-2 MJ

Tau Ceti contains one of the biggest family of planets in the solar neighborhood as it is orbited by 13 planets, fully five more than our Sun has. The planets range in orbital periods from about two weeks to seven centuries and distances ranging from 0.1 AU to 73 AU, considerably wider in range than our solar system in both categories. This system has similar average eccentricity amongst the planets (0.0652) as there are in our solar system (0.0603). There are two trojan planets in this system, a pair at 0.86 AU and a pair at 12.13 AU, both orbiting 60° from each other.

This system contains seven terrestrial planets, all within 1.35 AU, and one giant planet, all orbiting beyond 2 AU. There is a debris disk between 6–52 AU.

Detected planets[]


Penthus (tentatively Tau Ceti b) is the innermost planet of the Tau Ceti system. It is the super-Earth massing 2.1 Earth masses, just above the lower limit of super-Earth. It is also larger and denser than Earth. The planet is tidally-locked to the sun, meaning its rotation period is exactly the same as its orbital period. It takes 13.7 days to rotate once on its axis while it takes 13.7 days to orbit the sun.

As is the case for most innermost planets, Penthus is the hottest planet in this system with the surface temperature of 962°F. Tidal forces of the parent star may melt the interior of Penthus, making the world volcanically active with lava covering much of the surface.


Zeus (tentatively Tau Ceti c) is a barren world about 50% bigger than Earth and over three times the mass of our planet. The density of Zeus is similar to that of the three innermost terrestrial planets in our solar system. It takes around 5 weeks to revolve around the star and about 412 weeks to rotate once on its axis, which is similar to the just under 4 weeks, one month, rotation period of our Moon.


Hades (tentatively Tau Ceti d) is a super-Earth orbiting at Mercury-like distance from the star and is almost seven times the density of water. It is a desert planet with the surface temperature of 165°F, surrounding by the atmosphere about half as thick as Earth's. Its surface temperature of 5°F warmer than its equilibrium suggests that the atmosphere of Hades contains very little greenhouse gases like carbon dioxide. The planet appears tan from space, reflecting about half of all sunlight falling onto the planet.


Aita (tentatively Tau Ceti e) is a 4.5-Earth mass planet orbiting at 55% the distance between Earth and the Sun or roughly halfway between the orbits of Mercury and Venus. The density is a bit more than Earth's and surface gravity approaching twice that of Earth.

Aita is qualified as a sulfur planet because sulfur is abundant on the surface, interior, and atmosphere. The atmosphere is 25 times thicker than Earth's comprising mainly of CO2, SO2, and H2S. The average surface temperature is 142°F.


Poseidon (tentatively Tau Ceti f) is a super-Earth orbiting at 1.35 AU from Tau Ceti, taking 21 months to orbit the star compared to 12 months for Earth. It weighs seven times as much as Earth while it is ¾ bigger than Earth. Poseidon's density 6.9 g/cm3, compared to 5.5 g/cm3 for Earth. The weight of an average person would weigh about 330 pounds on Poseidon, compared to just 150 pounds here on Earth. Poseidon rotates in the opposite direction of its orbit, taking 74 hours 51.8 minutes to complete one backward spin.

Water covers about 54% of the surface of Poseidon, named after the God of the Sea. The surface temperature is 107°F, but the equilibrium temperature is just –133°F. The greenhouse effect must have raised the temperature this much and the atmosphere contains abundant amounts of carbon dioxide, methane, and water vapor. The atmosphere of Poseidon is nine times thicker than Earth's.

Tau Ceti g[]

Tau Ceti g is a planet 75% more massive than Earth orbiting at 0.133 AU from the star. The planet takes 20 days to revolve around the star. The planet rotates on its side with an axial tilt of 79.4° and complete its rotation in 3.66 days (87.8 hours).

Tau Cet g is an hot planet. Its appearance is unknown.

Tau Ceti h[]

Tau Ceti h is a planet with an eccentric orbit.

Tau Ceti h is a little bit bigger than Earth and the mass classification still places this at the same category as Earth, mid-Earth, as the planet's mass of 1.83 Earth masses is short of the upper limit at 2 Earth masses.

Tau Ceti i[]

Tau Ceti i is a gas giant orbiting at 3-20 AU, taking more than 1813 years to revolve once around the star. The upper clouds are made predominantly of ammonia with water clouds and ammonium hydrosulfide clouds underneath it. The planet's surface temperature (at 1-bar layer) is about the same as Jupiter's. Tau Cet i is bigger and more massive than Jupiter, though still placed in the same mass category as Jupiter.

Radial velocity monitoring of the parent star loosely rule out any giant planets with orbital periods less than 15 years, corresponding to the semimajor axis of under 5.6 AU. This planet's orbital period is just beyond that limit, so hopefully this planet is confirmed soon.

Conjectured planets[]