PlanetStar Wiki
Artist's conception of Poseidon as an oceanic rocky planet
Date December 19, 2012
Discoverers Tuomi et al.
Detection method Radial velocity (HARPS)
Site La Silla Observatory
Name & designations
Pronunciation /'pōs•ī•din/
Adjective Poseidian
Planet numbers P850, Tau Ceti P5,
Cetus P34, Hippocampus P102,
2012 P161, 2012 Cet-12,
2012 Hip-19
Star designations 52 Ceti f, BF 1315 f,
PH 629 f, Pi Hippocampi f,
376 Hippocampi f, P22 Ceti f,
P73 Hippocampi f, HD 10700 f,
HIP 8102 f, HR 509 f,
Gliese 71 f, SAO 147986 f
System Tau Ceti
Constellation Cetus
Caelregio Hippocampus
Right ascension 01h 44m 04.08s (26.017 01°)
Declination −15° 56' 14.9" (−15.937 48°)
Distance 3.650 pc (11.905 ly)
Orbital characteristics
Semimajor axis 1.342 934 AU (200.900 0 Gm)
Periastron 1.307 034 AU (195.529 6 Gm)
Apastron 1.378 833 AU (206.270 5 Gm)
Eccentricity 0.026 731 9
Orbital circumference 8.436 80 AU (1 262.127 Gm)
Orbital area 5.663 75 AU² (126 751.9 Gm²)
Orbital period 642.298 388 d (1.758 517 15 yr)
Avg. velocity 22.820 km/s (4.798 AU/yr)
Max. velocity 23.123 km/s (4.842 AU/yr)
Min. velocity 22.513 km/s (4.753 AU/yr)
Direction of orbit
relative to star's rotation
Inclination 74.664° to ecliptic
−3.437° to star's equator
4.331° to invariable plane
Argument of periastron 273.631°
Longitude of ascending node 222.316°
Longitude of periastron 135.946°
Angular separation 367.907 mas
Observing the parent star
Mean angular star size 0.316 02° (18.961')
Max. angular star size 0.324 70° (19.482')
Min. angular star size 0.307 79° (18.467')
Mean star magnitude −25.248
Max. star magnitude −25.306
Min. star magnitude −25.190
Bulk characteristics
Mean radius 1.768 5 R (11.267 Mm
Equatorial radius 1.767 4 E (11.273 Mm)
Polar radius 1.770 7 P (11.256 Mm)
Mean circumference 70.793 Mm
Equatorial circumference 70.828 Mm
Polar circumference 70.723 Mm
Surface area 3.127 6 S (1 595.3 Mm²)
Volume 5.531 1 V (5 991.3 Mm³)
Flattening 0.001 48 (1:677.1)
Angular diameter 41.267 μas
Mass 6.920 4 M
Reciprocal mass
relative to star
37 680
Density 6.901 g/cm³
Gravitational influence
Surface gravity 2.213 g (21.70 m/s²)
Weight on Poseidon
(150 lb on Earth)
332 lb
Standard gravitational parameter 2.759 × 106 km³/s²
Escape velocity 22.13 km/s
Hill radius 10.941 LD (4.205 8 Gm)
Roche limit
(3 g/cm3 satellite)
0.048 75 LD (18.739 Mm)
Stationary orbit 0.418 22 LD (160.762 Mm)
Stationary velocity 4.011 km/s (0.901 LD/d)
Rotation characteristics
Rotation period 74.862 8 h (3.119 285 d)
Rotation velocity 946 kph (4.81°/h)
Direction of rotation
relative to orbit
Axial tilt 177.620°
Longitude of vernal equinox 84.512°
North pole right ascension 06h 14m 49s (93.706°)
North pole declination +07° 06' 27" (+7.107°)
North polar constellation Orion
North polar caelregio Araneus
South pole right ascension 18h 14m 49s (273.706°)
South pole declination −07° 06' 27" (−7.107°)
South polar constellation Serpens
South polar caelregio Tarandus
Thermal characteristics
Surface temperature 315 K (42°C, 107°F, 567°R)
Mean irradiance 349 W/m² (0.255 I)
Irradiance at periastron 368 W/m² (0.269 I)
Irradiance at apastron 331 W/m² (0.242 I)
Albedo 0.275 (bond), 0.214 (geom.)
Scale height 17.32 km
Volume 6.699 ae (28.05 Mm³)
Total mass 1.218 atmu (6.26 Eg)
Surface pressure 9.313 atm (943.6 kPa,
136.86 psi)
Surface density 0.223 g/m³
Molar mass 29.22 g/mol
Composition 46.756% N2, 26.885% Ar,
12.988% O2, 8.138% He,
4.883% H2O, 0.296% H2,
316 ppm CO2, 131 ppm Ne,
46.6 ppm CO, 26.9 ppm CH4,
8.99 ppm PH3, 6.78 ppm SO2,
133 ppb NH3, 47.6 ppb H2S,
13.9 ppb NO
Dipole strength 2.04 μT (20.4 mG)
Magnetic moment 5.22 × 1018 T•m³
Dipole tilt 63.93°
Satellite system
Number of moons 2
Number of rings 3

Poseidon (Tau Ceti f, P850) is the fifth exoplanet in orbit around Tau Ceti, a star just 12 light-years away. It is the outermost of the five detected planets discovered on December 19, 2012 and is one of roughly thirteen in this planetary system. Poseidon is an ocean world weighing seven times as much as Earth's.

Poseidon was named after Greek god of the sea, equivalent to the Roman god Neptune.

Discovery and chronology[]

Poseidon was discovered on December 19, 2012, together with four other planets in this system. This discovery was made by carefully watching the wobble of Tau Ceti caused by gravitational tug of planets. It was successfully done using high resolution HARPS spectrograph mounted on the 3.6-meter telescope in La Silla Observatory located in the Atacama Desert in Chile. Poseidon became the 842nd exoplanet discovered since 1992 and is the 161st planet discovered in 2012. It is also the 34th planet discovered in Cetus and 102nd in Hippocampus.

Orbit and rotation[]


Poseidon orbits at 201 gigameters, between the orbits of Earth (150 Gm) and Mars (228 Gm). Due to its slight elliptical orbit with an eccentricity of 0.027, it varies in planet–star distance by over 5% throughout its orbit. At periastron, the closest point to its sun, is at 196 Gm, while at apastron, the farthest point, is at 206 Gm. Periastron lies at 274° to its reference point.

Poseidon takes finite amount of time to go from one point in its orbit, move around and return to the same point again. The amount of time is 55.5 megaseconds (1.76 years, 642 days).


Poseidon takes 75 hours or 3.12 days to spin 360° around its axis, rotating at 178° relative to orbital motion. However, it rotates in the opposite direction to its orbit like Venus and Uranus in the Solar System, hence its tilt between 90° to 180°.

Poseidon's angle of rotation, orbit, and inclination to line of sight would combine to have planet's poles pointing to different directions relative to Earth's, resulting in having different pole stars. On this planet, north pole points to the constellation Orion, which is an equatorial constellation from Earth with most of the constellation occupying north of the celestial equator. The south pole points to the opposite side of the celestial sphere to north pole. The south pole points to Serpens Cauda, which is also an equatorial constellation again mostly north for Earth.

Parent star observation and irradiance[]

Since Poseidon orbits over a third farther away from Tau Ceti than Earth is to the Sun, plus the parent star is less than half the Sun's brightness, then Tau Ceti seen from Poseidon is ¼ the brightness of Sun seen from Earth. The magnitude of Tau Ceti seen from Poseidon is −25.25, about 1½ magnitudes different from the Sun seen from Earth. Even that, parent star would be too bright to be seen directly after few seconds and would be blinded temporarily. Also its apparent diameter of the star seen from the planet is 19 arcminutes or about ⅓ of a degree, about ⅔ the apparent diameter of the Sun seen from Earth.

Poseidon receives 349 W/m² of energy from the parent star, which is ¼ the energy received by Earth from the Sun.

Structure and composition[]

Mass and size[]

Poseidon weighs 6.92 Earth masses, meaning this planet has nearly seven times more stuff than Earth has. Poseidon has a diameter of 11.27 megameters, which is 1.77 times the size of Earth. Its surface area relative to Earth is square of its size relative to Earth while its volume is cubed. Its surface area is 3.13 times Earth's while its volume is 5.53 Earths. We see that volume is 80% the value of mass, meaning the planet is 25% denser than Earth.

Gravitational influence[]

Poseidon's surface gravity is higher than Earth's due to its greater mass. Surface gravity usually don't differ from Earth as great as mass because size is inversely proportional to gravity and more massive planets tend to be bigger. The planet's surface gravity is about 2.2 times that of Earth's and objects fall at 21.7 m/s². However, due to planet's thick atmosphere to be mentioned below, falling objects would accelerate far shy of this value. Gravity causes objects within the atmosphere to fall, while it also cause objects beyond that to orbit the planet. Gravitational influence in the orbit of Poseidon is called its hill sphere and it extends 11 lunar distances or 4.2 Gm. The farther the object is from the planet, the longer it takes to orbit. For an object to have an orbital period identical to Poseidon's rotation period, which is about 3.12 days, it would have to orbit at exactly the right distance, called its stationary orbit. It must orbit at 0.418 LD or about 14.26 planetary radii. Such an orbit would be stable since it is 126 the way to the outermost limit of hill sphere.


Poseidon has three main layers: crust, mantle, and core. The crust is the uppermost layer made of solid rocks as well as liquid water. The mantle is comprised of molten rocks and core is the densest part of the planet and is the source of internal heating. The core is relatively large and is made of 89% iron and 8% nickel, 2% sulfur, and 1% carbon. The core is formed during differentiation in which denser materials sink to form the core after the planet formed.


Water covers about 54% of the planet's surface with deep oceans couple dozen kilometers deep. On the solid surface, there are rugged, rocky terrain. Most of the surface are covered in mountain ranges.


Nitrogen is the most abundant gas in the atmosphere of Poseidon, with water vapor making up 5% of the atmosphere. Oxygen, which is a gas we breathe, makes up 13% of the atmosphere, a bit less than 21% for Earth. Carbon dioxide also makes up a bit less than Earth's.

Atmospheric pressure of Poseidon is 9.3 times greater than Earth's and 10% Venus' atmospheric pressure. Due to its high pressure and amount of water vapor, which is a potent greenhouse gas, Poseidon is a greenhouse, raising the temperature by roughly 134 C°, from −92°C to 42°C, thus allowing liquid water to exist.

Magnetic field[]

Poseidon has a relatively weak magnetic field, at over 2 microteslas, under 1% the Earth's strength, even though Poseidon takes just over three times longer to rotate once than Earth. Such a weak magnetic field is due to lack of electric currents in the planet's liquid outer core. Poseidon's magnetosphere is very tilted, at roughly 64°. It is generally unknown why it is tilted this much. The planet's axial tilt is only a couple of degrees. One possible reason is that the planet's magnetic core is misaligned with the rotation caused by an unknown mechanism. It could be that geodynamo is not affected by Coriolis effect due to planet's dense interior and high pressure pressing the core.

Moons and rings[]

Poseidon has two moons orbiting in a 3:1 mean motion resonance between the two. The inner moon takes 2.7 days while outer moon takes 9.1 days to orbit the planet. The outer moon is bigger than the inner moon, 5.6 times the size and about the size of our Moon. The outer moon has a diameter of 3.39 Mm (2107 miles) compared to 3.47 Mm (2159 miles) for our Moon, while the inner moon is 0.61 Mm (377 miles) across. The outer moon has 80% the mass of our Moon and the inner has about 0.4% of the mass of the outer moon. The inner moon is brown while the outer moon is gray and both surfaces are pockmarked with craters like Luna.

In addition to the moons, Poseidon has a ring system comprising of three closely spaced, dusty, wispy rings surrounding close to the planet. The rings formed when a small asteroid passing by so close to the planet (within its roche limit) that the tidal forces broke apart into dust. The planet's gravity then rearrange dust into rings. The existing rings will not last much longer until dust particles that make up rings will enter the atmosphere and distinegrate like micrometeoroids.

Future studies[]

Poseidon poses a challenge since it does not transit its star. An alternative is to observe reflected light, which is difficult as it only been done for Jupiter-size planets. Future generations of telescopes can pick up reflected light from Poseidon and study its atmosphere as well as physical characteristics such as its actual mass and size. In addition to reflected light, this planet can be studied using direct imaging, which is difficult given that planet orbits close to the glare of its star and is small, though future generations of technologies can make it lot easier. Direct imaging can be used to what planet appears like as well as if moons actually exist. This method can see if Poseidon is actually an oceanic world.

Related links[]