|Date||December 19, 2012|
|Discoverers||Tuomi et al.|
|Detection method||Radial velocity (HARPS)|
|Site||La Silla Observatory|
|Name & designations|
|Planet numbers||P850, Tau Ceti P5,|
Cetus P34, Hippocampus P102,
2012 P161, 2012 Cet-12,
|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
|Right ascension||01h 44m 04.08s (26.017 01°)|
|Declination||−15° 56' 14.9" (−15.937 48°)|
|Eccentricity||0.026 731 9|
|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|
|Flattening||0.001 48 (1:677.1)|
|Angular diameter||41.267 μas|
relative to star
|Weight on Poseidon
(150 lb on Earth)
|Standard gravitational parameter||2.759 × 106 km³/s²|
(3 g/cm3 satellite)
|Direction of rotation
relative to orbit
|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|
|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.)|
|Volume||6.699 ae (28.05 Mm³)|
|Total mass||1.218 atmu (6.26 Eg)|
|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|
|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.
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.
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 1⁄26 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.
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.
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.