| Discovery | |
|---|---|
| Date | December 19, 2012 |
| Discoverers | Tuomi et al. |
| Detection method | Radial velocity (HARPS) |
| Site | La Silla Observatory |
| Name & designations | |
| Pronunciation | /'züs/ |
| Adjective | Zeusian |
| Planet numbers | P847, Tau Ceti P2, Cetus P31, Hippocampus P99, 2012 P158, 2012 Cet-9, 2012 Hip-16 |
| Star designations | 52 Ceti c, BF 1315 c, PH 629 c, Pi Hippocampi c, 376 Hippocampi c, P22 Ceti c, P73 Hippocampi c, HD 10700 c, HIP 8102 c, HR 509 c, Gliese 71 c, SAO 147986 c |
| Location | |
| 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 | 0.194 352 AU (29.074 7 Gm) |
| Periastron | 0.187 720 AU (28.082 5 Gm) |
| Apastron | 0.200 984 AU (30.066 8 Gm) |
| Eccentricity | 0.034 124 2 |
| Orbital circumference | 1.220 89 AU (182.642 8 Gm) |
| Orbital area | 0.118 60 AU² (2 654.2 Gm²) |
| Orbital period | 35.362 218 d (0.096 816 48 yr) |
| Avg. velocity | 59.982 km/s (12.610 AU/yr) |
| Max. velocity | 60.997 km/s (13.304 AU/yr) |
| Min. velocity | 58.950 km/s (11.876 AU/yr) |
| Direction of orbit relative to star's rotation |
Prograde |
| Inclination | 73.226° to ecliptic 1.941° to star's equator −3.863° to invariable plane |
| Argument of periastron | 170.026° |
| Longitude of ascending node | 119.134° |
| Longitude of periastron | 289.159° |
| Angular separation | 53.244 mas |
| Observing the parent star | |
| Mean angular star size | 2.183 62° (131.017') |
| Max. angular star size | 2.260 77° (135.646') |
| Min. angular star size | 2.111 57° (126.694') |
| Mean star magnitude | −29.445 |
| Max. star magnitude | −29.520 |
| Min. star magnitude | −29.372 |
| Classification | |
| Bulk characteristics | |
| Mean radius | 1.491 2 R⊕ (9.500 Mm) |
| Equatorial radius | 1.490 0 E⊕ (9.503 Mm) |
| Polar radius | 1.493 6 P⊕ (9.494 Mm) |
| Mean circumference | 59.693 Mm |
| Equatorial circumference | 59.712 Mm |
| Polar circumference | 59.655 Mm |
| Surface area | 2.223 7 S⊕ (1 134.2 Mm²) |
| Volume | 3.316 0 V⊕ (3 591.9 Mm³) |
| Flattening | 0.000 95 (1:1 055) |
| Angular diameter | 34.796 μas |
| Mass | 3.243 3 M⊕ |
| Reciprocal mass relative to star |
80 401 |
| Density | 5.394 g/cm³ |
| Gravitational influence | |
| Surface gravity | 1.459 g (14.30 m/s²) |
| Weight on Zeus (150 lb on Earth) |
219 lb (1.46 wa) |
| Standard gravitational parameter | 1.293 × 106 km³/s² |
| Escape velocity | 16.50 km/s |
| Hill radius | 1.221 LD (0.469 2 Gm) |
| Roche limit (3 g/cm3 satellite) |
0.037 87 LD (14.556 Mm) |
| Stationary orbit | 1.610 59 LD (619.110 Mm) |
| Stationary velocity | 1.436 km/s (0.323 LD/d) |
| Rotation characteristics | |
| Rotation period | 763.823 9 h (31.825 996 d) |
| Rotation velocity | 78 kph (0.47°/h) |
| Direction of rotation relative to orbit |
Prograde |
| Axial tilt | 4.516° |
| Longitude of vernal equinox | 274.810° |
| North pole right ascension | 13h 02m 25s (195.602°) |
| North pole declination | −08° 24' 29" (−8.408°) |
| North polar constellation | Virgo |
| North polar caelregio | Noctua |
| South pole right ascension | 01h 02m 25s (15.602°) |
| South pole declination | +08° 24' 29" (+8.408°) |
| South polar constellation | Pisces |
| South polar caelregio | Hippocampus |
| Thermal characteristics | |
| Surface temperature | 655 K (382°C, 720°F, 1180°R) |
| Mean irradiance | 16 643 W/m² (12.170 I⊕) |
| Irradiance at periastron | 17 840 W/m² (13.045 I⊕) |
| Irradiance at apastron | 15 563 W/m² (11.380 I⊕) |
| Albedo | 0.621 (bond), 0.655 (geom.) |
| Atmosphere | |
| Scale height | 2.89 km |
| Volume | 0.830 ae (3.48 Mm³) |
| Total mass | 0.120 atmu (0.617 Eg) |
| Surface pressure | 1.254 atm (127.1 kPa, 18.43 psi) |
| Surface density | 0.178 g/m³ |
| Molar mass | 42.56 g/mol |
| Composition | 92.551% CO2, 6.193% CO, 0.867% He, 0.251% CH4, 762 ppm H2, 437 ppm H2O, 186 ppm Ar |
| Magnetosphere | |
| Dipole strength | 0.746 nT (7.46 μG) |
| Magnetic moment | 7.11 × 1013 T•m³ |
| Dipole tilt | 2.79° |
| Satellite system | |
| Number of moons | 0 |
| Number of rings | 0 |
Zeus (Tau Ceti c, P847) is the second exoplanet in orbit around Tau Ceti, a star just 12 light-years away. It is one of the five planets discovered on December 19, 2012. It is a rocky planet 1½ times the size of Earth orbiting five times closer to its star than where Earth orbits the star.
Zeus was named after the Greek god of sky and thunder.
Discovery and chronology
Zeus was discovered on December 19, 2012, together with four other planets in this system. It was obtained by using high resolution HARPS spectrograph mounted on the 3.6-meter telescope in La Silla Observatory located in the Atacama Desert in Chile. Zeus became the 839th exoplanet discovered since 1992 and is the 158th planet discovered in 2012. It is also the 31st planet discovered in Cetus and 99th in Hippocampus.
Orbit and rotation
Zeus orbits at 29.07 gigameters, doubling the distance of the innermost planet from the star. Zeus takes five weeks (over three megaseconds) to revolve once around the star, a year lasting one-tenth as long as an Earth year. It has a circular orbit with an eccentricity of 0.034, still twice as eccentric as Earth's. When making one round trip, Zeus moves by 182⅔ Gm, that is the orbital circumference.
Zeus rotates quite slowly due to its tidal influence of the star. It takes roughly 2.75 megaseconds or one month to complete the rotation, similar to the rotation of our Moon. Every hour, celestial objects in the Zeus' sky move by about the diameter of the full moon. The rotational axis tilts 4.5° to the orbital plane and longitude of vernal equinox is 277.1° with 0° denoting First Point of Aries and going eastward. When combining axial tilt, longitude of vernal equinox, inclination to line of sight, and coordinates seen from Earth would imply that the planet's north pole points to the constellation Virgo at right ascension 13h 02m and declination −08° 24' while the south pole points to the constellation Pisces at right ascension 01h 02m and declination +08° 24'.
Parent star observation and irradiance
As seen from the surface of Zeus, the parent star would appear brighter than the Sun as seen from Earth because the planet orbits much closer according to the law of inverse proportion. The parent star would have a magnitude of −29.44 compared to −26.74 for Sun as seen from Earth. Sun appearing brighter usually means it is bigger. In this case, the angular diameter of the star is 2.18°, compared to about 0.5° as the angular diameter of the Sun as viewed from our homeworld.
Since the planet orbits more than five times closer, irradiance would assume to be more than 25 times greater, but it should be noted that its parent star is less luminous than our Sun. Zeus receives 12 times more insolation than Earth receives from our own star.
Structure and composition
Mass and size
Zeus' mean diameter is 19 megameters or 1.5 times Earth. From its size, Zeus has a surface area 2.2 times greater and volume 3.3 times greater than Earth's. It masses 3¼ times greater than Earth's, classifying this as super-Earth since the mass is between 2 and 10 Earth masses. This results in the density of 5.4 g/cm³, which is similar to Earth, Venus, and Mercury.
Gravitational influence
Zeus' surface gravity is 1.46 g, with 1 g is the surface gravity of Earth. Since 1 g acceleration is 9.8 m/s², then 1.46 g acceleration would be 14.3 m/s². The planet's gravity influence the space in the vicinity of the planet, keeping any of its possible moons in orbit. This vicinity is called its hill sphere with outer limit being equal in gravitational influence from planet and star, located 469 Mm or 49.4 planetary radii from the planet's center. Distance from the planet where an orbiting object has the same orbital period as the planet's rotation is just beyond the hill sphere and thus this orbit is not possible for long until it settles into the orbit around the star. If the planet's rotation is a bit faster, then such an orbit would be stable as it would lie within the hill sphere.
Interior
Like other terrestrial planets, Zeus underwent differentiation, an event in which denser materials sink to form the core. The planet's core is made of iron and nickel with small amount of sulfur. Surrounding the core is mantle, where rocks are semisolid or molten and above it is the crust where the planet's surface lies at the top.
Surface
Zeus' surface is mostly rocky plains but there are prominent terrains like mountains, canyons, ridges, volcanoes, and others. Because of the planet's surface feature, Zeus is a barren planet. The planet's tallest mountain is just 5.4 kilometers high, over half the height of the Earth's tallest mountain, Mount Everest, located around 20°N latitude. There are also moderate number of impact craters, the largest has a diameter of 293 km, located near the south pole.
Atmosphere
Zeus' atmosphere is composed almost exclusively of carbon dioxide at 92.5% and carbon monoxide making up 83% of the remaining atmosphere. Abundant amount of carbon monoxide is produced by volcanoes together with carbon dioxide due to its carbon-rich interior. Besides these carbon oxides, there trace amounts of water vapor, argon, helium, and hydrogen. Unlike solar system rocky planets with considerable atmospheres, nitrogen is absent in Zeus' atmosphere. It could either be that the planet does not acquire nitrogen during its formation or was reacted or transmuted due to intense UV radiation and then either escaped into space due to stellar winds or sank into the interior.
Zeus' atmospheric pressure is 25% greater than Earth's but it has total mass just 12% that of Earth's. Due to great amount of heavy gases, the molar mass is high, 42.56 g/mol.
Magnetic field
The planet's magnetic field strength is 7.46 μG, which is weak due to its slow rotation.
Moons and rings
Zeus has no moons nor rings. Like the inner planet Penthus, Zeus once had moons. There were three small moons with sizes of 23 km, 29 km, and 153 km. All three escaped the orbit 3.1, 2.7, and 0.8 billion years ago, respectively.
Future studies
Zeus 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 Zeus 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 extremely difficult given that planet orbits close to the glare of its star and is small, though future generations of technologies can make it whole lot easier. Direct imaging can be used to what planet appears like as well as if moons actually exist. Looking for signatures of volcanism can be done using reflected light or direct imaging.
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