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Date December 19, 2012
Discoverers Tuomi et al.
Detection method Radial velocity (HARPS)
Site La Silla Observatory
Name & designations
Pronunciation /'pen•this/
Adjective Penthusian
Planet numbers P846, Tau Ceti P1,
Cetus P30, Hippocampus P98,
2012 P157, 2012 Cet-8,
2012 Hip-15
Star designations 52 Ceti b, BF 1315 b,
PH 629 b, Pi Hippocampi b,
376 Hippocampi b, P22 Ceti b,
P73 Hippocampi b, HD 10700 b,
HIP 8102 b, HR 509 b,
Gliese 71 b, SAO 147986 b
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 0.103 262 AU (15.447 7 Gm)
Periastron 0.086 986 AU (13.013 0 Gm)
Apastron 0.119 537 AU (17.882 5 Gm)
Eccentricity 0.157 612 9
Orbital circumference 0.645 85 AU (96.617 9 Gm)
Orbital area 0.033 08 AU² (740.3 Gm²)
Orbital period 13.695 062 d (0.037 495 04 yr)
Avg. velocity 81.931 km/s (17.225 AU/yr)
Max. velocity 88.152 km/s (17.592 AU/yr)
Min. velocity 75.198 km/s (16.850 AU/yr)
Direction of orbit
relative to star's rotation
Inclination 107.245° to ecliptic,
2.338° to star's equator,
6.439° to invariable plane
Argument of periastron 85.592°
Longitude of ascending node 59.220°
Longitude of periastron 144.812°
Angular separation 40.112 mas
Observing the parent star
Mean angular star size 4.109 87° (246.592')
Max. angular star size 4.898 84° (292.730')
Min. angular star size 3.550 30° (213.018')
Mean star magnitude −30.818
Max. star magnitude −31.191
Min. star magnitude −30.500
Bulk characteristics
Mean radius 1.221 4 R (7.781 Mm)
Equatorial radius 1.220 5 E (7.784 Mm)
Polar radius 1.223 2 P (7.776 Mm)
Mean circumference 48.892 Mm
Equatorial circumference 48.910 Mm
Polar circumference 48.855 Mm
Surface area 1.491 8 S (760.89 Mm²)
Volume 1.822 0 V (1 973.6 Mm³)
Flattening 0.001 12 (1:893.0)
Angular diameter 28.500 μas
Mass 2.096 6 M
Reciprocal mass
relative to star
124 370
Density 6.347 g/cm³
Gravitational influence
Surface gravity 1.405 g (13.78 m/s²)
Weight on Penthus
(150 lb (1 wa) on Earth)
211 lb (1.41 wa)
Standard gravitational parameter 8.357 × 105 km³/s²
Escape velocity 14.66 km/s
Hill radius 0.489 LD (0.188 0 Gm)
Roche limit
(3 g/cm3 satellite)
0.032 74 LD (12.586 Mm)
Stationary orbit 0.785 69 LD (302.018 Mm)
Stationary velocity 1.645 km/s (0.370 LD/d)
Rotation characteristics
Rotation period 328.681 5 h (13.695 062 d)
Rotation velocity 149 kph (1.10°/h)
Direction of rotation
relative to orbit
Axial tilt 0.521°
Longitude of vernal equinox 320.600°
North pole right ascension 14h 34m 30s (218.624°)
North pole declination +08° 50' 57" (+8.849°)
North polar constellation Boötes
North polar caelregio Noctua
South pole right ascension 02h 34m 30s (38.624°)
South pole declination −08° 50' 57" (−8.849°)
South polar constellation Cetus
South polar caelregio Hippocampus
Thermal characteristics
Surface temperature 790 K (516°C, 962°F, 1421°R)
Mean irradiance 58 958 W/m² (43.110 I)
Irradiance at periastron 83 084 W/m² (60.752 I)
Irradiance at apastron 43 996 W/m² (32.170 I)
Albedo 0.297 (bond), 0.203 (geom.)
Scale height 3.23 km
Volume 0.627 ae (2.63 Mm³)
Total mass 0.069 atmu (0.354 Eg)
Surface pressure 0.086 9 atm (8.81 kPa,
1.277 psi)
Surface density 0.134 g/m³
Molar mass 43.00 g/mol
Composition 89.515% CO2, 5.283% Ar,
4.315% CO, 0.632% H2O,
0.216% SO2, 0.162% He,
612 ppm O2, 216 ppm Ne,
62.8 ppm Kr, 7.62 ppm H2,
31.4 ppb Hg2
Dipole strength 36.3 nT (363 μG)
Magnetic moment 8.69 × 1016 T•m³
Dipole tilt 5.74°
Satellite system
Number of moons 0
Number of rings 0

Penthus (Tau Ceti b, P846) is the innermost exoplanet in orbit around Tau Ceti, a star just 12 light-years away. It is the innermost of the five planets discovered on December 19, 2012. It is a rocky planet twice the mass of Earth orbiting 10 times closer to its star than where Earth orbits the star.

Penthus was named after the Greek god of grief.

Discovery and chronology Edit

Penthus 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. Penthus became the 838th exoplanet discovered since the early 1992 and is the 157th planet discovered in 2012. It is also the 30th planet discovered in Cetus and 98th in Hippocampus.

Orbit and rotation Edit

Penthus is the innermost planet of the Tau Ceti system. Penthus orbits at 15.45 gigameters, which is 10% the distance between Sun and Earth and 30% the distance between Mercury and Sun. However, this planet don't stay at this distance throughout the orbit. The orbital distance varies from 13.01 to 17.88 Gm, with the closest approach called periastron and farthest called apastron. This corresponds to the eccentricity of 0.157, which is about halfway between Mars (0.093) and Mercury (0.204). The amount of time taken to complete one round trip around the star is 1.183 megaseconds, or about one fortnight (two weeks). Penthus takes 13.7 days to move 360°, its moves at about 26.3°/day, obtained by dividing 360 by its orbital period.

The planet's orbital plane tilts about 2.3° to the parent star's equator and the planet's equator tilts at 0.5° to the plane of its orbit. The planet got to rotate in order to have its equator, in this case it takes 13.7 days to rotate once on its axis, identical to its orbital period because the planet is tidally locked. The planet's rotation rate is just 1.1°/h, compared to 15°/h, on Earth, equating to the conventional speed of 149 kph (92 mph) when taking planet's equatorial circumference into account. Penthus' tilt of rotation relative to the line of sight is such that the north pole points to the constellation Boötes while the south pole points to the constellation Cetus. On Earth, Boötes lies from +7 to +55° in declination while Cetus is a equatorial constellation with northernmost boundary at +10½° and goes down to −25°.

Parent star observation and irradiance Edit

As seen from the surface of Penthus, 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 −30.82, over four magnitudes brighter than the Sun seen from Earth. But because the planet varies in distance from its star, magnitude varies from −30.50 to −31.19. In addition to being brighter, the star appears much bigger than the Sun seen from Earth and thus fills more of the sky. The apparent angular diameter of the star is 4.1° and the size varies throughout its orbit.

If a star is bright as seen from the planet, it means that the planet is receiving more radiation and thus have higher irradiance. Since the star is 43 times brighter than Sun we see from Earth, then irradiance would be around 43 times greater than Earth's. During the closest approach to the star, it receives as much as 61 times more radiation than Earth receives from the Sun and during the farthest approach, it receives just 32 times more.

Structure and composition Edit

Mass and size Edit

Penthus contains 12.5 yottagrams of matter, which is more than twice as much as Earth's, classifying Penthus as super-Earth in the planetary mass classification scheme. The planet's mean diameter is 15.56 megameters, which is 119 that of Earth's, although the equatorial diameter is larger and polar diameter is smaller by a slight margin due to its rotation. The density of Penthus is 6.35 g/cm³, which is denser than Earth, implying that Penthus is a rocky planet containing slightly greater proportion of heavy materials than Earth contains.

Gravitational influence Edit

Since gravity is caused by the presence of matter, then more matter would produce more gravity. That is the case on Penthus that its gravitational pull is 1.4 g. Since 1 g acceleration is 9.8 m/s², then 1.4 g acceleration would 13.8 m/s². The force of gravity extends for indefinite distance, but there is the limit called its hill limit where planet's gravitational influence equals parent star's gravitational influence. The hill limit is located at 188 megameters from the planet's center or over 24 planetary radii. The distance from the planet where the orbital revolution equals the revolution around the planet's axis is well beyond the hill sphere at 302 Mm (38.8 planetary radii).

Interior Edit

Like other terrestrial planets, Penthus 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 Edit

Penthus' surface is littered with active volcanoes and fissures, meaning there are lavas in a lot of places. The planet is also hot (516°C), allowing rocks to stay molten for a while. To make matters more volcanically active, tidal forces of the nearby star cause planet to stretch and squeeze as it orbits in variable distance throughout its year. Penthus maybe as volcanically active as Jupiter's moon Io due to this same mechanism. In addition to volcanoes and fissures, there are canyons, plateaus, mountains, ridges and other features. There are very few craters due to craters erasing constantly by being filled with lava.

Atmosphere Edit

Due to the planet's weak magnetic field due to tidal forces slowing down the rotation, the atmosphere would be exposed to space weather. Stellar radiation and flares would blow away gases surrounding the planet, this results in Penthus having just a thin atmosphere despite its greater gravitational pull than Earth's. The atmospheric pressure is just 9% that of Earth's, but still over a dozen times thicker than Mars'. The atmosphere is made almost exclusively of gases emitted by many active volcanoes. The chief gas is carbon dioxide, making up roughly 89½% of the atmosphere by volume. Most of the remaining atmosphere is made of argon (5.3%) and carbon monoxide (4.3%). There is also water vapor making up 0.6% by volume. Compared to most other planets, the atmosphere contains a lot of sulfur dioxide (0.2%), believed to be emitted from volcanoes.

Magnetic field Edit

As mentioned above, the planet has a weak magnetic field due to its slow rotation. Rotation causes dynamics in the planet's core by motions of liquids producing electricity and magnetism. Slow rotation wouldn't have much effects and leaves only with weak magnetic field. Penthus' magnetic field is just over 0.1% the Earth's strength and magnetic poles tilt 5.7° relative to geographic poles.

Moons and rings Edit

Penthus has no moons nor rings. Planet once had a small moon that torn apart that formed temporary rings and then impacted the planet 3.68 billion years ago.

Future studies Edit

Penthus 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 Penthus 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 very 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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