Penthus

Penthus (Tau Ceti b, P846) is the innermost in orbit around, a star just 12 s 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.

was named after the of grief.

Discovery and chronology
Penthus was discovered on December 19, 2012, together with four other planets in this system. It was obtained by using high resolution spectrograph mounted on the 3.6-meter telescope in  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 and 98th in Hippocampus.

Orbit and rotation
Penthus is the innermost planet of the Tau Ceti system. Penthus orbits at 15.45 s, 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 and farthest called. This corresponds to the of 0.157, which is about halfway between  (0.093) and  (0.204). The amount of time taken to complete one round trip around the star is 1.183 s, or about one (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.° 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 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
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. 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.

Mass and size
Penthus contains 12.5 s 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 s, which is 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
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 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
Like other terrestrial planets, Penthus underwent, an event in which denser materials sink to form the core. The planet's core is made of and  with small amount of. 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
Penthus' surface is littered with active es 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 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
Due to the planet's weak magnetic field due to tidal forces slowing down the rotation, the atmosphere would be exposed to. 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. The atmosphere is made almost exclusively of gases emitted by many active volcanoes. The chief gas is, making up roughly 89½% of the atmosphere by volume. Most of the remaining atmosphere is made of (5.3%) and  (4.3%). There is also water vapor making up 0.6% by volume. Compared to most other planets, the atmosphere contains a lot of (0.2%), believed to be emitted from volcanoes.

Magnetic field
As mentioned above, the planet has a weak 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 s tilt 5.7° relative to geographic poles.

Moons and rings
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
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.