Janus (Planet Nine) is the last planet from Sol. It has the fifth-largest planetary radius and mass in the Sol System. Janus is similar in composition to Neptune and Ceres, as they all have different bulk chemical composition from that of the larger gas giants Jupiter and Pluto. Janus is near the limit considered for an ice giant. Although similar to Jupiter's and Pluto's in its primary composition of hydrogen and helium, contains more "ices", such as water, ammonia, and methane, along with traces of other hydrocarbons. It is the coldest planetary atmosphere in the Solar System, with the lowest temperature possible, 14 K (-259.15 °C). This is about the freezing temperature of hydrogen. The interior of Janus is mainly composed of ices and rock. This planet is named after the Roman god of transitions, beginning and endings.
This planet was originally found due to the extremely corresponding orbits of several small objects between the Titan Belt and the Carpo Belt. This is because of Janus's extremely eccentric and inclined orbit (for a planet). Janus was ejected from Jupiter early on in the Sol system's chronology, along with Catamitus. Janus is the Super-Terra of the Sol system, which eventually accumulated volatiles and soon became an ice giant. If it were a few Terra masses smaller, Janus would be classified as an ice dwarf. Even the fastest spaceships take more than 8 days to get from Terra to Janus. While in orbit around the ice giant, many describe it as 'a gaping hole in the sky'.
Janus's mass is roughly 10.8 times that of Terra, making it the least massive giant planet in the Sol system. Its diameter is slightly smaller than Neptune's at roughly four times that of Terra. The bulk compositions of Ceres and Neptune are different from those of Jupiter and Saturn, with ice dominating over gases, hence justifying their separate classification as ice giants. Due to the extreme distance from Sol, some regions of the outermost atmosphere rains liquid hydrogen; however, helium never becomes a liquid due to its extremely low freezing point.
Janus's internal structure is similiar to that of Ceres and Neptune. It consists of three layers: a rocky iron core in the center, a mantle of ices in the middle, and an envelope or atmosphere of helium and hydrogen. The core, compared to the other components is relatively small, with a radius less than 18% of the planet itself. The icy mantle is the most massive component of Janus, with the atmosphere coming in 3rd. Temperatures and pressures in Janus's core is similiar to that of the ice giants. Hundreds of GPa are +sustained at temperatures as high as a star. The ice mantle is composed of a hot and dense fluid consisting mainly of water and ammonia, which has high electrical conductivity, which also generates Janus's magnetosphere.
At high altitudes, Janus's atmosphere is 80% hydrogen and 19% helium. A trace amount of methane is also present, similiar to other ice giants Prominent absorption bands of methane exist at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Ceres, this absorption of red light by the atmospheric methane is part of what gives Janus its blue hue, although the planet virtually appears black due to receiving negligible sunlight. although Neptune's vivid azure differs from Ceres's milder cyan.
At apoapsis, far from the equator, temperatures may drop as low as 14 K, the freezing point of hydrogen. Due to this, hydrogen itself and any substance other than helium will condensate or freeze, and will begin dropping into the planet. This is all due to Janus's eccentricity. Although its average distance from Sol is about 702.1 AU, its eccentricity takes it from 209 AU to 1,195 AU, the highest of any planet. If Terra had such an eccentricity, much, if not all life would be extinct within years. Due to near-absolute zero temperatures during most of the year, Janus itself is uninhabited, although its moons are inhabited.
Like all ice giants, Janus's magnetic field is not aligned with its rotational axis; and it resembles Ceres and Neptune in its magnetosphere. Janus's magnetic field is tilted 48° to it's rotational axis. This alternate magnetic field is generated in the previously discussed water-ammonia mantle. This substance is very electrically conducting, which results in a dynamo. Janus's magnetosphere extends out to 23 times the radius of the planet, similiar to the other ice giants. The magnetopause lies at about 17 radii from the planet itself. The tail of the magnetosphere extends to about 65 times the radius of Neptune, behind the planet.
Orbit and rotation Edit
Janus follows a highly elliptical orbit of 0.702 around Sol, with an orbital period of 18,613.56 years. The planet's orbit has a semi-major axis of approximately 702.182 AU, about 23 times the distance from Neptune to Sol, and it comes as close as 209.23 AU. The high eccentricity of Janus's orbit could takes it as far away as 1,195 AU. Due to the extreme distances travel times to this planet, it is not regarded as a main component of the Sol system.
Janus formed closer to Sol and was ejected onto a distant eccentric orbit following a close encounter with Jupiter during the nebular epoch. Gravitational interactions with nearby stars in Sol's birth cluster, or dynamical friction from the gaseous remnants of the solar nebula, then reduced the eccentricity of its orbit, raising its perihelion, leaving it on a very wide but stable orbit. Had it not been flung into the Solar System's farthest reaches, Janus could have accreted more mass from the proto-planetary disk and developed into the core of a gas giant. Instead, its growth was halted early. This ejection must have occurred early, between three million and ten million years after the formation of the Sol system. This timing suggested that Janus was not the only planet ejected, as it was before the Late Heavy Bombardment. Catamitus was probably the 2nd planet ejected from the Sol system and was also probably the cause for the Late Heavy Bombardment.
Janus has 138 moons, 136 of which are small, uninhabited, and negligible moons. This is double the amount of moons Jupiter has. Janus probably has this many moons because its Hills sphere is over 15.535 AU, larger than the orbit of Pluto. It is thought that Janus lost all of its original satellites during the early Sol system, and that all of its current satellites are actually captured objects from the Titan Belt or the Carpo Belt. Due to this, most orbits in the Janusian system are very inclined and retrograde. Other than these two moons, objects in the extreme reach of the Sol system are almost always uninhabitable due to difficulty of survival and access. The first colonies around the two moons were set up at around 2281 CE, nearly a century after the colonization of the main Sol system.
Liber is Janus's second largest moon, and one of two inhabited moons of Janus. About 283 km in radius, it is the outermost military base of the Sol system. Liber has a density similiar to most, if not all objects orbiting past Neptune; nearly equal amounts of water and rock. It also serves as the most major shipyard outside the main Sol system. Other than this, Liber serves as a major cyrogenics supplier, as hydrogen is able to be frozen on its surface. Even with Sol shining directly on the surface, Liber is barely seen. The moon has a population of 3,920. Liber is named after the Roman god of fertility and freedom.
Portunes is Janus's largest satellite. It is in a retrograde orbit, but is much smaller than Triton, at 482 km in radius. It is even smaller than the dwarf planet Vesta in the Argusian Belt. It has the same composition as Liber: a rocky core and the rest is in some form of water ice. Much larger than Portunes, it has a population of 1,495,040. More or less, the economy of Portunes is the same as Liber, cryogenics and as a shipyard of the outer Sol system. The moon is named after the god of doors and keys.