|Other Names:||Sol IX, Planet Nine, Planet X|
|Moons:||≥12 (Thanatos, Typhon, others)|
|Orbital Period:||14,201 years|
|Semimajor Axis:||594 AU|
|Radius:||13,000 km (approximate)|
|Gravity:||2.36 g ±0.5 g|
|Rotational Period:||46 hours ±1 hour|
|Age:||Unknown (estimated ~4 billion years)|
|Average Temperature:||-250 °C|
|Atmosphere Pressure:||0.04 atm|
(percent makeup unknown)
|Exploration:||- 2098 (Orpheus, first fly-by)|
Tartarus, formerly known as Planet Nine and Planet X, is the ninth and most distant planet of Sol, and the last planet to be discovered. Prior to 1999, Pluto was considered the 9th planet after its discovery by Clyde Tombaugh, but it was later downgraded to the status of a minor planet. Until its discovery in 2064 by the Deep Sight space telescope, it was assumed that the Solar System had just 8 planets until observations of an unusual alignment of Kuiper Belt objects pointed toward the presence of a massive planet in the distant Solar System.
A mission to explore Tartarus via probe was quickly organized and took the form of the Orpheus probe that made a fly-by of the system in 2098. Initial observations via Deep Sight suggested that Tartarus exhibited physical properties consistent with an ejected Ice Giant planet similar to Uranus or Neptune, but later focused observations found it to be more likely to be similar to a type of planet known as a "Super Earth", common in other solar systems but until recently seemingly absent from our own. Current consensus is that Tartarus is a Super-Earth ejected from the inner solar system at some point in the distant past, but further study will be necessary.
Naturally, the discovery of a new planet in the Solar System has spurred a wave of interest in Tartarus, but as of 2110 very little is fully known about the planet. As of 2110, there are several more probe missions planned as a fly-by to the planet, along with the Lethe mission currently in transit to the planet as its first orbiter and expected to arrive in 2125 with intent to study the planet's surface and interior via use of a scattered fleet of "micro-landers" However, owing to the planet's distance, exploration and study of the planet remains a somewhat niche interest for the space programs of Earth, with far more focus in the inner Solar System.
The information currently known about Tartarus suggest that the planet has at least 12 moons, two of which are large enough to take round shapes: Thanatos, and Typhon, though Thanatos is by far the larger of the two and composes what is likely to be 95% of all mass orbiting Tartarus. Its moon system is likely to be composed primarily of captured material from the Scattered Disc and Kuiper Belt, with Thanatos in particular likely to be a captured dwarf planet. Observations of the planet during the transit of a nearby star also reveal that Tartarus likely has a fairly large system of rings, but an unusually thick one that may be as much as 100 km thick as compared to the far thinner ring systems of the four gaseous planets in the Sol System. The currently accepted theory is that the ring system is formed of a disrupted former moon or dwarf planet that was either destroyed via impact or by Tartarus' gravitational forces in the recent past, likely within the last several thousand years.
The circumstances of its origins are debated heavily. The Grand Tack Model does suggest disruption of forming Super-Earths in the early solar system, but no current models provide evidence for the outwards movement of the inner planets consistent with the currently observed four inner planets. Another suggestion, based on its highly elliptical orbit, is that Tartarus was at some point ejected from a different solar system and later captured by Sol, with repeated fly-bys of and collisions with Kuiper Belt and Scattered Disc objects slowing it enough to provide for a capture orbit, however most simulations do not adequately explain how such a captured body could form resonant orbits as those seen with objects like Nerrivik. Until further study can be carried out with the Lethe probe, it is likely these questions will remain unanswered.