Planet Nine (sometimes called Planet X) is a hypothetical, unconfirmed planet which is said to exist in the outer region of the Solar System. Its gravity could explain the unlikely clustering of orbits for a group of extreme trans-Neptunian objects (eTNOs) that orbit beyond Neptune with average distances greater than 250 AU. These eTNOs tend to have their points of closest approach to the Sun clustered in one direction, and many of their orbits have a similar tilt. This evidence suggests that the gravity from a large undiscovered object is influencing the orbits of the most distant known Solar System objects.
2016 Estimate[]
In 2016, when Planet Nine was first hypothesized, Planet Nine's orbital perimeters were
Aphelion: 1,200 AU
Perihelion: 200 AU
SMA: 700 AU
Orbital Period: ~10,233 y
Eccentricity: 0.6
Inclination: 30 degrees relative to Earth's orbital plane.
AOP: 150 degrees
2019 Estimate[]
SMA: 400-500 AU
Eccentricity: 0.15-0.3
Inclination: 20 degrees relative to Earth's orbital plane.
Mass: 5 Earth masses
2021 Estimate[]
SMA: 380 AU
Perihelion: 300 AU
Inclination: 16 degrees relative to Earth's orbital plane.
Mass: 6.2-6.3 Earths.
Criticism[]
Planet Nine has been criticized many times. The most famous example is a scientist saying that the eTNOs' orbital clustering is simply a coincidence.
Conspiracy[]
In 2012, conspiracy theorists said that a planet 2-4 times the size of Earth, known as "Planet X" or "Nibiru" was on a collision course with Earth. Nibiru's existence has since been debunked by astronomers and scientists alike.
Possible Origin[]
Planet Nine could be captured by the Sun from another star, or, more likely, a planet that was ejected by Jupiter and Saturn in the early solar system. Though it is not yet known if Planet Nine and the Fifth Gas Giant are the same object, this cannot be ruled out.