Planet X

**Planet Nine** is a hypothetical planet in the outer region of the Solar System. Its gravitational effects could explain the peculiar clustering of orbits for a group of extreme trans-Neptunian objects (ETNOs), bodies beyond Neptune that orbit the Sun at distances averaging more than 250 times that of the Earth. These ETNOs tend to make their closest approaches to the Sun in one sector, and their orbits are similarly tilted. These alignments suggest that an undiscovered planet may be shepherding the orbits of the most distant known Solar System objects. Nonetheless, some astronomers question the idea that the hypothetical planet exists and instead assert that the clustering of the ETNOs orbits is due to observing biases, resulting from the difficulty of discovering and tracking these objects during much of the year.

Based on earlier considerations, this hypothetical super-Earth-sized planet would have had a predicted mass of five to ten times that of the Earth, and an elongated orbit 400 to 800 times as far from the Sun as the Earth. The orbit estimation was refined in 2021, resulting in a somewhat smaller semimajor axis of ~300 AU. This was shortly thereafter updated to 460 AU. Konstantin Batygin and Michael E. Brown suggested that Planet Nine could be the core of a giant planet that was ejected from its original orbit by Jupiter during the genesis of the Solar System. Others proposed that the planet was captured from another star, was once a rogue planet, or that it formed on a distant orbit and was pulled into an eccentric orbit by a passing star.

Although sky surveys such as Wide-field Infrared Survey Explorer (WISE) and Pan-STARRS did not detect Planet Nine, they have not ruled out the existence of a Neptune-diameter object in the outer Solar System. The ability of these past sky surveys to detect Planet Nine was dependent on its location and characteristics. Further surveys of the remaining regions are ongoing using NEOWISE and the 8-meter Subaru Telescope. Unless Planet Nine is observed, its existence is purely conjectural. Several alternative hypotheses have been proposed to explain the observed clustering of trans-Neptunian objects (TNOs).

## History

Following the discovery of Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The best-known of these theories predicted the existence of a distant planet that was influencing the orbits of Uranus and Neptune. After extensive calculations Percival Lowell predicted the possible orbit and location of the hypothetical trans-Neptunian planet and began an extensive search for it in 1906. He called the hypothetical object *Planet X*, a name previously used by Gabriel Dallet. Clyde Tombaugh continued Lowell's search and in 1930 discovered Pluto, but it was soon determined to be too small to qualify as Lowell's Planet X. After Voyager 2's flyby of Neptune in 1989, the difference between Uranus' predicted and observed orbit was determined to have been due to the use of a previously inaccurate mass of Neptune.

The discovery of Sedna's peculiar orbit in 2004 led to speculation that it had encountered a massive body other than one of the known planets. Sedna's orbit is detached, with a perihelion distance of 76 AU that is too large to be due to gravitational interactions with Neptune. Several authors proposed that Sedna entered this orbit after encountering a massive body such as an unknown planet on a distant orbit, a member of the open cluster that formed with the Sun, or another star that later passed near the Solar System. The announcement in March 2014 of the discovery of a second sednoid with a perihelion distance of 80 AU, 2012 VP113, in a similar orbit led to renewed speculation that an unknown super-Earth remained in the distant Solar System.

## Hypothesis

One hypothetical path through the sky of Planet Nine near aphelion crossing Orion west to east with about 2,000 years of motion. It is derived from that employed in the artistic conception on Brown's blog.
In early 2016, California Institute of Technology's Batygin and Brown described how the similar orbits of six ETNOs could be explained by Planet Nine and proposed a possible orbit for the planet. This hypothesis could also explain ETNOs with orbits perpendicular to the inner planets and others with extreme inclinations, and had been offered as an explanation of the tilt of the Sun's axis.

### Orbit

Planet Nine was initially hypothesized to follow an elliptical orbit around the Sun with an eccentricity of 0.2 to 0.5, and its semi-major axis was estimated to be 400 to 800 AU, roughly 13 to 26 times the distance from Neptune to the Sun. It would take the planet between 10,000 and 20,000 years to make one full orbit around the Sun, and its inclination to the ecliptic, the plane of the Earth's orbit, was projected to be 15° to 25°. The aphelion, or farthest point from the Sun, would be in the general direction of the constellation of Taurus, whereas the perihelion, the nearest point to the Sun, would be in the general direction of the southerly areas of Serpens (Caput), Ophiuchus, and Libra. Brown thinks that if Planet Nine exists, a probe could reach it in as little as 20 years by using a powered slingshot trajectory around the Sun.

### Mass and radius

The planet is estimated to have 5 to 10 times the mass of Earth and a radius of 2 to 4 times Earth's. Brown thinks that if Planet Nine exists, its mass is sufficient to clear its orbit of large bodies in 4.5 billion years, the age of the Solar System, and that its gravity dominates the outer edge of the Solar System, which is sufficient to make it a planet by current definitions. Astronomer Jean-Luc Margot has also stated that Planet Nine satisfies his criteria and would qualify as a planet if and when it is detected.

### Primordial black hole

In 2019, Jakub Scholtz and James Unwin proposed that a primordial black hole was responsible for the clustering of the orbits of the ETNOs. Their analysis of OGLE gravitational lensing data revealed a population of planetary mass objects in the direction of the galactic bulge more numerous than the local population of stars. They propose that instead of being free floating planets, these objects are primordial black holes. Since their estimate of the size of this population is greater than the estimated population of free floating planets from planetary formation models they argue that the capture of a hypothetical primordial black hole would be more probable than the capture of a rogue planet.

## Naming

Planet Nine does not have an official name and will not receive one unless its existence is confirmed via imaging. Only two planets, Uranus and Neptune, have been discovered in the Solar System during recorded history. However, many minor planets, including dwarf planets such as Pluto, asteroids, and comets have been discovered and named. Consequently, there is a well-established process for naming newly discovered solar system objects. If Planet Nine is observed, the International Astronomical Union will certify a name, with priority usually given to a name proposed by its discoverers. It is likely to be a name chosen from Roman or Greek mythology.

## Nibiru cataclysm

In context, The **Nibiru cataclysm** is a supposed disastrous encounter between Earth and a large planetary object (either a collision or a near-miss) that certain groups believed would take place in the early 21st century. Believers in this doomsday event usually refer to this object as *Nibiru* or Planet X. The idea was first put forward in 1995 by Nancy Lieder, founder of the website ZetaTalk. Lieder claims she is a contactee with the ability to receive messages from extraterrestrials from the Zeta Reticuli star system through an implant in her brain. She states that she was chosen to warn mankind that the object would sweep through the inner Solar System in May 2003 (though that date was later postponed) causing Earth to undergo a physical pole shift that would destroy most of humanity.

The prediction has subsequently spread beyond Lieder's website and has been embraced by numerous Internet doomsday groups. In the late 2000s, it became closely associated with the 2012 phenomenon. Since 2012, the Nibiru cataclysm has frequently reappeared in the popular media, usually linked to newsmaking astronomical objects such as Comet ISON or Planet Nine. Although the name "Nibiru" is derived from the works of the "ancient astronaut" writer Zecharia Sitchin and his interpretations of Babylonian and Sumerian mythology, he denied any connection between his work and various claims of a coming apocalypse. A prediction by self-described "Christian numerologist" David Meade that the Nibiru cataclysm would occur on 23 September 2017 received extensive media coverage.

The idea that a planet-sized object will collide with or closely pass by Earth in the near future is not supported by any scientific evidence and has been rejected by astronomers and planetary scientists as pseudoscience and an Internet hoax.