Planet Nine

This article is about the hypothetical planet first suggested in 2014. For other uses, see Ninth planet (disambiguation).

Not to be confused with the hypothetical Planet X first proposed in 1906 by Percival Lowell.

Planet Nine

Artist's impression of Planet Nine eclipsing the central Milky Way, with the Sun in the distance; Neptune's orbit is shown as a small ellipse around the Sun
Orbital characteristics
Semi-major axis	400–800 AU
Eccentricity	0.2–0.5
Inclination	15°–25°
Argument of perihelion	150° (est.)
Physical characteristics
Mass	5–10 Template:Earth mass (est.)
Apparent magnitude	>22.5 (est.)

Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravitational effects could explain the unusual 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 improbable alignments suggest that an undiscovered planet may be shepherding the orbits of the most distant known Solar System objects.

|last         = Burdick
|first        = Alan
|date         = 20 January 2016
|title        = Discovering Planet Nine
|url          = http://www.newyorker.com/tech/elements/discovering-planet-nine
|magazine     = The New Yorker
|accessdate   = 20 January 2016
|archive-url  = https://web.archive.org/web/20160121034701/http://www.newyorker.com/tech/elements/discovering-planet-nine
|archive-date = 21 January 2016
|dead-url     = no
|df           = dmy-all

}}</ref>

This undiscovered super-Earth-sized planet would have 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. Konstantin Batygin and Michael E. Brown suggest 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 propose 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.

As of the end of 2018, no observation of Planet Nine had been announced. While 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 were 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 theories have been proposed to explain the observed clustering of TNOs.