Planet Nine is a hypothetical planet in the outer region of the Solar System.[2][4] 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.[4][5][6] Nonetheless, some astronomers question this conclusion and instead assert that the clustering of the ETNOs orbits is due to observational biases, resulting from the difficulty of discovering and tracking these objects during much of the year.[7]
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 380+140
−80 AU.[3] This was shortly thereafter updated to 460 +160
−100 AU.[1] Konstantin Batygin and Michael E. Brown suggested that Planet Nine may 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,[8] was once a rogue planet, or that it formed on a distant orbit and was pulled into an eccentric orbit by a passing star.[4]
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.[9][10] 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.[11][12] Unless Planet Nine is observed, its existence remains purely conjectural. Several alternative hypotheses have been proposed to explain the observed clustering of trans-Neptunian objects (TNOs).
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 (See labelled version)
Batygin and Brown nicknamed their predicted object "Planet Nine," but the actual naming rights of an object go to the person who actually discovers it. The name used during previous hunts for the long suspected giant, undiscovered object beyond Neptune is "Planet X."
If the predicted world is found, the name must be approved by the International Astronomical Union. Planets are traditionally named for mythological Roman gods.
At the brightest possible values, Planet Nine would be detectable with multiple current sky surveys. Since it hasn’t yet been detected, the planet is likely not this bright. For most of the possible brightness range, the planet would be detectable by a telescope like the upcoming LSST telescope at the Rubin Observatory. For the dimmer end of the distribution, 10-meter or larger telescopes would be required for detection.
While the existence of a Planet Nine is still controversial, the statistical significance of KBO clustering remains, for which P9 would be the simplest explanation. The continued search for such a planet will strengthen our understanding of the outer Solar system and, if successful, allow for the detailed study of a sub-Neptune mass planet, a size very common among known exoplanets
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