Planet 9
Planet Nine is a hypothetical planet in the outer region of the Solar System.[2][3] Its gravitational effects could explain the unlikely 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.[3][4][5] 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.[6]
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 somewhat smaller semimajor axis of 380+140
−80 AU.[1] 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,[7] was once a rogue planet, or that it formed on a distant orbit and was pulled into an eccentric orbit by a passing star.[3]
As of August 2021, no observation of Planet Nine had been announced.[8][9] 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.[10][11] 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.[8][12] 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).
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.[13][14] 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.[15] 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.[16]
Attempts to detect planets beyond Neptune by indirect means such as orbital perturbation date back to before the discovery of Pluto. Among the first was George Forbes who postulated the existence of two trans-Neptunian planets in 1880. One would have an average distance from the Sun, or semi-major axis, of 100 astronomical units (AU), 100 times that of the Earth. The second would have a semi-major axis of 300 AU. His work is considered similar to more recent Planet Nine theories in that the planets would be responsible for a clustering of the orbits of several objects, in this case the clustering of aphelion distances of periodic comets near 100 and 300 AU. This is similar to how the aphelion distances of Jupiter-family comets cluster near its orbit.[17][18]
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.[19][20] 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.[21][22]
At a conference in 2012, Rodney Gomes proposed that an undetected planet was responsible for the orbits of some ETNOs with detached orbits and the large semi-major axis Centaurs, small Solar System bodies that cross the orbits of the giant planets.[23][24] The proposed Neptune-massed planet would be in a distant (1500 AU), eccentric (eccentricity 0.4), and inclined (inclination 40°) orbit. Like Planet Nine it would cause the perihelia of objects with semi-major axes greater than 300 AU to oscillate, delivering some into planet-crossing orbits and others into detached orbits like that of Sedna. An article by Gomes, Soares, and Brasser was published in 2015, detailing their arguments.[25]
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