The Nineth Planet

What do think? Is there a nineth planet lurking out in the depths of interplanetary space making its way back to the sun to intersect with Earth’s orbit? Type yes if you think this is the case.

Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and culminated at the start of the 20th with Percival Lowell‘s quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.

In 2014, astronomers announced the discovery of 2012 VP113, a large object with a Sedna-like 4200-year orbit and a perihelion of roughly 80 AU, which led them to suggest that it offered evidence of a potential trans-Neptunian planet. Trujillo and Sheppard argued that the orbital clustering of arguments of perihelia for VP113 and other extremely distant TNOs suggests the existence of a “super-Earth” of between 2 and 15 Earth masses beyond 200 AU and possibly on an inclined orbit at 1500 AU.

In 2014 astronomers at the Universidad Complutense in Madrid suggested that the available data actually indicate more than one trans-Neptunian planet.

On January 20, 2016, Brown and Konstantin Batygin published an article corroborating Trujillo and Sheppard’s initial findings; proposing a super-Earth (dubbed Planet Nine) based on a statistical clustering of the arguments of perihelia (noted before) near zero and also ascending nodes near 113° of six distant trans-Neptunian objects. They estimated it to be ten times the mass of Earth (about 60% the mass of Neptune) with a semimajor axis of approximately 400–1500 AU.

Probability

Even without gravitational evidence, Mike Brown, the discoverer of Sedna, has argued that Sedna’s 12,000-year orbit means that probability alone suggests that an Earth-sized object exists beyond Neptune. Sedna’s orbit is so eccentric that it spends only a small fraction of its orbital period near the Sun, where it can be easily observed. This means that unless its discovery was a freak accident, there is probably a substantial population of objects roughly Sedna’s diameter yet to be observed in its orbital region. Mike Brown noted that “Sedna is about three-quarters the size of Pluto. If there are sixty objects three-quarters the size of Pluto [out there] then there are probably forty objects the size of Pluto … If there are forty objects the size of Pluto, then there are probably ten that are twice the size of Pluto. There are probably three or four that are three times the size of Pluto, and the biggest of these objects … is probably the size of Mars or the size of the Earth.” However, he notes that, should such an object be found, even though it might approach Earth in size, it would still be a dwarf planet by the current definition, because it would not have cleared its neighbourhood sufficiently.

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