Still, legitimate hunts are on for planet-like objects beyond Neptune that are at least bigger than Pluto. Eris, which Caltech's Brown discovered in 2005, is a bit smaller but more massive than Pluto. And another dwarf planet called 2012 VP113, thought to be about 280 miles wide, might be the most distant object of its kind known—the closest it gets to the sun is about 7.2 billion miles. Ernest W. Brown (1931). "On a criterion for the prediction of an unknown planet". Monthly Notices of the Royal Astronomical Society. 92: 80–100. Bibcode: 1931MNRAS..92...80B. doi: 10.1093/mnras/92.1.80.

In the 1980s and 1990s, Robert Harrington led a search to determine the real cause of the apparent irregularities. [44] He calculated that any PlanetX would be at roughly three times the distance of Neptune from the Sun; its orbit would be highly eccentric, and strongly inclined to the ecliptic—the planet's orbit would be at roughly a 32-degree angle from the orbital plane of the other known planets. [45] This hypothesis was met with a mixed reception. Noted PlanetX skeptic Brian G. Marsden of the Minor Planet Center pointed out that these discrepancies were a hundredth the size of those noticed by Le Verrier, and could easily be due to observational error. [46]de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (11 October 2017). "Evidence for a possible bimodal distribution of the nodal distances of the extreme trans-Neptunian objects: avoiding a trans-Plutonian planet or just plain bias?". Monthly Notices of the Royal Astronomical Society Letters. 471 (1): L61–L65. arXiv: 1706.06981. Bibcode: 2017MNRAS.471L..61D. doi: 10.1093/mnrasl/slx106. S2CID 55469849. Brady, Joseph L. (1972). "The Effect of a Trans-Plutonian Planet on Halley's Comet". Publications of the Astronomical Society of the Pacific. 84 (498): 314–322. Bibcode: 1972PASP...84..314B. doi: 10.1086/129290. S2CID 122053270. Although some astronomers, such as Renu Malhotra and David Jewitt, have cautiously supported these claims, others, such as Alessandro Morbidelli, have dismissed them as "contrived". [63] Malhotra & Volk (2017) [83] argued that an unexpected variance in inclination for KBOs farther than the cliff at 50AU (7.5billionkm; 4.6billionmi) provided evidence of a possible Mars-sized planet, possibly up to 2.4 M Earth, residing at the edge of the Solar System, which many news sources began referring to as "PlanetTen". [84] [83] [85] [86] Shortly after it was proposed, Lorenzo Iorio showed that the hypothetical planet's existence cannot be ruled out by Cassini ranging data. [87] However, Brown notes that even though it might approach or exceed Earth in size, should such an object be found it would still be a "dwarf planet" by the current definition, because it would not have cleared its neighbourhood sufficiently. [78] Kuiper cliff and "Planet Ten" [ edit ]

In 1984, paleontologists claimed that a dim companion star to the sun would explain the periodic occurrence of mass extinctions on Earth. A massive body could theoretically disturb objects in the Oort Cloud surrounding the solar system, sending comets hurtling toward Earth with a deadly frequency. Known as Nemesis, scientists suggested the star could be a red dwarf, or a brown dwarf too dim to observe. During this phase, players will have a chance to submit theories as to where they think objects are located on the board. Each player can submit one theory in Standard Mode and up to two in Expert Mode. There are a couple of other game points, which keep things interesting. Every few moves you get to submit theories based on what you know, and there are conferences where the app gives all players more information about the location of Planet X – this can be a real leveller! You can draw a line through any two points," he says. "If they had three I might say this is interesting." Even then, the burden of proof would be high. Sheppard isn't a stranger to speculation about massive objects beyond Neptune—he has done some estimates based on perturbations of dwarf planet orbits. However, he's never floated the idea of one explicitly. Locate Planet X – Follow the usual steps for locating Planet X, but don’t advance your telescope 5 spaces.

Score: 8/10

a b J. K. Davies; J. McFarland; M. E. Bailey; B. G. Marsden; etal. (2008). "The Early Development of Ideas Concerning the Transneptunian Region" (PDF). In M. Antonietta Baracci; Hermann Boenhardt; Dale Cruikchank; Alissandro Morbidelli (eds.). The Solar System Beyond Neptune. University of Arizona Press. pp.11–23. Archived from the original (PDF) on 2015-02-20 . Retrieved 2014-11-05.

However, further observations showed that the object, now called Pluto, wasn't heavy enough to affect Neptune's orbit. And when the Voyager 2 mission got better estimates of Neptune's mass in 1989, it became clear that Planet X was unnecessary anyway.We have played it 4 times now. I love it, I would say hubby likes it. (He most enjoyed the first game, when he won.) Note: The above values represent a Standard Mode game. When playing in Expert Mode, there will be 5 Truly Empty Sectors and 4 Dwarf Planets. These Dwarf Planets always appear in one of 6 possible patterns, which are shown on your player screen.

In December 2015, astronomers at the Atacama Large Millimeter Array (ALMA) detected a brief series of 350GHz pulses that they concluded must either be a series of independent sources, or a single, fast moving source. Deciding that the latter was the most likely, they calculated based on its speed that, were it bound to the Sun, the object, which they named "Gna" after a fast-moving messenger goddess in Norse mythology, [97] would be about 12–25 AU distant and have a dwarf planet-sized diameter of 220 to 880km. However, if it were a rogue planet not gravitationally bound to the Sun, and as far away as 4000 AU, it could be much larger. [98] The paper was never formally accepted, and has been withdrawn until the detection is confirmed. [98] Scientists' reactions to the notice were largely sceptical; Mike Brown commented that, "If it is true that ALMA accidentally discovered a massive outer Solar System object in its tiny, tiny, tiny, field of view, that would suggest that there are something like 200,000 Earth-sized planets in the outer Solar System ... Even better, I just realized that this many Earth-sized planets existing would destabilize the entire Solar System and we would all die." [97] Constraints on additional planets [ edit ]Even before Neptune's discovery, some speculated that one planet alone was not enough to explain the discrepancy. On 17 November 1834, the British amateur astronomer the Reverend Thomas John Hussey reported a conversation he had had with French astronomer Alexis Bouvard to George Biddell Airy, the British Astronomer Royal. Hussey reported that when he suggested to Bouvard that the unusual motion of Uranus might be due to the gravitational influence of an undiscovered planet, Bouvard replied that the idea had occurred to him, and that he had corresponded with Peter Andreas Hansen, director of the Seeberg Observatory in Gotha, about the subject. Hansen's opinion was that a single body could not adequately explain the motion of Uranus, and postulated that two planets lay beyond Uranus. [11] Iorio, Lorenzo (2017). "Is the Recently Proposed Mars-Sized Perturber at 65–80 AU Ruled Out by the Cassini Ranging Data?". Frontiers in Astronomy and Space Sciences. 4: 28. arXiv: 1407.5894. Bibcode: 2017FrASS...4...28I. doi: 10.3389/fspas.2017.00028. S2CID 26844167. a b c d e f g h i j k Morton Grosser (1964). "The Search For A Planet Beyond Neptune". Isis. 55 (2): 163–183. doi: 10.1086/349825. JSTOR 228182. S2CID 144255699.