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. Lowell's first search focused on the ecliptic, the plane encompassed by the zodiac where the other planets in the Solar System lie. Using a 5-inch photographic camera, he manually examined over 200 three-hour exposures with a magnifying glass, and found no planets. At that time Pluto was too far above the ecliptic to be imaged by the survey. [18] After revising his predicted possible locations, Lowell conducted a second search from 1914 to 1916. [18] In 1915, he published his Memoir of a Trans-Neptunian Planet, in which he concluded that PlanetX had a mass roughly seven times that of Earth—about half that of Neptune [20]—and a mean distance from the Sun of 43AU. He assumed PlanetX would be a large, low-density object with a high albedo, like the giant planets. As a result, it would show a disc with diameter of about one arcsecond and an apparent magnitude between 12 and 13—bright enough to be spotted. [18] [21] In 1850 James Ferguson, Assistant Astronomer at the United States Naval Observatory, noted that he had "lost" a star he had observed, GR1719k, which Lt. Matthew Maury, the superintendent of the Observatory, claimed was evidence that it must be a new planet. Subsequent searches failed to recover the "planet" in a different position, and in 1878, CHF Peters, director of the Hamilton College Observatory in New York, showed that the star had not in fact vanished, and that the previous results had been due to human error. [11]

Schwamb, Megan (2007-09-18). "Searching for Sedna's Sisters: Exploring the inner Oort cloud" (PDF). Caltech. Archived from the original (PDF) on 2013-05-12 . Retrieved 2010-08-06. Brown, M.E.; Trujillo, C. & Rabinowitz, D. (2004). "Discovery of a candidate inner Oort Cloud planetoid". Astrophysical Journal. 617 (1): 645–649. arXiv: astro-ph/0404456. Bibcode: 2004ApJ...617..645B. doi: 10.1086/422095. S2CID 7738201. 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.

Achenbach, Joel; Feltman, Rachel (2016-01-20). "New evidence suggests a ninth planet lurking at the edge of the solar system". The Washington Post . Retrieved 2016-01-20. de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (1 May 2022). "Twisted extreme trans-Neptunian orbital parameter space: statistically significant asymmetries confirmed". Monthly Notices of the Royal Astronomical Society Letters. 512 (1): L6–L10. arXiv: 2202.01693. Bibcode: 2022MNRAS.512L...6D. doi: 10.1093/mnrasl/slac012.

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 continued at the start of the 20th with Percival Lowell's quest for Planet X. Lowell proposed the PlanetX hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune, [1] speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities. [2] NASA-Funded Scientists Discover Tenth Planet". Jet Propulsion Laboratory. 2005. Archived from the original on 2011-07-21 . Retrieved 2007-02-22.

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The game’s attachment to the real subject continues into its components, with player markers provided as charming plastic minis of real-life observatories, and the player screens used to hide your notes and deductions depicting those observatories as well as a couple of data points about them. Unfortunately, the components also relate to the first minor mark against the game – which is that there’s fairly few of them for the RRP of the game. While the cost obviously covers the app as well, it makes The Search for Planet X a little harder to recommend. For instance, one of the topics might be ‘Comets & Asteroids’. You can then write this on your sheet to help with deductions. The recent data isn't the first evidence against the theoretical bodies. The link between Planet X and mass extinctions was ruled out some time ago. Several infrared sky surveys in addition to WISE have also examined the space around the solar system and found no signs of a companion star to the sun. Comets (2) – Comets only appear in certain sectors (noted on your sheet). When surveying for them, you must start and end in one of these designated sectors. Throughout the mid-20th century, estimates of Pluto's mass were revised downward. In 1931, Nicholson and Mayall calculated its mass, based on its supposed effect on the giant planets, as roughly that of Earth; [34] a value somewhat in accord with the 0.91 Earth mass calculated in 1942 by Lloyd R. Wylie at the US Naval Observatory, using the same assumptions. [35] In 1949, Gerard Kuiper's measurements of Pluto's diameter with the 200-inch telescope at Mount Palomar Observatory led him to the conclusion that it was midway in size between Mercury and Mars and that its mass was most probably about 0.1 Earth mass. [36]

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. Paul Schlyter. "Hypothetical Planets". The Swedish Amateur Astronomical Society . Retrieved 2019-12-07. Fienga, A; Laskar, J; Manche, H; Gastineau, M (23 February 2016). "Constraints on the location of a possible 9th planet derived from the Cassini data". Astronomy and Astrophysics. 587: L8. arXiv: 1602.06116. Bibcode: 2016A&A...587L...8F. doi: 10.1051/0004-6361/201628227. S2CID 119116589. 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. 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.

Using these rules, you will gradually be able to work out the pattern of the night sky and where each object is. 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]