In 2014 astronomers at the Universidad Complutense in Madrid suggested that the available data actually indicates more than one trans-Neptunian planet; [71] subsequent work further suggests that the evidence is robust enough but rather than connected with Ω and ω, semi-major axes and nodal distances could be the signposts. [72] [73] Additional work based on improved orbits of 39 objects still indicates that more than one perturber could be present and that one of them could orbit the Sun at 300-400 AU. [74] 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. 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. a b c Trujillo, C. A.; Sheppard, S. S. (2014). "A Sedna-like body with a perihelion of 80 astronomical units" (PDF). Nature. 507 (7493): 471–474. Bibcode: 2014Natur.507..471T. doi: 10.1038/nature13156. PMID 24670765. S2CID 4393431. Archived from the original (PDF) on 2014-12-16 . Retrieved 2016-01-25. Planet X" redirects here. For the conspiracy theory, see Nibiru cataclysm. For the hypothetical planet first proposed in 2014, see Planet Nine. For other uses, see Planet X (disambiguation).

After the discovery of Pluto and Charon, no more trans-Neptunian objects (TNOs) were found until 15760 Albion in 1992. [56] Since then, thousands of such objects have been discovered. Most are now recognized as part of the Kuiper belt, a swarm of icy bodies left over from the Solar System's formation that orbit near the ecliptic plane just beyond Neptune. Though none were as large as Pluto, some of these distant trans-Neptunian objects, such as Sedna, were initially described in the media as "new planets". [57] It’s also worth understanding the concept of the ‘visible sky’– at all times, the Earth board will be covering up half of the sectors in the game. Whenever you Survey/Target, you can only ask for information in the half that is currently uncovered. 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.Rodgers, Paul (February 13, 2011). "Up telescope! Search begins for giant new planet". The Independent . Retrieved February 14, 2011. Eris was never officially classified as a planet, and the 2006 definition of planet defined both Eris and Pluto not as planets but as dwarf planets because they have not cleared their neighbourhoods. [4] They do not orbit the Sun alone, but as part of a population of similarly sized objects. Pluto itself is now recognized as being a member of the Kuiper belt and the largest dwarf planet, larger than the more massive Eris. Ken Croswell (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. New York: The Free Press. ISBN 978-0684832524. You also receive one bonus point for each sector in which you were the first person to submit a correct theory. If more than one player submitted correct theories at the same time, you both receive a point. The Final

Although its mission did not involve a search for PlanetX, the IRAS space observatory made headlines briefly in 1983 due to an "unknown object" that was at first described as "possibly as large as the giant planet Jupiter and possibly so close to Earth that it would be part of this Solar System". [49] Further analysis revealed that of several unidentified objects, nine were distant galaxies and the tenth was " interstellar cirrus"; none were found to be Solar System bodies. [50]Patryk S., Lykawka; Tadashi, Mukai (2008). "An Outer Planet Beyond Pluto and the Origin of the Trans-Neptunian Belt Architecture". The Astronomical Journal. 135 (4): 1161–1200. arXiv: 0712.2198. Bibcode: 2008AJ....135.1161L. doi: 10.1088/0004-6256/135/4/1161. S2CID 118414447. Mark Littman (1990). Planets Beyond: Discovering the Outer Solar System. New York: Wiley. ISBN 978-0471510536. Some of these include extremely close stars, such as one located only 20 light-years away in the constellation Norma. A study that looked at WISE data last year found a pair of brown dwarfs just 6.5 light-years from Earth, making it the closest star system discovered in almost 100 years. A number of astronomers, most notably Alan Stern, the head of NASA's New Horizons mission to Pluto, contend that the IAU's definition is flawed, and that Pluto and Eris, and all large trans-Neptunian objects, such as Makemake, Sedna, Quaoar, Gonggong and Haumea, should be considered planets in their own right. [60] However, the discovery of Eris did not rehabilitate the PlanetX theory because it is far too small to have significant effects on the outer planets' orbits. [61] Subsequently proposed trans-Neptunian planets [ edit ]

Central Bureau for Astronomical Telegrams, International Astronomical Union (2006). "Circular No. 8747" (PDF). Archived from the original on February 5, 2007 . Retrieved 2011-07-05. 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.While the astronomical community widely agrees that PlanetX, as originally envisioned, does not exist, the concept of an as-yet-unobserved planet has been revived by a number of astronomers to explain other anomalies observed in the outer Solar System. [5] As of March 2014, observations with the WISE telescope have ruled out the possibility of a Saturn-sized object (95 Earth masses) out to 10,000 AU, and a Jupiter-sized (≈318 Earth masses) or larger object out to 26,000 AU. [6] Clyde W. Tombaugh (1946). "The Search for the Ninth Planet, Pluto". Astronomical Society of the Pacific Leaflets. 5 (209): 73–80. Bibcode: 1946ASPL....5...73T. 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.