Editing Pluto (section)

=== Planet X disproved ===
Once Pluto was found, its faintness and lack of a [[Angular diameter|viewable disc]] cast doubt on the idea that it was Lowell's [[planets beyond Neptune|Planet X]].<ref name="Tombaugh1946" /> Estimates of Pluto's mass were revised downward throughout the 20th century.<ref>{{cite book|last1 = Stern|first1 = Alan|last2 = Tholen|first2 = David James|title = Pluto and Charon|date = 1997|publisher=University of Arizona Press|isbn = 978-0-8165-1840-1|pages=206–208}}</ref>

{| class="wikitable plainrowheaders floatright" style="clear:right;"
|+ Mass estimates for Pluto
!scope="col"| Year
!scope="col"| Mass
!scope="col"| Estimate by
|-
!scope="row"| 1915
| 7 Earths
| [[Percival Lowell|Lowell]] (prediction for [[Planet X]])<ref name="Tombaugh1946" />
|-
!scope="row"| 1931
| 1 Earth
| [[Seth Barnes Nicholson|Nicholson]] & [[Nicholas U. Mayall|Mayall]]<ref name="RAS1931.91" /><ref name="Nicholson et al 1930">{{cite journal
 | bibcode = 1930PASP...42..350N
 | title = The Probable Value of the Mass of Pluto
 | first1 = Seth B.
 | last1 = Nicholson
 | author-link1 = Seth Barnes Nicholson
 | first2 = Nicholas U.
 | last2 = Mayall
 | author-link2 = Nicholas U. Mayall
 | journal = Publications of the Astronomical Society of the Pacific
 | volume = 42
 | issue = 250
 | page = 350
 | date = December 1930
 | doi = 10.1086/124071
 | doi-access = free
}}</ref><ref name="Nicholson et al. 1931" />
|-
!scope="row"| 1948
| 0.1 (1/10) Earth
| [[Gerard Kuiper|Kuiper]]<ref name="Kuiper 10.1086/126255" />
|-
!scope="row"| 1976
| 0.01 (1/100) Earth
| [[Dale Cruikshank|Cruikshank]], <!-- Carl -->Pilcher, & [[David Morrison (astrophysicist)|Morrison]]{{sfn|Croswell|1997|p=57}}
|-
!scope="row"| 1978
| 0.0015 (1/650) Earth
| [[James W. Christy|Christy]] & [[Robert Sutton Harrington|Harrington]]<ref name="ChristyHarrington1978" />
|-
!scope="row"| 2006
| 0.00218 (1/459) Earth
| [[Marc W. Buie|Buie]] et al.<!-- William M. Grundy, Eliot F. Young, Leslie A. Young, S. Alan Stern --><ref name="BuieGrundyYoung_2006" />
|}
Astronomers initially calculated its mass based on its presumed effect on Neptune and Uranus. In 1931, Pluto was calculated to be roughly the mass of [[Earth]], with further calculations in 1948 bringing the mass down to roughly that of [[Mars]].<ref name="Nicholson et al 1930" /><ref name="Kuiper 10.1086/126255" /> In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the [[University of Hawaiʻi]] calculated Pluto's [[Albedo#Astronomical albedo|albedo]] for the first time, finding that it matched that for methane ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of Earth.{{sfn|Croswell|1997|p=57}} (Pluto's albedo is {{nowrap|1.4–1.9}} times that of Earth.<ref name="Pluto Fact Sheet" />)

In 1978, the discovery of Pluto's moon [[Charon (moon)|Charon]] allowed the measurement of Pluto's mass for the first time: roughly 0.2% that of Earth, and far too small to account for the discrepancies in the orbit of Uranus. Subsequent searches for an alternative Planet X, notably by [[Robert Sutton Harrington]],<ref name="SeidelmannHarrington1988" /> failed. In 1992, [[E. Myles Standish|Myles Standish]] used data from ''[[Voyager 2]]'''s flyby of Neptune in 1989, which had revised the estimates of Neptune's mass downward by 0.5%—an amount comparable to the mass of Mars—to recalculate its gravitational effect on Uranus. With the new figures added in, the discrepancies, and with them the need for a Planet X, vanished.<ref name="Standish1993" /> {{as of|2000}} the majority of scientists agree that Planet X, as Lowell defined it, does not exist.<ref name="Standage2000" /> Lowell had made a prediction of Planet X's orbit and position in 1915 that was fairly close to Pluto's actual orbit and its position at that time; [[Ernest W. Brown]] concluded soon after Pluto's discovery that this was a coincidence.<ref>Ernest W. Brown, [https://www.pnas.org/content/16/5/364 On the predictions of trans-Neptunian planets from the perturbations of Uranus] {{Webarchive|url=https://web.archive.org/web/20220118073827/https://www.pnas.org/content/16/5/364 |date=January 18, 2022 }}, PNAS May 15, 1930, 16 (5) 364–371.</ref>