Editing Planets beyond Neptune (section)

=== Pluto loses Planet X title ===
[[File:Charon Discovery.jpg|thumb|Discovery image of [[Charon (moon)|Charon]]]]
To the observatory's disappointment and surprise, Pluto showed no visible disc; it appeared as a point, no different from a star, and, at only 15th magnitude, was six times dimmer than Lowell had predicted, which meant it was either very small, or very dark.<ref name=tombaugh/> Because of Lowell's predictions, astronomers thought that Pluto would be massive enough to [[Perturbation (astronomy)|perturb]] planets. This led them to assume that its [[Albedo#Astronomical albedo|albedo]] could be no less than 0.07 (meaning that, at minimum, it would reflect 7% of the light that hit it), which would have made Pluto about as dark as asphalt, and similar in reflectivity to the least reflective planet, which is [[Mercury (planet)|Mercury]].<ref name="bower">{{Cite journal |last=Bower |first=Ernest Clare |date=1931 |title=On the orbit and mass of Pluto with an ephemeris for 1931-1932 |journal=Lick Observatory Bulletins |language=en |volume=15 |issue=437 |pages=171–178 |bibcode=1931LicOB..15..171B |doi=10.5479/ADS/bib/1931LicOB.15.171B |issn=0075-9317}}</ref> This would have given Pluto an estimated mass of no more than 70% that of Earth.<ref name=bower/> Observations also revealed that Pluto's orbit was very elliptical, far more than that of any other planet.<ref name="lauch">{{Cite book |last1=Davies |first1=John K. |title=The Solar System Beyond Neptune |last2=McFarland |first2=John |last3=Bailey |first3=Mark E. |last4=Marsden |first4=Brian G. |last5=Ip |first5=Wing-Huen |date=2008 |publisher=University of Arizona Press |editor-last=Baracci |editor-first=M. Antonietta |pages=11–23 |chapter=The Early Development of Ideas Concerning the Transneptunian Region |display-authors=4 |access-date=2014-11-05 |editor-last2=Boenhardt |editor-first2=Hermann |editor-last3=Cruikchank |editor-first3=Dale |editor-last4=Morbidelli |editor-first4=Alissandro |chapter-url=http://www.arm.ac.uk/preprints/2008/522.pdf |archive-url=https://web.archive.org/web/20150220182134/http://www.arm.ac.uk/preprints/2008/522.pdf |archive-date=2015-02-20 |url-status=dead}}</ref>

Almost immediately, some astronomers questioned Pluto's status as a planet. Barely a month after its discovery was announced, on April 14, 1930, in an article in ''[[The New York Times]]'', [[Armin Otto Leuschner|Armin O. Leuschner]] suggested that Pluto's dimness and high orbital eccentricity made it more similar to an asteroid or comet: "The Lowell result confirms the possible high eccentricity announced by us on April 5. Among the possibilities are a large asteroid greatly disturbed in its orbit by close approach to a major planet such as Jupiter, or it may be one of many long-period planetary objects yet to be discovered, or a bright cometary object."<ref name=lauch/><ref name=nyt/> In that same article, [[Harvard Observatory]] director [[Harlow Shapley]] wrote that Pluto was a "member of the Solar System not comparable with known asteroids and comets, and perhaps of greater importance to cosmogony than would be another major planet beyond Neptune."<ref name=nyt>{{cite news|journal=The New York Times|date=April 14, 1930|url= https://timesmachine.nytimes.com/timesmachine/1930/04/14/96096499.pdf|title="Planet X" Orbit Raises More Doubt}}</ref> In 1931, after examining the structure of the [[Errors and residuals|residuals]] of Uranus' longitude using a trigonometric formula, [[Ernest W. Brown]] asserted (in agreement with [[E. C. Bower]]) that the presumed irregularities in the orbit of Uranus could not be due to the gravitational effect of a more distant planet, and thus that Lowell's supposed prediction was "purely accidental".<ref>{{Cite journal |last=Brown |first=Ernest W. |date=November 1931 |title=On a Criterion for the Prediction of an Unknown Planet |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=92 |issue=1 |pages=80–100 |bibcode=1931MNRAS..92...80B |doi=10.1093/mnras/92.1.80 |issn=0035-8711 |doi-access=free}}</ref>

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;<ref name="The Discovery of Pluto">{{cite journal|title= The Discovery of Pluto |journal= Monthly Notices of the Royal Astronomical Society |volume=91 |issue= 4 |date=February 1931 |pages= 380–385 |bibcode= 1931MNRAS..91..380. |doi=10.1093/mnras/91.4.380|doi-access= free }}</ref> 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.<ref name="wylie">{{Cite book |last=Weintraub |first=David A. |url=https://books.google.com/books?id=dW1_AwAAQBAJ |title=Is Pluto a Planet?: A Historical Journey through the Solar System |date=2014 |publisher=Princeton University Press |isbn=978-1-4008-5297-0 |page=141 |language=en}}</ref> 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.<ref name="Gerard P. Kuiper 1950 133–137">{{cite journal|title=The Diameter of Pluto |first=Gerard P. |last=Kuiper |journal=Publications of the Astronomical Society of the Pacific |volume=62 |pages= 133–137 |date=August 1950 |bibcode= 1950PASP...62..133K|doi= 10.1086/126255 |issue=366|doi-access=free }}</ref>

In 1973, based on the similarities in the periodicity and amplitude of brightness variation with [[Triton (moon)|Triton]], Dennis Rawlins conjectured Pluto's mass must be similar to Triton's. In retrospect, the conjecture turns out to have been correct; it had been argued by astronomers [[Walter Baade]] and E.C. Bower as early as 1934.<ref>{{Cite journal |last=Baade |first=W. |author-link=Walter Baade |year=1934 |title=The Photographic Magnitude and Color Index of Pluto |journal=Publications of the Astronomical Society of the Pacific |language=en |volume=46 |page=218 |bibcode=1934PASP...46..218B |doi=10.1086/124467 |issn=0004-6280 |doi-access=free |number=272}}</ref> However, because Triton's mass was then believed to be roughly 2.5% of the Earth–Moon system (more than ten times its actual value), Rawlins's determination for Pluto's mass was similarly incorrect. It was nonetheless a meagre enough value for him to conclude Pluto was not Planet X.<ref name="Mass and Position Limits for an Hypothetical Tenth Planet of the Solar System">{{Cite journal |last1=Rawlins |first1=D. |last2=Hammerton |first2=M. |date=June 1973 |title=Mass and Position Limits for an Hypothetical Tenth Planet of the Solar System |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=162 |issue=3 |pages=261–270 |bibcode=1973MNRAS.162..261R |doi=10.1093/mnras/162.3.261 |issn=0035-8711 |doi-access=free}} Rawlins also took into account Pluto's stellar occultation failure as reported by {{Cite journal |last1=Halliday |first1=Ian |last2=Hardie |first2=R. H. |last3=Franz |first3=Otto G. |last4=Priser |first4=John B. |year=1966 |title=An Upper Limit for the Diameter of Pluto |journal=Publications of the Astronomical Society of the Pacific |language=en |volume=78 |issue=461 |pages=113 |bibcode=1966PASP...78..113H |doi=10.1086/128307 |issn=0004-6280 |s2cid=121483531}}</ref> In 1976, Dale Cruikshank, Carl Pilcher, and David Morrison of the [[University of Hawaii]] analysed spectra from Pluto's surface and determined that it must contain [[methane]] [[Volatile (astrogeology)|ice]], which is highly reflective. This meant that Pluto, far from being dark, was in fact exceptionally bright, and thus was probably no more than {{frac|1|100}}&nbsp;Earth mass.<ref>{{cite journal |doi=10.1126/science.194.4267.835-a |pmid=17744185 |journal=Science |volume=194 |issue=4267 |pages=835–837 |year=1976 |title=Pluto: Evidence for methane frost }}</ref><ref name="Croswell 1997, p. 57">Croswell (1997), p. 57.</ref>

<div style="float:right; margin:2px;">
{| class=wikitable style="text-align:center; font-size:11px"
|+ Mass estimates for Pluto
!Year!! Mass!!Notes
|-
|1931 || 1 Earth ||Nicholson & Mayall<ref name="The Discovery of Pluto"/>
|-
|1942 || 0.91 Earth || Wylie<ref name=wylie/>
|-
|1948||  0.1 (1/10 Earth) || Kuiper<ref name="Gerard P. Kuiper 1950 133–137"/>
|-
|1973|| 0.025 (1/40 Earth) || Rawlins<ref name="Mass and Position Limits for an Hypothetical Tenth Planet of the Solar System" />
|-
|1976||0.01 (1/100 Earth)||Cruikshank, Pilcher, & Morrison<ref name="Croswell 1997, p. 57"/>
|-
|1978||0.002 (1/500 Earth) || Christy & Harrington<ref name="Christy">{{Cite journal |last1=Christy |first1=J. W. |last2=Harrington |first2=R. S. |name-list-style=amp |date=August 1978 |title=The satellite of Pluto |journal=The Astronomical Journal |volume=83 |issue=8 |pages=1005 |bibcode=1978AJ.....83.1005C |doi=10.1086/112284}}</ref>
|-
|2006||0.00218 (1/459 Earth) || Buie et al.<ref name="Buie">{{Cite journal |last1=Buie |first1=Marc W. |last2=Grundy |first2=William M. |last3=Young |first3=Eliot F. |last4=Young |first4=Leslie A. |last5=Stern |first5=S. Alan |name-list-style=amp |date=July 2006 |title=Orbits and Photometry of Pluto's Satellites: Charon, S/2005 P1, and S/2005 P2 |journal=The Astronomical Journal |language=en |volume=132 |issue=1 |pages=290–298 |arxiv=astro-ph/0512491 |bibcode=2006AJ....132..290B |doi=10.1086/504422 |issn=0004-6256 |s2cid=119386667}}</ref>
|}
</div>

Pluto's size was finally determined conclusively in 1978, when American astronomer [[James W. Christy]] discovered its moon [[Charon (moon)|Charon]]. This enabled him, together with [[Robert Sutton Harrington]] of the U.S. Naval Observatory, to measure the mass of the Pluto–Charon system directly by observing the moon's orbital motion around Pluto.<ref name="Christy" /> They determined Pluto's mass to be 1.31×10<sup>22</sup>&nbsp;kg; roughly one five-hundredth that of Earth or one-sixth that of the Moon, and far too small to account for the observed discrepancies in the orbits of the outer planets. Lowell's prediction had been a coincidence: If there was a Planet&nbsp;X, it was not Pluto.<ref>Croswell (1997), pp. 57–58.</ref>