Editing Planet (section)

=== Scientific Revolution and discovery of outer planets ===
{{See also|Heliocentrism}}
[[File:The_Solar_System,_with_the_orbits_of_5_remarkable_comets._LOC_2013593161.jpg|thumb|True-scale Solar System poster made by [[Emanuel Bowen]] in 1747. At that time, Uranus, Neptune, and the asteroid belts had all not yet been discovered.]]
With the advent of the [[Scientific Revolution]] and the [[heliocentric model]] of [[Copernicus]], [[Galileo]], and [[Kepler]], use of the term "planet" changed from something that moved around the sky relative to the [[fixed star]] to a body that orbited the Sun, directly (a primary planet) or indirectly (a secondary or satellite planet). Thus the Earth was added to the roster of planets,<ref name="galileo_project" /> and the Sun was removed. The Copernican count of primary planets stood until 1781, when [[William Herschel]] discovered [[Uranus]].<ref name="Dreyer">{{cite book|first=J. L. E. | last=Dreyer |year=1912|title=The Scientific Papers of Sir William Herschel|publisher=Royal Society and Royal Astronomical Society|volume=1|page=100|author-link=J. L. E. Dreyer|url=https://archive.org/details/scientificpapers032804mbp/page/100/mode/2up}}</ref>

When four satellites of Jupiter (the [[Galilean moons]]) and five of Saturn were discovered in the 17th century, they joined Earth's Moon in the category of "satellite planets" or "secondary planets" orbiting the primary planets, though in the following decades they would come to be called simply "satellites" for short. Scientists generally considered planetary satellites to also be planets until about the 1920s, although this usage was not common among non-scientists.<ref name=metzger22>{{cite journal |last1=Metzger |first1=Philip T. |author-link1=Philip T. Metzger |last2=Grundy |first2=W. M. |first3=Mark V. |last3=Sykes |first4=Alan |last4=Stern |first5=James F. |last5=Bell III |first6=Charlene E. |last6=Detelich |first7=Kirby |last7=Runyon |first8=Michael |last8=Summers |date=2022 |title=Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science |url=https://www.sciencedirect.com/science/article/pii/S0019103521004206 |journal=Icarus |volume=374 |issue= |page=114768 |doi=10.1016/j.icarus.2021.114768 |arxiv=2110.15285 |bibcode=2022Icar..37414768M |s2cid=240071005 |access-date=8 August 2022 |archive-date=11 September 2022 |archive-url=https://web.archive.org/web/20220911060134/https://www.sciencedirect.com/science/article/pii/S0019103521004206 |url-status=live }}</ref>

In the first decade of the 19th century, four new 'planets' were discovered: [[Ceres (dwarf planet)|Ceres]] (in 1801), [[2 Pallas|Pallas]] (in 1802), [[3 Juno|Juno]] (in 1804), and [[4 Vesta|Vesta]] (in 1807). It soon became apparent that they were rather different from previously known planets: they shared the same general region of space, between Mars and Jupiter (the [[asteroid belt]]), with sometimes overlapping orbits. This was an area where only one planet had been expected, and they were much smaller than all other planets; indeed, it was suspected that they might be shards of a larger planet that had broken up. Herschel called them ''[[asteroid]]s'' (from the Greek for "starlike") because even in the largest telescopes they resembled stars, without a resolvable disk.<ref name="asteroids" /><ref>{{cite OED|asteroid}}</ref>

The situation was stable for four decades, but in the 1840s several additional asteroids were discovered ([[5 Astraea|Astraea]] in 1845; [[6 Hebe|Hebe]], [[7 Iris|Iris]], and [[8 Flora|Flora]] in 1847; [[9 Metis|Metis]] in 1848; and [[10 Hygiea|Hygiea]] in 1849). New "planets" were discovered every year; as a result, astronomers began tabulating the asteroids ([[minor planet]]s) separately from the major planets and assigning them numbers instead of abstract [[planetary symbol]]s,<ref name="asteroids">{{cite web | last =Hilton |first =James L. |date = 17 September 2001 |url =http://aa.usno.navy.mil/faq/docs/minorplanets.php |title =When Did the Asteroids Become Minor Planets? |publisher =U.S. Naval Observatory |access-date = 8 April 2007 |archive-url = https://web.archive.org/web/20070921162818/http://aa.usno.navy.mil/faq/docs/minorplanets.php |archive-date = 21 September 2007}}</ref> although they continued to be considered as small planets.<ref name=metzger19/>

[[Discovery of Neptune|Neptune was discovered in 1846]], its position having been predicted thanks to its gravitational influence upon Uranus. Because the orbit of Mercury appeared to be affected in a similar way, it was believed in the late 19th century that there might be [[Vulcan (hypothetical planet)|another planet even closer to the Sun]]. However, the discrepancy between Mercury's orbit and the predictions of Newtonian gravity was instead explained by an improved theory of gravity, Einstein's [[general relativity]].<ref>{{cite book | first1=Richard P. | last1=Baum  | first2=William | last2=Sheehan | title=In Search of Planet Vulcan: The Ghost in Newton's Clockwork | publisher=Basic Books | year=2003 | page=264 | isbn=978-0738208893 }}</ref><ref>{{cite journal | last1 = Park | first1 = Ryan S. |  last2 = Folkner  | first2= William M. |last3 = Konopliv |first3 = Alexander S. | last4 = Williams | first4 = James G. |last5 = Smith |first5 = David E. |last6 =  Zuber | first6 = Maria T.| display-authors = 4 | year = 2017 | title = Precession of Mercury's Perihelion from Ranging to the MESSENGER Spacecraft | journal = The Astronomical Journal | volume = 153 | issue = 3| page = 121 | doi=10.3847/1538-3881/aa5be2| bibcode = 2017AJ....153..121P | hdl = 1721.1/109312 | s2cid = 125439949 | hdl-access = free | doi-access = free }}</ref>

[[Pluto]] was discovered in 1930. After initial observations led to the belief that it was larger than Earth,<ref>{{cite book |title=Planet Quest: The Epic Discovery of Alien Solar Systems |first=Ken |last=Croswell |author-link=Ken Croswell |publisher=The Free Press |date=1997 |page=57 |isbn=978-0-684-83252-4 }}</ref> the object was immediately accepted as the ninth major planet. Further monitoring found the body was actually much smaller: in 1936, [[Raymond Arthur Lyttleton|Ray Lyttleton]] suggested that Pluto may be an escaped satellite of [[Neptune]],<ref>{{cite journal | last=Lyttleton |first=Raymond A. |date=1936 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=97 |issue=2 |pages=108–115 |title= On the possible results of an encounter of Pluto with the Neptunian system |bibcode=1936MNRAS..97..108L |doi=10.1093/mnras/97.2.108|doi-access=free }}</ref> and [[Fred Lawrence Whipple|Fred Whipple]] suggested in 1964 that Pluto may be a comet.<ref>{{cite journal | journal=Proceedings of the National Academy of Sciences of the United States of America |volume=52 |pages=565–594 |last=Whipple |first=Fred |date=1964 |bibcode=1964PNAS...52..565W |title= The History of the Solar System |doi= 10.1073/pnas.52.2.565 | pmid=16591209 | issue=2 | pmc=300311|doi-access=free }}</ref> The discovery of its large moon [[Charon (moon)|Charon]] in 1978 showed that Pluto was only 0.2% the mass of Earth.<ref name="ChristyHarrington1978">{{cite journal| first1 = James W.| last1 = Christy| first2 = Robert Sutton| last2 = Harrington| author-link2 = Robert Sutton Harrington| title = The Satellite of Pluto| journal = Astronomical Journal| date = 1978| volume = 83| issue = 8| pages = 1005–1008| bibcode = 1978AJ.....83.1005C| doi = 10.1086/112284| s2cid = 120501620}}</ref> As this was still substantially more massive than any known asteroid, and because no other [[trans-Neptunian objects]] had been discovered at that time, Pluto kept its planetary status, only officially losing it in 2006.<ref>{{cite journal | journal=Scientific American |date=1996 |pages=46–52 |last1=Luu |first1=Jane X. |last2=Jewitt |first2=David C. |title=The Kuiper Belt |volume=274 |issue=5 |doi=10.1038/scientificamerican0596-46|bibcode = 1996SciAm.274e..46L }}</ref><ref name="Pluto loses status as a planet-2006">{{cite news |date=24 August 2006 |title=Pluto loses status as a planet |url=http://news.bbc.co.uk/1/hi/world/5282440.stm |url-status=live |archive-url=https://web.archive.org/web/20120530155226/http://news.bbc.co.uk/2/hi/5282440.stm |archive-date=30 May 2012 |access-date=23 August 2008 |department=[[BBC News]] |publisher=[[British Broadcasting Corporation]]}}</ref>

In the 1950s, [[Gerard Kuiper]] published papers on the origin of the asteroids. He recognized that asteroids were typically not spherical, as had previously been thought, and that the [[asteroid family|asteroid families]] were remnants of collisions. Thus he differentiated between the largest asteroids as "true planets" versus the smaller ones as collisional fragments. From the 1960s onwards, the term "minor planet" was mostly displaced by the term "asteroid", and references to the asteroids as planets in the literature became scarce, except for the geologically evolved largest three: Ceres, and less often Pallas and Vesta.<ref name=metzger19>{{cite journal |last1=Metzger |first1=Philip T. |author-link1=Philip T. Metzger |last2=Sykes |first2=Mark V. |last3=Stern |first3=Alan |last4=Runyon |first4=Kirby |date=2019 |title=The Reclassification of Asteroids from Planets to Non-Planets |journal=Icarus |volume=319 |pages=21–32 |doi=10.1016/j.icarus.2018.08.026|arxiv=1805.04115 |bibcode=2019Icar..319...21M |s2cid=119206487 }}</ref>

The beginning of Solar System exploration by space probes in the 1960s spurred a renewed interest in planetary science. A split in definitions regarding satellites occurred around then: planetary scientists began to reconsider the large moons as also being planets, but astronomers who were not planetary scientists generally did not.<ref name=metzger22/> (This is not exactly the same as the definition used in the previous century, which classed ''all'' satellites as secondary planets, even non-round ones like Saturn's [[Hyperion (moon)|Hyperion]] or Mars's [[Phobos (moon)|Phobos]] and [[Deimos (moon)|Deimos]].)<ref>{{cite book |last=Hind |first=John Russell |author-link=John Russell Hind |date=1863 |title=An introduction to astronomy, to which is added an astronomical vocabulary |url=https://books.google.com/books?id=d9aWKHamz8sC&dq=%22hyperion%22+%22secondary+planet%22&pg=PA205 |location=London |publisher=Henry G. Bohn |page=204 |isbn= |access-date=25 October 2023 |archive-date=30 October 2023 |archive-url=https://web.archive.org/web/20231030064739/https://books.google.com/books?id=d9aWKHamz8sC&dq=%22hyperion%22+%22secondary+planet%22&pg=PA205 |url-status=live }}</ref><ref>{{cite book |author=<!--Staff writer(s); no by-line.--> |editor-first1=Robert |editor-last1=Hunter |editor-first2=John A. |editor-last2=Williams |editor-first3=S. J. |editor-last3=Heritage |date=1897 |title=The American Encyclopædic Dictionary |url=https://books.google.com/books?id=P_VOAAAAYAAJ&dq=%22phobos%22+%22secondary+planet%22&pg=PA3553 |location=Chicago and New York |publisher=R. S. Peale and J. A. Hill |volume=8 |pages=3553–3554 |isbn= |access-date=25 October 2023 |archive-date=30 October 2023 |archive-url=https://web.archive.org/web/20231030063236/https://books.google.com/books?id=P_VOAAAAYAAJ&dq=%22phobos%22+%22secondary+planet%22&pg=PA3553 |url-status=live }}</ref> All the eight major planets and their planetary-mass moons have since been explored by spacecraft, as have many asteroids and the dwarf planets Ceres and Pluto; however, so far the only planetary-mass body beyond Earth that has been explored by humans is the Moon.{{efn|See [[Timeline of Solar System exploration]].}}