Editing Definition of planet (section)

=== Modern planets ===
[[File:Uranuslauf November Dezember2022.gif|thumb|Motion of Uranus across the sky as seen by two images taken 25 days apart]]
In 1781, the astronomer [[William Herschel]] was searching the sky for elusive [[stellar parallax]]es when he observed what he termed a [[comet]] in the constellation of [[Taurus (constellation)|Taurus]]. Unlike stars, which remained mere points of light even under high magnification, this object's size increased in proportion to the power used. That this strange object might have been a planet simply did not occur to Herschel; the five planets beyond Earth had been part of humanity's conception of the universe since antiquity. As the asteroids had yet to be discovered, comets were the only moving objects one expected to find in a telescope.<ref name=ken/> However, unlike a comet, this object's orbit was nearly circular and within the ecliptic plane. Before Herschel announced his discovery of his "comet,” his colleague, British [[Astronomer Royal]] [[Nevil Maskelyne]], wrote to him, saying "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any [[Coma (cometary)|coma]] or tail to it."<ref>{{cite book|title=William Herschel: Astronomer and Musician of 19 New King Street, Bath|author=Patrick Moore|publisher=PME Erwood|year=1981|page=8|isbn=978-0-907322-06-1}}</ref> The "comet" was also very far away, too far away for a mere comet to resolve itself. Eventually it was recognized as the seventh planet and named [[Uranus]] after the father of Saturn.

Gravitationally induced irregularities in Uranus's observed orbit led eventually to the discovery of [[Neptune]] in 1846, and presumed irregularities in Neptune's orbit subsequently led to a search which did not find the perturbing object (it was later found to be a mathematical artifact caused by an overestimation of Neptune's mass) but did find [[Pluto]] in 1930. Initially believed to be roughly the mass of the Earth, observation gradually shrank Pluto's estimated mass until it was revealed to be a mere five hundredth as large; far too small to have influenced Neptune's orbit at all.<ref name=ken>
{{cite book | author=Croswell, Ken | year=1999 | title=Planet Quest: The Epic Discovery of Alien Solar Systems| publisher= Oxford University Press|pages=48, 66 |isbn=978-0-19-288083-3}}</ref> In 1989, [[Voyager 2]] determined the irregularities to be due to an overestimation of Neptune's mass.<ref>{{cite web|title=Hopes Fade in hunt for Planet X|author=Ken Croswell|year=1993|url=http://kencroswell.com/HopesFadeInHuntForPlanetX.html |access-date=November 4, 2007}}</ref>