Editing Pluto (section)

=== Relationship with Neptune <span class="anchor" id="Orbits of Pluto and Neptune"></span> ===
[[File:Plutoorbit1.5sideview.gif|left|thumb|Orbit of Pluto&nbsp;– ecliptic view. This "side view" of Pluto's orbit (in red) shows its large inclination to the [[ecliptic]]. Neptune is seen orbiting close to the ecliptic.]]

Despite Pluto's orbit appearing to cross that of Neptune when viewed from north or south of the Solar System, the two objects' orbits do not intersect. When Pluto is closest to the Sun, and close to Neptune's orbit as viewed from such a position, it is also the farthest north of Neptune's path. Pluto's orbit passes about 8 [[astronomical unit|AU]] north of that of Neptune, preventing a collision.<ref name="huainn01" /><ref name="Hunter2004" /><ref name="malhotra-9planets" />{{efn|Because of the eccentricity of Pluto's orbit, some have theorized that it was once a [[satellite of Neptune]].<ref>{{cite book|last1=Sagan|first1=Carl|url=https://books.google.com/books?id=LhkoowKFaTsC|title=Comet|last2=Druyan|first2=Ann|publisher=Random House|year=1997|isbn=978-0-3078-0105-0|location=New York|page=223|author-link1=Carl Sagan|author-link2=Ann Druyan|name-list-style=amp|access-date=October 18, 2021|archive-date=February 26, 2024|archive-url=https://web.archive.org/web/20240226151129/https://books.google.com/books?id=LhkoowKFaTsC|url-status=live}}</ref>}}

This alone is not enough to protect Pluto; [[perturbation (astronomy)|perturbations]] from the planets (especially Neptune) could alter Pluto's orbit (such as its [[apsidal precession|orbital precession]]) over millions of years so that a collision could happen. However, Pluto is also protected by its 2:3 [[orbital resonance]] with [[Neptune]]: for every two orbits that Pluto makes around the Sun, Neptune makes three, in a frame of reference that rotates at the rate that Pluto's perihelion precesses (about {{value|0.97e-4}} degrees per year<ref name=williams71/>). Each cycle lasts about 495 years. (There are many other objects in this same resonance, called [[plutino]]s.) At present, in each 495-year cycle, the first time Pluto is at [[perihelion]] (such as in 1989), Neptune is 57° ahead of Pluto. By Pluto's second passage through perihelion, Neptune will have completed a further one and a half of its own orbits, and will be 123° behind Pluto.<ref name=Horizons>The [https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27134340%27&START_TIME=%271700-1-7%27&STOP_TIME=%272097-12-31%27&STEP_SIZE=%271461%20days%27&QUANTITIES=%2718%2019%27 ecliptic longitude of Pluto] {{Webarchive|url=https://web.archive.org/web/20240213103809/https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27134340%27&START_TIME=%271700-1-7%27&STOP_TIME=%272097-12-31%27&STEP_SIZE=%271461%20days%27&QUANTITIES=%2718%2019%27 |date=February 13, 2024 }} and [https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27899%27&START_TIME=%271700-1-7%27&STOP_TIME=%272097-12-31%27&STEP_SIZE=%271461%20days%27&QUANTITIES=%2718%2019%27 of Neptune] {{Webarchive|url=https://web.archive.org/web/20240213103809/https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27899%27&START_TIME=%271700-1-7%27&STOP_TIME=%272097-12-31%27&STEP_SIZE=%271461%20days%27&QUANTITIES=%2718%2019%27 |date=February 13, 2024 }} are available from the [[JPL Horizons On-Line Ephemeris System]].</ref> Pluto and Neptune's minimum separation is over 17&nbsp;AU, which is greater than Pluto's minimum separation from Uranus (11&nbsp;AU).<ref name="malhotra-9planets" /> The minimum separation between Pluto and Neptune actually occurs near the time of Pluto's aphelion.<ref name=williams71 />

[[File:Neptune-Pluto longitude difference.png|thumb|400px|Ecliptic longitude of Neptune minus that of Pluto (blue), and rate of change of Pluto's distance from the Sun (red). The red curve crosses zero at perihelion and aphelion.]]
The 2:3 resonance between the two bodies is highly stable and has been preserved over millions of years.<ref name="sp-345" /> This prevents their orbits from changing relative to one another, so the two bodies can never pass near each other. Even if Pluto's orbit were not inclined, the two bodies could never collide.<ref name="malhotra-9planets" /> When Pluto's period is slightly different from 3/2 of Neptune's, the pattern of its distance from Neptune will drift. Near perihelion Pluto moves interior to Neptune's orbit and is therefore moving faster, so during the first of two orbits in the 495-year cycle, it is approaching Neptune from behind. At present it remains between 50° and 65° behind Neptune for 100 years (e.g. 1937–2036).<ref name=Horizons/> The gravitational pull between the two causes [[angular momentum]] to be transferred to Pluto. This situation moves Pluto into a slightly larger orbit, where it has a slightly longer period, according to [[Kepler's third law]]. After several such repetitions, Pluto is sufficiently delayed that at the second perihelion of each cycle it will not be far ahead of Neptune coming behind it, and Neptune will start to decrease Pluto's period again. The whole cycle takes about 20,000 years to complete.<ref name="malhotra-9planets" /><ref name="sp-345" /><ref name="Cohen_Hubbard_1965">{{cite journal|last1=Cohen|first1=C. J.|last2=Hubbard|first2=E. C.|title=Libration of the close approaches of Pluto to Neptune|journal=Astronomical Journal|date=1965|volume=70|page=10|doi=10.1086/109674|bibcode=1965AJ.....70...10C|doi-access=free}}</ref>

==== Other factors ====
Numerical studies have shown that over millions of years, the general nature of the alignment between the orbits of Pluto and Neptune does not change.<ref name="huainn01" /><ref name="williams71" /> There are several other resonances and interactions that enhance Pluto's stability. These arise principally from two additional mechanisms (besides the 2:3 mean-motion resonance).

First, Pluto's [[argument of perihelion]], the angle between the point where it crosses the ecliptic (or the [[invariant plane]]) and the point where it is closest to the Sun, [[libration|librates]] around 90°.<ref name="williams71" /> This means that when Pluto is closest to the Sun, it is at its farthest north of the plane of the Solar System, preventing encounters with Neptune. This is a consequence of the [[Kozai mechanism]],<ref name="huainn01" /> which relates the eccentricity of an orbit to its inclination to a larger perturbing body—in this case, Neptune. Relative to Neptune, the amplitude of libration is 38°, and so the angular separation of Pluto's perihelion to the orbit of Neptune is always greater than 52° {{nowrap|(90°–38°)}}. The closest such angular separation occurs every 10,000 years.<ref name="sp-345" />

Second, the longitudes of ascending nodes of the two bodies—the points where they cross the [[invariant plane]]—are in near-resonance with the above libration. When the two longitudes are the same—that is, when one could draw a straight line through both nodes and the Sun—Pluto's perihelion lies exactly at 90°, and hence it comes closest to the Sun when it is furthest north of Neptune's orbit. This is known as the ''1:1 superresonance''. All the [[Jovian planets]] (Jupiter, Saturn, Uranus, and Neptune) play a role in the creation of the superresonance.<ref name="huainn01" />