Editing Planet (section)

==== Orbit ====
{{Main|Orbit|orbital elements}}
{{see also|Kepler's laws of planetary motion|Exoplanetology#Orbital parameters}}
[[File:TheKuiperBelt Orbits Pluto Ecliptic.svg|thumb|right|upright=1.35|The orbit of the planet Neptune compared to that of [[Pluto]]. Note the elongation of Pluto's orbit in relation to Neptune's ([[orbital eccentricity|eccentricity]]), as well as its large angle to the ecliptic ([[inclination]]).]]

In the Solar System, all the planets orbit the Sun in the same direction as the [[Solar rotation|Sun rotates]]: [[counter-clockwise]] as seen from above the Sun's north pole. At least one exoplanet, [[WASP-17b]], has been found to orbit in the opposite direction to its star's rotation.<ref>{{cite journal | first1 = D. R. |last1=Anderson | title = WASP-17b: an ultra-low density planet in a probable retrograde orbit | arxiv = 0908.1553 | date = 2009 | last2 = Hellier | first2 = C. | last3 = Gillon | first3 = M. | last4 = Triaud | first4 = A. H. M. J. | last5 = Smalley | first5 = B. | last6 = Hebb | first6 = L. | last7 = Collier Cameron | first7 = A. | last8 = Maxted | first8 = P. F. L. | last9 = Queloz | first9 = D.| last10 =  West | first10 = R. G. | last11 =  Bentley | first11 = S. J. | last12 =  Enoch | first12 = B. | last13 =  Horne | first13 = K. | last14 =  Lister | first14 = T. A. | last15 =  Mayor | first15 = M. | last16 =  Parley | first16 = N. R. | last17 =  Pepe | first17 = F. | last18 =  Pollacco | first18 = D. | last19 =  Ségransan | first19 = D. | last20 =  Udry | first20 = S. | last21 =  Wilson | first21 = D. M. | display-authors= 4| doi=10.1088/0004-637X/709/1/159 | volume=709 | issue = 1 | journal=The Astrophysical Journal | pages=159–167 | bibcode=2010ApJ...709..159A| s2cid = 53628741 }}</ref> The period of one revolution of a planet's orbit is known as its [[sidereal period]] or ''year''.<ref name="young">{{cite book | first=Charles Augustus |last=Young |date=1902 |title=Manual of Astronomy: A Text Book | url=https://archive.org/details/manualastronomy05youngoog |publisher=Ginn & company |pages=[https://archive.org/details/manualastronomy05youngoog/page/n342 324]–327}}</ref> A planet's year depends on its distance from its star; the farther a planet is from its star, the longer the distance it must travel and the slower its speed, since it is less affected by its star's [[gravity]].{{citation needed|date=April 2025}}

No planet's orbit is perfectly circular, and hence the distance of each from the host star varies over the course of its year. The closest approach to its star is called its [[periastron]], or [[perihelion]] in the Solar System, whereas its farthest separation from the star is called its [[apastron]] ([[aphelion]]). As a planet approaches periastron, its speed increases as it trades [[gravitational energy|gravitational potential energy]] for [[kinetic energy]], just as a falling object on Earth accelerates as it falls. As the planet nears apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its [[trajectory]].<ref>{{cite book | last1=Dvorak |first1=R. | last2=Kurths |first2= J. | last3=Freistetter |first3= F. |date=2005 |title=Chaos And Stability in Planetary Systems |publisher=Springer |location=New York |isbn=978-3-540-28208-2 |page=90}}</ref>

Each planet's orbit is delineated by a set of elements:
* The ''[[Orbital eccentricity|eccentricity]]'' of an orbit describes the elongation of a planet's elliptical (oval) orbit. Planets with low eccentricities have more circular orbits, whereas planets with high eccentricities have more elliptical orbits. The planets and large moons in the Solar System have relatively low eccentricities, and thus nearly circular orbits.<ref name="young"/> The comets and many Kuiper belt objects, as well as several exoplanets, have very high eccentricities, and thus exceedingly elliptical orbits.<ref>{{cite journal |title=Eccentricity evolution of giant planet orbits due to circumstellar disk torques |last1=Moorhead |first1=Althea V. |last2=Adams |first2= Fred C. |journal=Icarus |date=2008 |volume=193 |issue=2 |pages=475–484 |doi=10.1016/j.icarus.2007.07.009 |arxiv=0708.0335 |bibcode=2008Icar..193..475M|s2cid=16457143 }}</ref><ref>{{cite web |title=Planets – Kuiper Belt Objects |work=The Astrophysics Spectator |date=15 December 2004 | url=http://www.astrophysicsspectator.com/topics/planets/KuiperBelt.html |access-date=23 August 2008 | archive-url=https://web.archive.org/web/20210323161115/https://www.astrophysicsspectator.com/topics/planets/KuiperBelt.html |archive-date=23 March 2021}}</ref>
* The ''[[semi-major axis]]'' gives the size of the orbit. It is the distance from the midpoint to the longest diameter of its elliptical orbit. This distance is not the same as its apastron, because no planet's orbit has its star at its exact centre.<ref name="young" />
* The ''[[inclination]]'' of a planet tells how far above or below an established reference plane its orbit is tilted. In the Solar System, the reference plane is the plane of Earth's orbit, called the [[ecliptic]]. For exoplanets, the plane, known as the ''sky plane'' or ''plane of the sky'', is the plane perpendicular to the observer's line of sight from Earth.<ref>{{cite book |chapter-url=http://astrowww.phys.uvic.ca/~tatum/celmechs.html |title=Celestial Mechanics |date=2007 |chapter=17. Visual binary stars |first=J. B. |last=Tatum |access-date=2 February 2008 |publisher=Personal web page |archive-date=6 July 2007 |archive-url=https://web.archive.org/web/20070706031613/http://astrowww.phys.uvic.ca/~tatum/celmechs.html |url-status=live }}</ref> The orbits of the eight major planets of the Solar System all lie very close to the ecliptic; however, some smaller objects like Pallas, Pluto, and Eris orbit at far more extreme angles to it, as do comets.<ref>{{cite journal |title=A Correlation between Inclination and Color in the Classical Kuiper Belt | last1=Trujillo |first1=Chadwick A. | last2=Brown | first2=Michael E. |journal=Astrophysical Journal |date=2002 |bibcode=2002ApJ...566L.125T | volume=566 |issue=2 | page=L125 |doi=10.1086/339437|arxiv = astro-ph/0201040 | s2cid=11519263 }}</ref> The large moons are generally not very inclined to their parent planets' [[equator]]s, but Earth's Moon, Saturn's Iapetus, and Neptune's Triton are exceptions. Triton is unique among the large moons in that it orbits [[Retrograde and prograde motion|retrograde]], i.e. in the direction opposite to its parent planet's rotation.<ref>{{cite journal|last1=Peter Goldreich|title=History of the Lunar Orbit|journal=[[Reviews of Geophysics]]|volume=4|issue=4|pages=411–439|date=Nov 1966|doi=10.1029/RG004i004p00411|bibcode=1966RvGSP...4..411G}}</ref>
* The points at which a planet crosses above and below its reference plane are called its [[ascending node|ascending]] and [[descending node]]s.<ref name="young" /> The [[longitude of the ascending node]] is the angle between the reference plane's 0 longitude and the planet's ascending node. The [[argument of periapsis]] (or perihelion in the Solar System) is the angle between a planet's ascending node and its closest approach to its star.<ref name="young" />