Editing Planet (section)

=== Dynamic characteristics ===

==== 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" />

==== Axial tilt ====
{{Main|Axial tilt}}
[[File:AxialTiltObliquity.png|thumb|Earth's [[axial tilt]] is about 23.4°. It oscillates between 22.1° and 24.5° on a 41,000-year cycle and is currently decreasing.]]

Planets have varying degrees of axial tilt; they spin at an angle to the [[reference plane|plane]] of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the [[Northern Hemisphere]] points away from its star, the [[Southern Hemisphere]] points towards it, and vice versa. Each planet therefore has [[season]]s, resulting in changes to the [[climate]] over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its [[solstice]]. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice with its day being the longest, the other has its winter solstice when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either continually in sunlight or continually in darkness around the time of [[Climate of Uranus|its solstices]].<ref name="Weather">{{cite web | last=Harvey |first=Samantha |date=1 May 2006 |url=http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=725 |archive-url=https://web.archive.org/web/20060831201346/http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=725 |archive-date=31 August 2006 |title=Weather, Weather, Everywhere? |publisher=NASA |access-date=23 August 2008}}</ref> In the Solar System, Mercury, Venus, Ceres, and Jupiter have very small tilts; Pallas, Uranus, and Pluto have extreme ones; and Earth, Mars, Vesta, Saturn, and Neptune have moderate ones.<ref name="factsheets">[https://web.archive.org/web/20160304052405/http://nssdc.gsfc.nasa.gov/planetary/planetfact.html Planetary Fact Sheets], NASA</ref><ref name="Schorghofer2016">{{Cite journal |last1=Schorghofer |first1=N. |last2=Mazarico |first2=E. |last3=Platz |first3=T. |last4=Preusker |first4=F. |last5=Schröder |first5=S. E. |last6=Raymond |first6=C. A. |last7=Russell |first7=C. T. |date=6 July 2016 |title=The permanently shadowed regions of dwarf planet Ceres |journal=Geophysical Research Letters |volume=43 |issue=13 |pages=6783–6789 |bibcode=2016GeoRL..43.6783S |doi=10.1002/2016GL069368 |doi-access=free}}</ref><ref name=Carry2009>{{Cite journal|title=Physical properties of (2) Pallas|author=Carry, B.|date=2009|doi=10.1016/j.icarus.2009.08.007 |arxiv=0912.3626|display-authors=etal|bibcode = 2010Icar..205..460C|volume=205|issue=2|journal=Icarus|pages=460–472|s2cid=119194526}}</ref><ref name="Thomas1997b">{{cite journal | title=Vesta: Spin Pole, Size, and Shape from HST Images | date=1997 | author=Thomas, P. C. | bibcode=1997Icar..128...88T | display-authors=etal | journal=Icarus | volume=128 | issue=1 | pages=88–94 | doi=10.1006/icar.1997.5736| doi-access=free }}</ref> Among exoplanets, axial tilts are not known for certain, though most hot Jupiters are believed to have a negligible axial tilt as a result of their proximity to their stars.<ref>{{cite journal |title=Obliquity Tides on Hot Jupiters | last1=Winn | first1=Joshua N. | last2=Holman | first2=Matthew J. |journal=The Astrophysical Journal |date=2005 | doi=10.1086/432834 | volume=628 |issue=2 |page=L159 |bibcode=2005ApJ...628L.159W|arxiv = astro-ph/0506468 |s2cid=7051928 }}</ref> Similarly, the axial tilts of the planetary-mass moons are near zero,<ref>{{cite book |title=Explanatory Supplement to the Astronomical Almanac |editor-first=P. Kenneth |editor-last=Seidelmann |publisher=University Science Books |date=1992 |page=384 }}</ref> with Earth's Moon at 6.687° as the biggest exception;<ref name="Lang2011">{{cite book |last=Lang |first=Kenneth R. |url=https://books.google.com/books?id=S4xDhVCxAQIC&pg=PA184 |title=The Cambridge Guide to the Solar System |publisher=Cambridge University Press |year=2011 |isbn=978-1139494175 |edition=2nd |archive-url=https://web.archive.org/web/20160101071141/https://books.google.com/books?id=S4xDhVCxAQIC&pg=PA184 |archive-date=1 January 2016}}</ref> additionally, Callisto's axial tilt varies between 0 and about 2 degrees on timescales of thousands of years.<ref name=galileantilt>{{cite journal |first=Bruce G. |last=Bills |title=Free and forced obliquities of the Galilean satellites of Jupiter |date=2005 |volume=175 |issue=1 |pages=233–247 |doi=10.1016/j.icarus.2004.10.028 |bibcode=2005Icar..175..233B |journal=Icarus |url=https://zenodo.org/record/1259023 |access-date=6 April 2023 |archive-date=27 July 2020 |archive-url=https://web.archive.org/web/20200727063125/https://zenodo.org/record/1259023 |url-status=live }}</ref>

==== Rotation ====
{{See also|Exoplanetology#Rotation and axial tilt}}
The planets rotate around invisible axes through their centres. A planet's [[rotation period]] is known as a [[day|stellar day]]. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the Sun's [[Poles of astronomical bodies#Geographic poles|north pole]]. The exceptions are Venus<ref>{{cite journal |title=Rotation of Venus: Period Estimated from Radar Measurements | last1=Goldstein | first1=R. M. | last2=Carpenter | first2=R. L. |date=1963 |journal =Science |volume=139 |doi=10.1126/science.139.3558.910 |pmid=17743054 |issue=3558 |bibcode=1963Sci...139..910G |pages=910–911|s2cid=21133097 }}</ref> and Uranus,<ref name="Belton-1984">{{cite conference |title=Rotational properties of Uranus and Neptune |first1=M. J. S. |last1=Belton | last2=Terrile | first2=R. J. |date=1984 |conference=Voyager "Uranus-Neptune" Workshop Pasadena 6–8 February 1984 |pages=327–347|bibcode=1984NASCP2330..327B |editor=Bergstralh, J. T.}}</ref> which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise.<ref>{{cite book |title=The Outer Worlds; Uranus, Neptune, Pluto, and Beyond |pages=195–206 |date=2006 |first=Michael P. |last=Borgia |publisher=Springer New York}}</ref> Regardless of which convention is used, Uranus has a [[retrograde rotation]] relative to its orbit.<ref name="Belton-1984" />

{{solar_system_bodies_rotation_animation.svg|upright}}

The rotation of a planet can be induced by several factors during formation. A net [[angular momentum]] can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets contributes to the angular momentum. Finally, during the last stages of planet building, a [[stochastic process]] of protoplanetary accretion can randomly alter the spin axis of the planet.<ref name="araa31">{{cite journal | title=Planet formation |last=Lissauer |first=Jack J. |journal=Annual Review of Astronomy and Astrophysics |volume=31 |pages=129–174 |date=September 1993 |doi=10.1146/annurev.aa.31.090193.001021 |bibcode=1993ARA&A..31..129L}}</ref> There is great variation in the length of day between the planets, with Venus taking 243&nbsp;[[Julian day|days]] to rotate, and the giant planets only a few hours.<ref name="planetcompare">{{cite web |title=Planet Compare |url=https://solarsystem.nasa.gov/planet-compare/ |url-status=live |archive-url=https://web.archive.org/web/20180309204400/https://solarsystem.nasa.gov/planet-compare/ |archive-date=9 March 2018 |access-date=12 July 2022 |website=Solar System Exploration |publisher=NASA}}</ref> The rotational periods of exoplanets are not known, but for [[hot Jupiter]]s, their proximity to their stars means that they are [[Tidal locking|tidally locked]] (that is, their orbits are in sync with their rotations). This means, they always show one face to their stars, with one side in perpetual day, the other in perpetual night.<ref>{{cite journal |title=Magnetically-Driven Planetary Radio Emissions and Application to Extrasolar Planets | last1=Zarka |first1=Philippe | last2=Treumann | first2=Rudolf A. | last3=Ryabov | first3=Boris P. | last4=Ryabov | first4=Vladimir B. |date=2001 |journal=Astrophysics and Space Science |volume=277 |issue=1/2 |pages=293–300 |doi = 10.1023/A:1012221527425|bibcode = 2001Ap&SS.277..293Z | s2cid=16842429 }}</ref> Mercury and Venus, the closest planets to the Sun, similarly exhibit very slow rotation: Mercury is tidally locked into a 3:2 spin–orbit resonance (rotating three times for every two revolutions around the Sun),<ref>{{cite journal |last1=Liu |first1=Han-Shou |last2=O'Keefe |first2=John A. |title=Theory of Rotation for the Planet Mercury |journal=Science |year=1965 |volume=150 |issue=3704 |page=1717 |doi=10.1126/science.150.3704.1717 |pmid=17768871 |bibcode=1965Sci...150.1717L|s2cid=45608770 }}</ref> and Venus's rotation may be in equilibrium between [[tidal force]]s slowing it down and [[atmospheric tide]]s created by solar heating speeding it up.<ref>{{cite journal |last1=Correia |first1=Alexandre C. M. |last2=Laskar |first2=Jacques |last3=De Surgy |first3=Olivier Néron |title=Long-Term Evolution of the Spin of Venus, Part I: Theory |journal=Icarus |volume=163 |issue=1 |pages=1–23 |date=May 2003 |url=http://www.imcce.fr/Equipes/ASD/preprints/prep.2002/venus1.2002.pdf |doi=10.1016/S0019-1035(03)00042-3 |bibcode=2003Icar..163....1C |access-date=9 September 2006 |archive-date=27 September 2019 |archive-url=https://web.archive.org/web/20190927122047/https://www.imcce.fr/Equipes/ASD/preprints/prep.2002/venus1.2002.pdf |url-status=live }}</ref><ref>{{cite journal |last1=Laskar |first1=Jacques |last2=De Surgy |first2=Olivier Néron |title=Long-Term Evolution of the Spin of Venus, Part II: Numerical Simulations |journal=Icarus |volume=163 |issue=1 |pages=24–45 |url=http://www.imcce.fr/Equipes/ASD/preprints/prep.2002/venus2.2002.pdf |doi=10.1016/S0019-1035(03)00043-5 |bibcode=2003Icar..163...24C |year=2003 |access-date=9 September 2006 |archive-date=2 May 2019 |archive-url=https://web.archive.org/web/20190502225637/https://www.imcce.fr/Equipes/ASD/preprints/prep.2002/venus2.2002.pdf |url-status=live }}</ref>

All the large moons are tidally locked to their parent planets;<ref>{{cite book|last1=Schutz|first1=Bernard|title=Gravity from the Ground Up|publisher=Cambridge University Press|isbn=978-0521455060|page=43|url=https://books.google.com/books?id=P_T0xxhDcsIC&pg=PA43|access-date=24 April 2017|date=2003|archive-date=6 August 2023|archive-url=https://web.archive.org/web/20230806164032/https://books.google.com/books?id=P_T0xxhDcsIC&pg=PA43|url-status=live}}</ref> Pluto and Charon are tidally locked to each other,<ref name="Young1997">{{cite web
| title = The Once and Future Pluto
| first = Leslie A.
| last = Young
| work = Southwest Research Institute, Boulder, Colorado
| url = http://www.boulder.swri.edu/~layoung/projects/talks03/IfA-jan03v1.ppt
| date = 1997
| access-date = 26 March 2007
| archive-date = 30 March 2004
| archive-url = https://web.archive.org/web/20040330212503/http://www.boulder.swri.edu/~layoung/projects/talks03/IfA-jan03v1.ppt
| url-status = live
}}</ref> as are Eris and Dysnomia,<ref name="Szakats2022">{{cite journal
  |display-authors = etal
  |first1 = R. |last1 = Szakáts
  |first2 = Cs. |last2 = Kiss
  |first3 = J. L. |last3 = Ortiz
  |first4 = N. |last4 = Morales
  |first5 = A. |last5 = Pál
  |first6 = T. G. |last6 = Müller
  |title      = Tidally locked rotation of the dwarf planet (136199) Eris discovered via long-term ground-based and space photometry
  |journal = Astronomy & Astrophysics |year = 2023 |volume = 669 |page = L3 |doi = 10.1051/0004-6361/202245234 |arxiv      = 2211.07987
|bibcode = 2023A&A...669L...3S |s2cid = 253522934 }}</ref> and probably {{dp|Orcus}} and its moon [[Vanth (moon)|Vanth]].<ref name="Brown2023"/> The other dwarf planets with known rotation periods rotate faster than Earth; Haumea rotates so fast that it has been distorted into a [[triaxial ellipsoid]].<ref name="Rabinowitz2005">
{{cite journal
| author = Rabinowitz, D. L.
| date = 2006
| title = Photometric Observations Constraining the Size, Shape, and Albedo of 2003&nbsp;EL<sub>61</sub>, a Rapidly Rotating, Pluto-Sized Object in the Kuiper Belt
| journal = [[Astrophysical Journal]]
| volume = 639
| issue = 2
| pages = 1238–1251
| doi = 10.1086/499575
| bibcode = 2006ApJ...639.1238R
| arxiv = astro-ph/0509401
| last2 = Barkume | first2 = Kristina
| last3 = Brown | first3 = Michael E.
| last4 = Roe | first4 = Henry
| last5 = Schwartz | first5 = Michael
| last6 = Tourtellotte | first6 = Suzanne
| last7 = Trujillo | first7 = Chad
| s2cid = 11484750
}}
</ref> The exoplanet [[Tau Boötis b]] and its parent star [[Tau Boötis]] appear to be mutually tidally locked.<ref>{{cite journal | title=Life on a tidally-locked planet | last=Singal | first=Ashok K. | journal=Planex Newsletter | volume=4 | issue=2 | page=8 | date=May 2014 | bibcode=2014arXiv1405.1025S | arxiv=1405.1025 }}</ref><ref>{{cite journal | title=MOST detects variability on tau Bootis possibly induced by its planetary companion | url=http://www.aanda.org/articles/aa/full/2008/17/aa8952-07/aa8952-07.html | last1=Walker | first1=G. A. H. | last2=Croll | first2=B. | last3=Matthews | first3=J. M. | last4=Kuschnig | first4=R. | last5=Huber | first5=D. | last6=Weiss | first6=W. W. | last7=Shkolnik | first7=E. | last8=Rucinski | first8=S. M. | last9=Guenther | first9=D. B. | display-authors=1 | year=2008 | journal=Astronomy and Astrophysics | volume=482 | issue=2 | pages=691–697 | doi=10.1051/0004-6361:20078952 | arxiv=0802.2732 | bibcode=2008A&A...482..691W | s2cid=56317105 | access-date=6 August 2022 | archive-date=25 February 2021 | archive-url=https://web.archive.org/web/20210225212508/https://www.aanda.org/articles/aa/full/2008/17/aa8952-07/aa8952-07.html | url-status=live }}</ref>

==== Orbital clearing ====
{{Main|Clearing the neighbourhood}}
The defining dynamic characteristic of a planet, according to the IAU definition, is that it has ''cleared its neighborhood''. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the [[planetesimal]]s in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. As described above, this characteristic was mandated as part of the [[International Astronomical Union|IAU]]'s official [[2006 definition of planet|definition of a planet]] in August 2006.<ref name="IAU" /> Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their [[circumstellar disc]]s.<ref>{{cite journal |title=The Total Number of Giant Planets in Debris Disks with Central Clearings |date=26 November 2007 | last1=Faber | first1=Peter | last2=Quillen | first2=Alice C. |journal=Monthly Notices of the Royal Astronomical Society |volume=382 |number=4 |pages=1823–1828 |doi=10.1111/j.1365-2966.2007.12490.x |doi-access=free |arxiv=0706.1684 |bibcode=2007MNRAS.382.1823F |s2cid=16610947 }}</ref>