Editing Planet (section)

== Attributes ==<!-- This section is linked from [[Earth radius]] -->
Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as [[ring system|rings]] or natural satellites, have only as yet been observed in planets in the Solar System, whereas others are commonly observed in exoplanets.<ref>{{Cite web |title=Extrasolar Planets |url=https://lasp.colorado.edu/outerplanets/exoplanets.php |access-date=13 May 2022 |website=lasp.colorado.edu |archive-date=5 April 2019 |archive-url=https://web.archive.org/web/20190405075609/https://lasp.colorado.edu/outerplanets/exoplanets.php |url-status=live }}</ref>

=== 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>

=== Physical characteristics ===

==== Size and shape ====
{{see also|Earth#Size and shape|Astronomical object#Shape|Planetary coordinate system}}
Gravity causes planets to be pulled into a roughly spherical shape, so a planet's size can be expressed roughly by an average radius (for example, [[Earth radius (unit)|Earth radius]] or [[Jupiter radius]]). However, planets are not perfectly spherical; for example, the [[Earth's rotation]] causes it to be slightly flattened at the poles with a [[equatorial bulge|bulge around the equator]].<ref name=milbert_smith96>{{cite web |last1=Milbert |first1=D. G. |last2=Smith |first2=D. A |url=http://www.ngs.noaa.gov/PUBS_LIB/gislis96.html |title=Converting GPS Height into NAVD88 Elevation with the GEOID96 Geoid Height Model |publisher=National Geodetic Survey, NOAA |access-date=7 March 2007 |archive-date=20 August 2011 |archive-url=https://web.archive.org/web/20110820090214/http://www.ngs.noaa.gov/PUBS_LIB/gislis96.html |url-status=live }}</ref> Therefore, a better approximation of Earth's shape is an [[oblate spheroid]], whose equatorial diameter is {{convert|43|km|mi|sp=us}} larger than the [[Geographical pole|pole]]-to-pole diameter.<ref name="ngdc2006">{{cite web |last1=Sandwell |first1=D. T. |last2=Smith |first2=Walter H. F. |author-link2=Walter H. F. Smith|date=7 July 2006 |url=http://www.ngdc.noaa.gov/mgg/bathymetry/predicted/explore.HTML |title=Exploring the Ocean Basins with Satellite Altimeter Data |publisher=NOAA/NGDC |access-date=21 April 2007|archive-url=https://archive.today/20140715142212/http://www.ngdc.noaa.gov/mgg/bathymetry/predicted/explore.HTML|archive-date=15 July 2014}}</ref> Generally, a planet's shape may be described by giving polar and equatorial radii of a [[spheroid]] or specifying a [[reference ellipsoid]]. From such a specification, the planet's flattening, surface area, and volume can be calculated; its [[normal gravity]] can be computed knowing its size, shape, rotation rate, and mass.<ref>{{Citation |last=Wieczorek |first=M. A. |title=10.05 – Gravity and Topography of the Terrestrial Planets |date=2015 |url=https://www.sciencedirect.com/science/article/pii/B978044453802400169X |work=Treatise on Geophysics |edition=2nd |pages=153–193 |editor-last=Schubert |editor-first=Gerald |place=Oxford |publisher=Elsevier |language=en |isbn=978-0-444-53803-1 |access-date=13 May 2022 |archive-date=13 May 2022 |archive-url=https://web.archive.org/web/20220513203935/https://www.sciencedirect.com/science/article/pii/B978044453802400169X |url-status=live }}</ref>

==== Mass ====
{{Main|Planetary mass}}
A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the [[electromagnetic force]]s binding its physical structure, leading to a state of [[hydrostatic equilibrium]]. This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere.<ref>{{cite web |title=The Dwarf Planets |url=http://www.gps.caltech.edu/~mbrown/dwarfplanets/ |author-link=Michael E. Brown |last=Brown |first=Michael E. |work=California Institute of Technology |date=2006 |access-date=1 February 2008 |archive-date=16 January 2011 |archive-url=https://web.archive.org/web/20110116181239/http://www.gps.caltech.edu/~mbrown/dwarfplanets/ |url-status=live }}</ref>

Mass is the prime attribute by which planets are distinguished from stars. No objects between the masses of the Sun and Jupiter exist in the Solar System, but there are exoplanets of this size. The lower [[stellar mass]] limit is estimated to be around 75 to 80 times that of Jupiter ({{Jupiter mass|link=yes}}). Some authors advocate that this be used as the upper limit for planethood, on the grounds that the internal physics of objects does not change between approximately one Saturn mass (beginning of significant self-compression) and the onset of hydrogen burning and becoming a [[red dwarf]] star.<ref name=ChenKipping/> Beyond roughly 13 {{Jupiter mass}} (at least for objects with solar-type [[isotopic abundance]]), an object achieves conditions suitable for [[nuclear fusion]] of [[deuterium]]: this has sometimes been advocated as a boundary,<ref name=exoworkdef/> even though deuterium burning does not last very long and most brown dwarfs have long since finished burning their deuterium.<ref name=Hatzes/> This is not universally agreed upon: the [[Extrasolar Planets Encyclopaedia|exoplanets Encyclopaedia]] includes objects up to 60 {{Jupiter mass}},<ref name=corot/> and the [[Exoplanet Data Explorer]] up to 24 {{Jupiter mass}}.<ref name=eod/>

The smallest known exoplanet with an accurately known mass is [[PSR B1257+12A]], one of the first exoplanets discovered, which was found in 1992 in orbit around a [[pulsar]]. Its mass is roughly half that of the planet Mercury.<ref name="konacki2003">{{cite journal | author=Konacki, M. | author2=Wolszczan, A. | title=Masses and Orbital Inclinations of Planets in the PSR B1257+12 System | journal=The Astrophysical Journal | volume=591 | issue=2 | pages=L147–L150 | date=2003 | doi=10.1086/377093 | bibcode=2003ApJ...591L.147K|arxiv = astro-ph/0305536 | s2cid=18649212 }}</ref> Even smaller is [[WD 1145+017 b]], orbiting a white dwarf; its mass is roughly that of the dwarf planet Haumea, and it is typically termed a minor planet.<ref>{{cite book |last=Veras |first=Dimitri |chapter=Planetary Systems Around White Dwarfs |date=2021 |url=https://oxfordre.com/planetaryscience/view/10.1093/acrefore/9780190647926.001.0001/acrefore-9780190647926-e-238 |title=Oxford Research Encyclopedia of Planetary Science |publisher=Oxford University Press |language=en |arxiv=2106.06550 |doi=10.1093/acrefore/9780190647926.013.238 |isbn=978-0-19-064792-6 |access-date=12 July 2022 |archive-date=6 June 2022 |archive-url=https://web.archive.org/web/20220606003104/https://oxfordre.com/planetaryscience/view/10.1093/acrefore/9780190647926.001.0001/acrefore-9780190647926-e-238 |url-status=live }}</ref> The smallest known planet orbiting a main-sequence star other than the Sun is [[Kepler-37b]], with a mass (and radius) that is probably slightly higher than that of the Moon.<ref name="Barclay-2013" /> The smallest object in the Solar System generally agreed to be a geophysical planet is Saturn's moon Mimas, with a radius about 3.1% of Earth's and a mass about 0.00063% of Earth's.<ref name="Jacobson2022">{{cite journal |last1=Jacobson |first1=Robert. A. |title=The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole* |journal=The Astronomical Journal |date=1 November 2022 |volume=164 |issue=5 |page=199 |doi=10.3847/1538-3881/ac90c9|bibcode=2022AJ....164..199J |s2cid=252992162 |doi-access=free }}</ref> Saturn's smaller moon [[Phoebe (moon)|Phoebe]], currently an irregular body of 1.7% Earth's radius<ref name=Thomas2010>{{cite journal| doi = 10.1016/j.icarus.2010.01.025| last1 = Thomas| first1 = P. C.| date = July 2010| title = Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission| journal = Icarus| volume = 208| issue = 1| pages = 395–401| url = http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| bibcode = 2010Icar..208..395T| access-date = 7 May 2023| archive-date = 23 December 2018| archive-url = https://web.archive.org/web/20181223003125/http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| url-status = dead}}</ref> and 0.00014% Earth's mass,<ref name="Jacobson2022"/> is thought to have attained hydrostatic equilibrium and differentiation early in its history before being battered out of shape by impacts.<ref name=planetlike>{{cite web |date=26 April 2012 |author=Jia-Rui C. Cook and Dwayne Brown |title=Cassini Finds Saturn Moon Has Planet-Like Qualities |url=http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20120426/ |publisher=JPL/NASA |archive-url=https://web.archive.org/web/20120427192715/http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20120426/ | archive-date=27 April 2012 |url-status=dead}}</ref> Some asteroids may be fragments of [[protoplanet]]s that began to accrete and differentiate, but suffered catastrophic collisions, leaving only a metallic or rocky core today,<ref name=Gaffey1984>{{cite journal
  |last=Gaffey |first=Michael
  |title=Rotational spectral variations of asteroid (8) Flora: Implications for the nature of the S-type asteroids and for the parent bodies of the ordinary chondrites
  |journal=Icarus
  |volume=60
  |issue=1 |pages=83–114 |date=1984
  |doi=10.1016/0019-1035(84)90140-4
  |bibcode=1984Icar...60...83G}}</ref><ref name=Hardersen-Gaffey-Abell-2005>
{{cite journal
 |first1=Paul S.     |last1=Hardersen
 |first2=Michael J.  |last2=Gaffey
 |first3=Paul A.     |last3=Abell
 |name-list-style=amp
 |year=2005
 |title=Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroid
 |journal=Icarus
 |volume=175 |issue=1 |page=141
 |bibcode=2005Icar..175..141H  |doi=10.1016/j.icarus.2004.10.017
}}
</ref><ref name=Asphaug-Reufer-2014>
{{cite journal
 |first1=E. |last1=Asphaug
 |first2=A. |last2=Reufer
 |year=2014
 |title=Mercury and other iron-rich planetary bodies as relics of inefficient accretion
 |journal=Nature Geoscience
 |volume=7  |issue=8  |pages=564–568
 |bibcode=2014NatGe...7..564A   |doi=10.1038/NGEO2189
}}</ref> or a reaccumulation of the resulting debris.<ref name=Yang2020/>

==== Internal differentiation ====
{{Main|Planetary differentiation}}
[[File:Jupiter interior.png|upright|thumb|Illustration of the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen]]

Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a [[Planetary differentiation|differentiated]] interior consisting of a dense [[planetary core]] surrounded by a [[Mantle (geology)|mantle]] that either is or was a [[fluid]]. The terrestrial planets' mantles are sealed within hard [[Crust (geology)|crusts]],<ref name="terrestrial">{{cite web |title=Planetary Interiors |work=Department of Physics, University of Oregon |url=http://abyss.uoregon.edu/~js/ast121/lectures/lec16.html |access-date=23 August 2008 |archive-date=8 August 2012 |archive-url=https://web.archive.org/web/20120808155809/http://abyss.uoregon.edu/~js/ast121/lectures/lec16.html |url-status=dead }}</ref> but in the giant planets the mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such as [[iron]] and [[nickel]] and mantles of [[silicate]]s. Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles of [[metallic hydrogen]].<ref>{{cite book | first=Linda T. |last=Elkins-Tanton |date=2006 |title=Jupiter and Saturn |publisher=Chelsea House |location=New York |isbn=978-0-8160-5196-0}}</ref> Uranus and Neptune, which are smaller, have rocky cores surrounded by mantles of water, [[ammonia]], [[methane]], and other [[Volatile (astrogeology)|ices]].<ref>{{cite journal| doi = 10.1016/0032-0633(95)00061-5| last1 = Podolak| first1 = M.| last2 = Weizman| first2 = A.| last3 = Marley| first3 = M.| date=December 1995 | title = Comparative models of Uranus and Neptune| journal = Planetary and Space Science| volume = 43| issue = 12| pages = 1517–1522| bibcode = 1995P&SS...43.1517P| ref = {{sfnRef|Podolak Weizman et al.|1995}}}}</ref> The fluid action within these planets' cores creates a [[geodynamo]] that generates a [[magnetic field]].<ref name="terrestrial" /> Similar differentiation processes are believed to have occurred on some of the large moons and dwarf planets,<ref name=Grundy2019/> though the process may not always have been completed: Ceres, Callisto, and Titan appear to be incompletely differentiated.<ref name="Neumann2015">{{Cite journal |last1=Neumann |first1=W. |last2=Breuer |first2=D. |last3=Spohn |first3=T. |date=2 December 2015 |title=Modelling the internal structure of Ceres: Coupling of accretion with compaction by creep and implications for the water-rock differentiation |url=http://www.aanda.org/articles/aa/pdf/2015/12/aa27083-15.pdf |url-status=live |journal=Astronomy & Astrophysics |volume=584 |page=A117 |bibcode=2015A&A...584A.117N |doi=10.1051/0004-6361/201527083 |archive-url=https://web.archive.org/web/20160822053141/http://www.aanda.org/articles/aa/pdf/2015/12/aa27083-15.pdf |archive-date=22 August 2016 |access-date=10 July 2016 |doi-access=free}}</ref><ref name=Monteux2014>{{cite journal |last1=Monteux |first1=J. |last2=Tobie |first2=G. |last3=Choblet |first3=G. |last4=Le Feuvre |first4=M. |title=Can large icy moons accrete undifferentiated? |journal=Icarus |year=2014 |volume=237 |pages=377–387 |doi=10.1016/j.icarus.2014.04.041 |bibcode=2014Icar..237..377M |s2cid=46172826 |url=https://hal.uca.fr/hal-01636068/file/Monteux-Icarus-V3-1-Final-2014.pdf |access-date=6 August 2022 |archive-date=9 October 2022 |archive-url=https://ghostarchive.org/archive/20221009/https://hal.uca.fr/hal-01636068/file/Monteux-Icarus-V3-1-Final-2014.pdf |url-status=live }}</ref> The asteroid Vesta, though not a dwarf planet because it was battered by impacts out of roundness, has a differentiated interior<ref name=Vestainterior>{{cite web|title=A look into Vesta's interior|url=https://www.mpg.de/877913/Vesta_asteroid|work=Max-Planck-Gesellschaft|date=6 January 2011|access-date=7 May 2023|archive-date=5 March 2023|archive-url=https://web.archive.org/web/20230305200352/https://www.mpg.de/877913/Vesta_asteroid|url-status=live}}</ref> similar to that of Venus, Earth, and Mars.<ref name=Asphaug-Reufer-2014/>

==== Atmosphere ====
{{Main|Atmosphere|extraterrestrial atmospheres}}
{{see also|Extraterrestrial skies}}
[[File:Top of Atmosphere.jpg|thumb|left|Earth's atmosphere]]
All of the Solar System planets [[Atmosphere of Mercury|except Mercury]]<ref>{{cite journal |last1=Zurbuchen |first1=Thomas H. |last2=Raines |first2=Jim M. |last3=Gloeckler |first3=George |last4=Krimigis |first4=Stamatios M. |last5=Slavin |first5=James A. |last6=Koehn |first6=Patrick L. |last7=Killen |first7=Rosemary M. |last8=Sprague |first8=Ann L. |last9=McNutt Jr. |first9=Ralph L. |last10=Solomon |first10=Sean C. |display-authors=4 |name-list-style=vanc |year=2008 |title=MESSENGER Observations of the Composition of Mercury's Ionized Exosphere and Plasma Environment |journal=Science |volume=321 |issue=5885 |pages=90–92 |bibcode=2008Sci...321...90Z |doi=10.1126/science.1159314 |pmid=18599777 |s2cid=206513512}}</ref> have substantial [[atmosphere]]s because their gravity is strong enough to keep gases close to the surface. Saturn's largest moon [[Titan (moon)|Titan]] also has a substantial atmosphere thicker than that of Earth;<ref>{{cite book|title=Titan: Exploring an Earthlike World|author1=Coustenis, Athéna|author2=Taylor, F. W.|name-list-style=amp|publisher=World Scientific|year=2008|page=130|url=https://books.google.com/books?id=j3O47dxrDAQC&q=Titan|access-date=25 March 2010|isbn=978-981-270-501-3|archive-date=14 December 2023|archive-url=https://web.archive.org/web/20231214142630/https://books.google.com/books?id=j3O47dxrDAQC&q=Titan#v=snippet&q=Titan&f=false|url-status=live}}</ref> Neptune's largest moon [[Triton (moon)|Triton]]<ref name="Solar System">{{cite web |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Nep_Triton |title=Neptune: Moons: Triton |work=Solar System Exploration |access-date=31 December 2007 |archive-url=https://web.archive.org/web/20080110095537/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Nep_Triton |archive-date=10 January 2008 }}</ref> and the dwarf planet [[Pluto]] have more tenuous atmospheres.<ref name=Lellouch_2015>{{cite journal
 |author1=Lellouch, E. |author2=de Bergh, C. |author3=Sicardy, B. |author4=Forget, F. |author5=Vangvichith, M. |author6=Käufl, H.-U. |title  =Exploring the spatial, temporal, and vertical distribution of methane in Pluto's atmosphere
 |journal=Icarus
 |date   =January 2015
 |doi    =10.1016/j.icarus.2014.03.027
 |bibcode=2015Icar..246..268L
 |arxiv  =1403.3208
 |volume=246 |pages=268–278
|s2cid=119194193 }}</ref> The larger giant planets are massive enough to keep large amounts of the light gases hydrogen and helium, whereas the smaller planets lose these gases into [[Interplanetary medium|space]].<ref>{{Cite journal | last1 = Sheppard | first1 = S. S. | last2 = Jewitt | first2 = D. | last3 = Kleyna | first3 = J. | title = An Ultradeep Survey for Irregular Satellites of Uranus: Limits to Completeness | doi = 10.1086/426329 | journal = The Astronomical Journal | volume = 129 | issue = 1 | pages = 518–525 | year = 2005 |arxiv = astro-ph/0410059 |bibcode = 2005AJ....129..518S | s2cid = 18688556 }}</ref> Analysis of exoplanets suggests that the threshold for being able to hold on to these light gases occurs at about {{val|2.0|0.7|0.6}} ''M''<sub>🜨</sub>, so that Earth and Venus are near the maximum size for rocky planets.<ref name=ChenKipping/>

The composition of Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular [[oxygen]].<ref name="zeilik">{{cite book | last1=Zeilik |first1=Michael A. |author2=Gregory, Stephan A. |title=Introductory Astronomy & Astrophysics |edition=4th |date=1998 |publisher=Saunders College Publishing |isbn=978-0-03-006228-5 |page=67}}</ref> The atmospheres of Mars and Venus are both dominated by [[carbon dioxide]], but differ drastically in density: the average surface pressure of [[Atmosphere of Mars|Mars's atmosphere]] is less than 1% that of Earth's (too low to allow liquid water to exist),<ref>{{Citation|last=Haberle|first=R. M.|title=Solar System/Sun, Atmospheres, Evolution of Atmospheres {{!}} Planetary Atmospheres: Mars|date=2015|encyclopedia=Encyclopedia of Atmospheric Sciences |edition=2nd|pages=168–177|editor-last=North|editor-first=Gerald R.|publisher=Academic Press|doi=10.1016/b978-0-12-382225-3.00312-1|isbn=978-0123822253|editor2-last=Pyle|editor2-first=John|editor3-last=Zhang|editor3-first=Fuqing}}</ref> while the average surface pressure of [[Atmosphere of Venus|Venus's atmosphere]] is about 92 times that of Earth's.<ref name=Basilevsky2003>{{cite journal |last=Basilevsky|first=Alexandr T.|author2=Head, James W.|title=The surface of Venus|journal=Rep. Prog. Phys.|date=2003|volume=66|issue=10|pages=1699–1734|doi=10.1088/0034-4885/66/10/R04 |bibcode= 2003RPPh...66.1699B|s2cid=250815558 }}</ref> It is likely that Venus's atmosphere was the result of a [[runaway greenhouse effect]] in its history, which today makes it the hottest planet by surface temperature, hotter even than Mercury.<ref>{{cite journal |author=S. I. Rasoonl|author2=C. de Bergh|name-list-style=amp |title=The Runaway Greenhouse Effect and the Accumulation of CO<sub>2</sub> in the Atmosphere of Venus |journal=Nature |volume=226 |pages=1037–1039 |date=1970 |pmid=16057644 |issue=5250 |doi=10.1038/2261037a0 |bibcode=1970Natur.226.1037R|s2cid=4201521}}</ref> Despite hostile surface conditions, temperature, and pressure at about 50–55&nbsp;km altitude in Venus's atmosphere are close to Earthlike conditions (the only place in the Solar System beyond Earth where this is so), and this region has been suggested as a plausible base for future [[Space exploration#Human spaceflight and habitation|human exploration]].<ref name="Badescu">{{cite book |author=Badescu, Viorel |editor=Zacny, Kris |title=Inner Solar System: Prospective Energy and Material Resources |url=https://www.springer.com/us/book/9783319195681 |location=Heidelberg |publisher=Springer-Verlag GmbH |page=492 |date=2015 |isbn=978-3319195681 |access-date=4 May 2023 |archive-date=21 August 2018 |archive-url=https://web.archive.org/web/20180821093729/https://www.springer.com/us/book/9783319195681 |url-status=live }}.<!--Based on ''Technica Molodezhi TM - 9 1971''--></ref> Titan has the only [[nitrogen]]-rich planetary atmosphere in the Solar System other than Earth's. Just as Earth's conditions are close to the [[triple point]] of water, allowing it to exist in all three states on the planet's surface, so Titan's are to the triple point of [[methane]].<ref>{{Cite journal|last=Horst|first=Sarah|date=2017|title=Titan's Atmosphere and Climate|journal=J. Geophys. Res. Planets|volume=122|issue=3|pages=432–482|doi=10.1002/2016JE005240|arxiv=1702.08611|bibcode=2017JGRE..122..432H|s2cid=119482985}}</ref>

Planetary atmospheres are affected by the varying [[insolation]] or internal energy, leading to the formation of dynamic [[weather system]]s such as [[hurricane]]s (on Earth), planet-wide [[dust storm]]s (on Mars), a greater-than-Earth-sized [[Anticyclonic storm|anticyclone]] on Jupiter (called the [[Great Red Spot]]), and [[Great Dark Spot|holes in the atmosphere]] (on Neptune).<ref name="Weather" /> Weather patterns detected on exoplanets include a hot region on [[HD 189733 b]] twice the size of the Great Red Spot,<ref name="knutson">{{cite journal | last1=Knutson |first1=Heather A. | last2=Charbonneau | first2=David | last3=Allen | first3=Lori E. |author3-link=Lori Allen (astronomer)| last4=Fortney | first4=Jonathan J. |title=A map of the day-night contrast of the extrasolar planet HD 189733 b |journal=Nature |date=2007 |volume=447 |doi=10.1038/nature05782 |pmid=17495920 |issue=7141 |bibcode=2007Natur.447..183K | pages=183–186|arxiv = 0705.0993|s2cid=4402268}}
* {{cite press release |date=9 May 2007 |title=First Map of an Extrasolar Planet |website=Center for Astrophysics |url=https://pweb.cfa.harvard.edu/news/first-map-extrasolar-planet |access-date=10 July 2022 |archive-date=5 December 2022 |archive-url=https://web.archive.org/web/20221205045421/https://pweb.cfa.harvard.edu/news/first-map-extrasolar-planet |url-status=live }}</ref> as well as [[cloud]]s on the hot Jupiter [[Kepler-7b]],<ref name="ArXiv-20130930">{{cite journal |last1=Demory |first1= Brice-Olivier |first2= Julien |last2= de Wit |first3= Nikole |last3= Lewis |first4= Jonathan |last4= Fortney |first5= Andras |last5= Zsom |first6= Sara |last6= Seager |display-authors=4 |year=2013 |title=Inference of Inhomogeneous Clouds in an Exoplanet Atmosphere |journal=The Astrophysical Journal Letters |volume=776 |issue=2 |page=L25 |arxiv=1309.7894 |bibcode=2013ApJ...776L..25D |doi=10.1088/2041-8205/776/2/L25 |s2cid=701011}}</ref> the super-Earth [[Gliese 1214 b]], and others.<ref name="NAT-20140101a">{{cite journal |last=Moses |first=Julianne |date=1 January 2014 |title=Extrasolar planets: Cloudy with a chance of dustballs |journal=[[Nature (journal)|Nature]] |volume=505 |issue=7481 |pages=31–32 |bibcode=2014Natur.505...31M |doi=10.1038/505031a |pmid=24380949|s2cid=4408861 }}</ref><ref>{{Cite journal |last1=Benneke |first1=Björn |last2=Wong |first2=Ian |last3=Piaulet |first3=Caroline |last4=Knutson |first4=Heather A. |last5=Lothringer |first5=Joshua |last6=Morley |first6=Caroline V. |last7=Crossfield |first7=Ian J. M. |last8=Gao |first8=Peter |last9=Greene |first9=Thomas P. |last10=Dressing |first10=Courtney |last11=Dragomir |first11=Diana |display-authors= 4 |date=10 December 2019 |title=Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b |journal=The Astrophysical Journal Letters |volume=887 |issue=1 |pages=L14 |doi=10.3847/2041-8213/ab59dc |arxiv=1909.04642 |bibcode=2019ApJ...887L..14B |s2cid=209324670 |issn=2041-8205 |doi-access=free }}</ref>

Hot Jupiters, due to their extreme proximities to their host stars, have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets.<ref>{{cite journal |journal=Nature |last1=Ballester |first1=Gilda E. |last2=Sing |first2=David K. |last3=Herbert |first3=Floyd |title=The signature of hot hydrogen in the atmosphere of the extrasolar planet HD 209458b |volume=445 |date=2007 |doi=10.1038/nature05525 |pmid=17268463 |issue=7127 |bibcode=2007Natur.445..511B |pages=511–514 |hdl=10871/16060 |s2cid=4391861 |url=https://ore.exeter.ac.uk/repository/bitstream/10871/16060/2/HD209458.nature.rev105.pdf |hdl-access=free |access-date=24 September 2019 |archive-date=28 July 2020 |archive-url=https://web.archive.org/web/20200728035216/https://repository/bitstream/handle/10871/16060/HD209458.nature.rev105.pdf;jsessionid=35C3149FC9764FBF9D4ADEA8F1DA25E4?sequence=2 |url-status=live }}
* {{cite press release |first1=Donna |last1=Weaver |first2=Ray |last2=Villard |url=https://hubblesite.org/contents/news-releases/2007/news-2007-07.html |title=Hubble Probes Layer-cake Structure of Alien World's Atmosphere |publisher=Space Telescope Science Institute |date=31 January 2007 |access-date=23 October 2011 |archive-date=9 July 2016 |archive-url=https://web.archive.org/web/20160709121743/http://hubblesite.org/newscenter/archive/releases/2007/07/full/ |url-status=live }}</ref><ref>{{Cite journal |last1=Villarreal D'Angelo |first1=Carolina |last2=Esquivel |first2=Alejandro |last3=Schneiter |first3=Matías |last4=Sgró |first4=Mario Agustín |date=21 September 2018 |title=Magnetized winds and their influence in the escaping upper atmosphere of HD 209458b |url=https://academic.oup.com/mnras/article/479/3/3115/5035846 |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=479 |issue=3 |pages=3115–3125 |doi=10.1093/mnras/sty1544 |doi-access=free |issn=0035-8711 |hdl=11336/86936 |hdl-access=free |access-date=10 July 2022 |archive-date=10 July 2022 |archive-url=https://web.archive.org/web/20220710233411/https://academic.oup.com/mnras/article/479/3/3115/5035846 |url-status=live }}</ref> These planets may have vast differences in temperature between their day and night sides that produce supersonic winds,<ref>{{cite journal | last1=Harrington |first1=Jason | last2=Hansen | first2=Brad M. | last3=Luszcz | first3=Statia H. | last4=Seager | first4=Sara |title=The phase-dependent infrared brightness of the extrasolar planet Andromeda b |journal=Science |volume=314 |date=2006 |doi=10.1126/science.1133904 |pmid=17038587 |issue=5799 |bibcode=2006Sci...314..623H | pages=623–626|arxiv = astro-ph/0610491 |s2cid=20549014}}
* {{cite press release |date=12 October 2006 |title=NASA's Spitzer Sees Day and Night on Exotic World |website=NASA |url=http://www.nasa.gov/vision/universe/starsgalaxies/spitzer-20061012.html |access-date=16 August 2007 |archive-date=13 July 2017 |archive-url=https://web.archive.org/web/20170713035307/https://www.nasa.gov/vision/universe/starsgalaxies/spitzer-20061012.html |url-status=dead }}</ref> although multiple factors are involved and the details of the atmospheric dynamics that affect the day-night temperature difference are complex.<ref>{{Cite journal |last1=Showman |first1=Adam P. |last2=Tan |first2=Xianyu |last3=Parmentier |first3=Vivien |date=December 2020 |title=Atmospheric Dynamics of Hot Giant Planets and Brown Dwarfs |url=http://link.springer.com/10.1007/s11214-020-00758-8 |journal=Space Science Reviews |language=en |volume=216 |issue=8 |page=139 |doi=10.1007/s11214-020-00758-8 |arxiv=2007.15363 |bibcode=2020SSRv..216..139S |s2cid=220870881 |issn=0038-6308 |access-date=10 July 2022 |archive-date=14 December 2023 |archive-url=https://web.archive.org/web/20231214142633/https://link.springer.com/article/10.1007/s11214-020-00758-8 |url-status=live }}</ref><ref>{{Cite journal |last1=Fortney |first1=Jonathan J. |last2=Dawson |first2=Rebekah I. |last3=Komacek |first3=Thaddeus D. |date=March 2021 |title=Hot Jupiters: Origins, Structure, Atmospheres |url=https://onlinelibrary.wiley.com/doi/10.1029/2020JE006629 |journal=Journal of Geophysical Research: Planets |language=en |volume=126 |issue=3 |doi=10.1029/2020JE006629 |arxiv=2102.05064 |bibcode=2021JGRE..12606629F |s2cid=231861632 |issn=2169-9097 |access-date=10 July 2022 |archive-date=14 December 2023 |archive-url=https://web.archive.org/web/20231214142634/https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JE006629 |url-status=live }}</ref>

==== Magnetosphere ====
{{Main|Magnetosphere}}
[[File:Structure_of_the_magnetosphere_LanguageSwitch.svg|lang=en|thumb|[[Earth's magnetic field|Earth's magnetosphere]] (diagram)]]

One important characteristic of the planets is their intrinsic [[magnetic moment]]s, which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of [[electrical conductivity|electrically conducting]] material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called the magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the [[ionosphere]] with the solar wind, which cannot effectively protect the planet.<ref name="Kivelson2007" />

Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field.<ref name="Kivelson2007" /> Of the magnetized planets, the magnetic field of Mercury is the weakest and is barely able to deflect the [[solar wind]]. Jupiter's moon [[Ganymede (moon)|Ganymede]] has a magnetic field several times stronger, and Jupiter's is the strongest in the Solar System (so intense in fact that it poses a serious health risk to future crewed missions to all its moons inward of Callisto<ref>{{Cite journal |last1=De Angelis |first1=G. |last2=Clowdsley |first2=M. S. |last3=Nealy |first3=J. E. |last4=Tripathi |first4=R. K. |last5=Wilson |first5=J. W. |display-authors=4 |date=January 2004 |title=Radiation analysis for manned missions to the Jupiter system |url=https://linkinghub.elsevier.com/retrieve/pii/S0273117704003205 |journal=Advances in Space Research |language=en |volume=34 |issue=6 |pages=1395–1403 |doi=10.1016/j.asr.2003.09.061 |pmid=15881781 |bibcode=2004AdSpR..34.1395D |access-date=13 July 2022 |archive-date=25 April 2022 |archive-url=https://web.archive.org/web/20220425152824/https://linkinghub.elsevier.com/retrieve/pii/S0273117704003205 |url-status=live }}</ref>). The magnetic fields of the other giant planets, measured at their surfaces, are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative to the planets' rotational [[Axis of rotation|axes]] and displaced from the planets' centres.<ref name="Kivelson2007">{{cite book |last1=Kivelson |first1=Margaret Galland | last2=Bagenal | first2=Fran |chapter=Planetary Magnetospheres |title=Encyclopedia of the Solar System |date=2007 |publisher=Academic Press |editor=Lucy-Ann McFadden |editor2=Paul Weissman |editor3=Torrence Johnson |isbn=978-0-12-088589-3 |page=[https://archive.org/details/encyclopediaofso0000unse_u6d1/page/519 519] |chapter-url=https://archive.org/details/encyclopediaofso0000unse_u6d1/page/519 }}</ref>

In 2003, a team of astronomers in Hawaii observing the star [[HD 179949]] detected a bright spot on its surface, apparently created by the magnetosphere of an orbiting hot Jupiter.<ref>{{cite web |last=Gefter |first=Amanda |date=17 January 2004 |title=Magnetic planet |url=https://astronomy.com/news-observing/news/2004/01/magnetic%20planet |access-date=29 January 2008 |work=Astronomy |archive-date=1 June 2019 |archive-url=https://web.archive.org/web/20190601224551/http://www.astronomy.com/news-observing/news/2004/01/magnetic%20planet |url-status=dead }}</ref><ref>{{Cite journal |last1=Shkolnik |first1=E. |last2=Walker |first2=G. A. H. |last3=Bohlender |first3=D. A. |date=10 November 2003 |title=Evidence for Planet-induced Chromospheric Activity on HD 179949 |url=https://iopscience.iop.org/article/10.1086/378583 |journal=The Astrophysical Journal |language=en |volume=597 |issue=2 |pages=1092–1096 |doi=10.1086/378583 |bibcode=2003ApJ...597.1092S |s2cid=15829056 |issn=0004-637X |access-date=10 July 2022 |archive-date=10 July 2022 |archive-url=https://web.archive.org/web/20220710171419/https://iopscience.iop.org/article/10.1086/378583 |url-status=live |arxiv=astro-ph/0303557 }}</ref>

===Secondary characteristics===
{{Main|Natural satellite|ring system}}
[[File:Voyager 2 - Saturn Rings - 3085 7800 2.png|thumb|upright|The [[rings of Saturn]]]]

Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in [[Orbital resonance|resonance]] with each other or with smaller bodies. This is common in satellite systems (e.g. the resonance between Io, [[Europa (moon)|Europa]], and Ganymede around Jupiter, or between Enceladus and Dione around Saturn). All except Mercury and Venus have [[natural satellite]]s, often called "moons". Earth has one, Mars has two, and the giant planets have numerous moons in complex planetary-type systems. Except for Ceres and Sedna, all the consensus dwarf planets are known to have at least one moon as well. Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa and Enceladus).<ref name="Grasset2000">{{cite journal | last1=Grasset |first1=O. | last2=Sotin | first2=C. | last3=Deschamps | first3=F. |title = On the internal structure and dynamic of Titan |date = 2000 |journal = Planetary and Space Science |volume = 48 | issue= 7–8 | pages = 617–636 |doi=10.1016/S0032-0633(00)00039-8 | bibcode=2000P&SS...48..617G}}</ref><ref name="Fortes2000">{{cite journal | journal = Icarus |volume= 146 |issue = 2 |pages = 444–452 |date= 2000 |doi = 10.1006/icar.2000.6400 |title = Exobiological implications of a possible ammonia-water ocean inside Titan | last=Fortes | first=A. D. |bibcode=2000Icar..146..444F}}</ref><ref>{{cite news |first=Nicola |last=Jones |date=11 December 2001 |work=New Scientist Print Edition |url=https://www.newscientist.com/article.ns?id=dn1647 |title=Bacterial explanation for Europa's rosy glow |access-date=23 August 2008 |archive-date=10 April 2008 |archive-url=https://web.archive.org/web/20080410053352/http://www.newscientist.com/article.ns?id=dn1647 }}</ref><ref name="Taubner et al. 2018">{{Cite journal| doi = 10.1038/s41467-018-02876-y| issn = 2041-1723| volume = 9| issue = 1| page = 748| last1 = Taubner| first1 = Ruth-Sophie| last2 = Pappenreiter| first2 = Patricia| last3 = Zwicker| first3 = Jennifer| last4 = Smrzka| first4 = Daniel| last5 = Pruckner| first5 = Christian| last6 = Kolar| first6 = Philipp| last7 = Bernacchi| first7 = Sébastien| last8 = Seifert| first8 = Arne H.| last9 = Krajete| first9 = Alexander| last10 = Bach| first10 = Wolfgang| last11 = Peckmann| first11 = Jörn| last12 = Paulik| first12 = Christian| last13 = Firneis| first13 = Maria G.| last14 = Schleper| first14 = Christa| last15 = Rittmann| first15 = Simon K.-M. R.| title = Biological methane production under putative Enceladus-like conditions| journal = Nature Communications| date = 27 February 2018| pmid = 29487311| pmc = 5829080| bibcode = 2018NatCo...9..748T}}</ref><ref name="Affholder et al. 2021">{{cite journal |author=Affholder, Antonin |display-authors=et al. |title=Bayesian analysis of Enceladus's plume data to assess methanogenesis |url=https://www.nature.com/articles/s41550-021-01372-6 |date=7 June 2021 |journal=[[Nature Astronomy]] |volume=5 |issue=8 |pages=805–814 |doi=10.1038/s41550-021-01372-6 |bibcode=2021NatAs...5..805A |s2cid=236220377 |access-date=7 July 2021 |archive-date=7 July 2021 |archive-url=https://web.archive.org/web/20210707121118/https://www.nature.com/articles/s41550-021-01372-6 |url-status=live }}</ref>

The four giant planets are orbited by [[planetary ring]]s of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny '[[Rings of Saturn#Propeller moonlets|moonlets]]' whose gravity shapes and maintains their structure. Although the origins of planetary rings are not precisely known, they are believed to be the result of natural satellites that fell below their parent planets' [[Roche limit]]s and were torn apart by [[tidal force]]s.<ref>{{cite journal | last1=Molnar | first1=L. A. | last2=Dunn | first2=D. E. |title=On the Formation of Planetary Rings |journal=Bulletin of the American Astronomical Society |date=1996 |volume=28 |pages=77–115 |bibcode=1996DPS....28.1815M }}</ref><ref>{{cite book | first=Encrenaz |last=Thérèse |author-link=Thérèse Encrenaz|date=2004 |title=The Solar System |edition=3rd |pages=388–390 |publisher=Springer |isbn=978-3-540-00241-3}}</ref> The dwarf planets Haumea<ref name="Ortiz2017">{{cite journal
| display-authors = etal
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}}</ref> and Quaoar also have rings.<ref name="Morgado2023">{{Cite Q|Q116754015|display-authors=1}}</ref>

No secondary characteristics have been observed around exoplanets. The [[sub-brown dwarf]] [[Cha 110913−773444]], which has been described as a [[rogue planet]], is believed to be orbited by a tiny [[protoplanetary disc]],<ref name="Luhman">{{cite journal | journal=Astrophysical Journal |last1=Luhman |first1=K. L. | last2=Adame | first2=Lucía | last3=D'Alessio | first3=Paola | last4=Calvet | first4=Nuria|author4-link=Nuria Calvet |title= Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk |volume=635 | issue=1 |page=L93 |doi=10.1086/498868 |date= 2005 |bibcode=2005ApJ...635L..93L|arxiv = astro-ph/0511807 |s2cid=11685964}}
* {{cite press release |author=Whitney Clavin |date=29 November 2005 |title=A Planet With Planets? Spitzer Finds Cosmic Oddball |website=NASA |url=http://www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html |access-date=10 September 2007 |archive-date=11 October 2012 |archive-url=https://web.archive.org/web/20121011011111/http://www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html |url-status=dead }}</ref> and the sub-brown dwarf [[OTS 44]] was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses.<ref name=joergens2013_AA558>{{cite journal|last1=Joergens|first1=V.|display-authors=4|last2=Bonnefoy|first2=M.|last3=Liu|first3=Y.|last4=Bayo|first4=A.|last5=Wolf|first5=S.|last6=Chauvin|first6=G.|last7=Rojo|first7=P.|title=OTS 44: Disk and accretion at the planetary border|journal=Astronomy & Astrophysics|volume=558|number=7|date=2013|doi=10.1051/0004-6361/201322432|bibcode=2013A&A...558L...7J|arxiv = 1310.1936|page=L7|s2cid=118456052}}</ref>