Editing Planet (section)

=== 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>