Editing Exoplanet (section)

== General features ==

=== Color and brightness ===
{{See also|Sudarsky's gas giant classification}}The apparent brightness ([[apparent magnitude]]) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with a high albedo that is far from the star.<ref>[http://phl.upr.edu/library/notes/theapparentbrightnessandsizeofexoplanetsandtheirstars The Apparent Brightness and Size of Exoplanets and their Stars] {{Webarchive|url=https://web.archive.org/web/20140812200814/http://phl.upr.edu/library/notes/theapparentbrightnessandsizeofexoplanetsandtheirstars|date=12 August 2014}}, Abel Mendez, updated 30 June 2012, 12:10&nbsp;pm</ref>[[File:Color HD 189733b vs solar system.jpg|thumb|upright=1.6|alt=Color-color diagram comparing the colors of Solar System planets to exoplanet HD 189733b. HD 189733b reflects as much green as Mars and almost as much blue as Earth.|This [[color–color diagram]] compares the colors of planets in the Solar System to exoplanet [[HD 189733b]]. The exoplanet's deep blue color is produced by [[silicate]] droplets, which scatter blue light in its atmosphere.]]
In 2013, the color of an exoplanet was determined for the first time. The best-fit [[albedo]] measurements of [[HD 189733b]] suggest that it is deep dark blue.<ref>{{Cite web|last=Garner|first=Rob|date=2016-10-31|title=NASA Hubble Finds a True Blue Planet|url=http://www.nasa.gov/content/nasa-hubble-finds-a-true-blue-planet|access-date=2022-01-17|website=NASA}}</ref><ref>{{Cite journal | doi = 10.1088/2041-8205/772/2/L16|arxiv=1307.3239| title = The Deep Blue Color of HD189733b: Albedo Measurements with Hubble Space Telescope/Space Telescope Imaging Spectrograph at Visible Wavelengths| journal = The Astrophysical Journal| volume = 772| issue = 2| pages = L16| year = 2013| last1 = Evans | first1 = T. M. | last2 = Pont | first2 = F. D. R. | last3 = Sing | first3 = D. K. | last4 = Aigrain | first4 = S.|author-link4=Suzanne Aigrain | last5 = Barstow | first5 = J. K. | last6 = Désert | first6 = J. M. | last7 = Gibson | first7 = N. | last8 = Heng | first8 = K. | last9 = Knutson | first9 = H. A. | last10 = Lecavelier Des Etangs | first10 = A. |bibcode=2013ApJ...772L..16E|s2cid=38344760}}</ref> Later that same year, the colors of several other exoplanets were determined, including [[GJ 504 b]] which visually has a magenta color,<ref name="Kuzuhara2013">{{Cite journal|arxiv=1307.2886|title=Direct Imaging of a Cold Jovian Exoplanet in Orbit around the Sun-like Star GJ 504|journal=The Astrophysical Journal|volume=774|issue=11|page=11|date=2013|display-authors=etal|doi=10.1088/0004-637X/774/1/11|bibcode = 2013ApJ...774...11K |last1=Kuzuhara|first1=M.|last2=Tamura|first2=M.|last3=Kudo|first3=T.|last4=Janson|first4=M.|last5=Kandori|first5=R.|last6=Brandt|first6=T. D.|last7=Thalmann|first7=C.|last8=Spiegel|first8=D.|last9=Biller|first9=B.|last10=Carson|first10=J.|last11=Hori|first11=Y.|last12=Suzuki|first12=R. |last13=Burrows |first13=Adam |last14=Henning|first14=T.|last15=Turner|first15=E. L.|last16=McElwain|first16=M. W.|last17=Moro-Martín|first17=A.|last18=Suenaga|first18=T.|last19=Takahashi|first19=Y. H.|last20=Kwon|first20=J.|last21=Lucas|first21=P.|last22=Abe|first22=L.|last23=Brandner|first23=W.|last24=Egner|first24=S.|last25=Feldt|first25=M.|last26=Fujiwara|first26=H.|last27=Goto|first27=M.|last28=Grady|first28=C. A.|last29=Guyon|first29=O.|last30=Hashimoto|first30=J.<!-- the rest: Y. Hayano, M. Hayashi, S. S. Hayashi, K. W. Hodapp, M. Ishii, M. Iye, G. R. Knapp, T. Matsuo, S. Mayama, S. Miyama, J.-I. Morino, J. Nishikawa, T. Nishimura, T. Kotani, N. Kusakabe, T. -S. Pyo, E. Serabyn, H. Suto, M. Takami, N. Takato, H. Terada, D. Tomono, M. Watanabe, J. P. Wisniewski, T. Yamada, H. Takami, T. Usuda -->|s2cid=53343537|url=https://pure.uva.nl/ws/files/2002826/150064_Direct_Imaging_of_a_Cold_Jovian_Exoplanet.pdf}}</ref> and [[Kappa Andromedae b]], which if seen up close would appear reddish in color.<ref name="arXiv1211.3744">{{cite journal|title=Direct Imaging Discovery of a 'Super-Jupiter' Around the late B-Type Star Kappa And|date=15 November 2012|arxiv=1211.3744|author1=Carson|author2=Thalmann|author3=Janson|author4=Kozakis|author5=Bonnefoy|author6=Biller|author7=Schlieder|author8=Currie|author9=McElwain|bibcode = 2013ApJ...763L..32C |doi = 10.1088/2041-8205/763/2/L32|volume=763|issue=2|journal=The Astrophysical Journal|pages=L32|s2cid=119253577}}</ref> [[Helium planet]]s are expected to be white or grey in appearance.<ref name=SpaceDaily-2015-06-16>{{cite news |url= http://www.spacedaily.com/reports/Helium_Shrouded_Planets_May_Be_Common_in_Our_Galaxy_999.html |title= Helium-Shrouded Planets May Be Common in Our Galaxy |publisher= SpaceDaily |date= 16 June 2015 |access-date=3 August 2015}}</ref>

The darkest known planet in terms of [[geometric albedo]] is [[TrES-2b]], a [[hot Jupiter]] that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres, but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.<ref name=darkest_news>{{cite web |url=http://www.space.com/12612-alien-planet-darkest-coal-black-kepler.html |title=Coal-Black Alien Planet Is Darkest Ever Seen |date=11 August 2011 |publisher=Space.com |access-date=12 August 2011}}</ref><ref>{{cite journal |arxiv=1108.2297 |bibcode=2011MNRAS.417L..88K |doi=10.1111/j.1745-3933.2011.01127.x |volume=417 |issue=1 |title=Detection of visible light from the darkest world |journal=Monthly Notices of the Royal Astronomical Society: Letters |pages=L88–L92 |year=2011 |last1=Kipping |first1=David M. |last2=Spiegel |first2=David S. |doi-access=free |s2cid=119287494}}</ref><ref>{{Cite journal |doi=10.1088/0004-637X/761/1/53 |arxiv=1210.4592 |title=Photometrically derived masses and radii of the planet and star in the TrES-2 system |journal=The Astrophysical Journal |volume=761 |issue=1 |page=53 |year=2012 |last1=Barclay |first1=T. |last2=Huber |first2=D. |last3=Rowe |first3=J. F. |last4=Fortney |first4=J. J. |last5=Morley |first5=C. V. |last6=Quintana |first6=E. V. |last7=Fabrycky |first7=D. C. |last8=Barentsen |first8=G. |last9=Bloemen |first9=S. |last10=Christiansen |first10=J. L. |last11=Demory |first11=B. O. |last12=Fulton |first12=B. J. |last13=Jenkins |first13=J. M. |last14=Mullally |first14=F. |last15=Ragozzine |first15=D. |last16=Seader |first16=S. E. |last17=Shporer |first17=A. |last18=Tenenbaum |first18=P. |last19=Thompson |first19=S. E. |bibcode=2012ApJ...761...53B |s2cid=18216065}}</ref>

For [[gas giant]]s, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.<ref name="wfirstreturn">{{cite arXiv|eprint=1412.6097 |last1=Burrows |first1=Adam |title=Scientific Return of Coronagraphic Exoplanet Imaging and Spectroscopy Using WFIRST |class=astro-ph.EP |year=2014 }}</ref>

There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully [[Planetary phase|phase]]-dependent, this is not always the case in the near infrared.<ref name="wfirstreturn" />

Temperatures of gas giants reduce over time and with distance from their stars. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures, ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.<ref name="wfirstreturn" />

=== Magnetic field ===

In 2014, a magnetic field around [[HD 209458 b]] was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one-tenth as strong as Jupiter's.<ref>{{Cite web|author1=Charles Q. Choi|date=2014-11-20|title=Unlocking the Secrets of an Alien World's Magnetic Field|url=https://www.space.com/27828-alien-planet-magnetic-field-strength.html|access-date=2022-01-17|website=Space.com|language=en}}</ref><ref>{{Cite journal|doi=10.1126/science.1257829|pmid=25414310 |title=Magnetic moment and plasma environment of HD 209458b as determined from Ly observations |journal=Science |volume=346 |issue=6212 |pages=981–984 |year=2014 |last1=Kislyakova |first1=K. G.|last2=Holmstrom |first2=M. |last3=Lammer |first3=H. |last4=Odert |first4=P. |last5=Khodachenko |first5=M. L. |bibcode=2014Sci...346..981K |arxiv = 1411.6875 |s2cid=206560188}}</ref>

The magnetic fields of exoplanets are thought to be detectable by their [[Aurora (astronomy)|auroral]] [[radio]] emissions with sensitive low-frequency radio telescopes such as [[Low-Frequency Array (LOFAR)|LOFAR]], although they have yet to be found.<ref>{{Cite journal | doi = 10.1111/j.1365-2966.2011.18528.x|arxiv=1102.2737| title = Magnetosphere-ionosphere coupling at Jupiter-like exoplanets with internal plasma sources: Implications for detectability of auroral radio emissions| journal = Monthly Notices of the Royal Astronomical Society| volume = 414| issue = 3| pages = 2125–2138| year = 2011| last1 = Nichols | first1 = J. D.|doi-access=free |bibcode=2011MNRAS.414.2125N|s2cid=56567587}}</ref><ref>{{Cite web|date=2011-04-18|title=Radio Telescopes Could Help Find Exoplanets|url=https://www.redorbit.com/news/space/2031221/radio_telescopes_could_help_find_exoplanets/|access-date=2022-01-17|website=Redorbit|language=en-US}}</ref> The radio emissions could measure the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.<ref>{{cite web|url=http://www.ece.vt.edu/swe/lwa/memo/lwa0013.pdf|title=Radio Detection of Extrasolar Planets: Present and Future Prospects|work=NRL, NASA/GSFC, NRAO, Observatoìre de Paris|access-date=15 October 2008|archive-date=30 October 2008|archive-url=https://web.archive.org/web/20081030022342/http://www.ece.vt.edu/swe/lwa/memo/lwa0013.pdf|url-status=dead}}</ref> However, the most sensitive radio search for [[Aurora|auroral]] emissions, thus far, from nine exoplanets with Arecibo also did not result in any discoveries.<ref>{{cite journal|last1=Route|first1=Matthew|title=ROME. IV. An Arecibo Search for Substellar Magnetospheric Radio Emissions in Purported Exoplanet-hosting Systems at 5 GHz|journal=The Astrophysical Journal|date=1 May 2024|volume=966|issue=1|page=55|doi=10.3847/1538-4357/ad30ff|arxiv=2403.02226|bibcode=2024ApJ...966...55R|doi-access=free }}</ref>

[[Earth's magnetic field]] results from its flowing liquid metallic core, but on massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures, which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as {{chem2|MgSi3O12}} could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.<ref name="Kean">{{cite journal|last1=Kean|first1=Sam|title=Forbidden plants, forbidden chemistry|journal=Distillations|date=2016|volume=2|issue=2|page=5|url=https://www.sciencehistory.org/distillations/magazine/forbidden-planet-forbidden-chemistry|access-date=22 March 2018|archive-date=23 March 2018|archive-url=https://web.archive.org/web/20180323154914/https://www.sciencehistory.org/distillations/magazine/forbidden-planet-forbidden-chemistry|url-status=dead}}</ref><ref>{{Cite web |author1=Choi |first=Charles Q. |date=2012-11-22 |title=Super-Earths Get Magnetic 'Shield' from Liquid Metal |url=https://www.space.com/18604-super-earth-planets-liquid-metal.html |access-date=2022-01-17 |website=Space.com |language=en}}</ref>

[[Hot Jupiter]]s have been observed to have a larger radius than expected. This could be caused by the interaction between the [[stellar wind]] and the planet's [[magnetosphere]] creating an electric current through the planet that heats it up ([[Joule heating]]) causing it to expand. The more magnetically active a star is, the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.<ref>{{Cite journal | doi = 10.1088/2041-8205/765/2/L25| title = Stellar Magnetic Fields As a Heating Source for Extrasolar Giant Planets| journal = The Astrophysical Journal| volume = 765| issue = 2| pages = L25| year = 2013| last1 = Buzasi | first1 = D.|arxiv = 1302.1466 |bibcode = 2013ApJ...765L..25B | s2cid = 118978422}}</ref>

In August 2018, scientists announced the transformation of gaseous [[deuterium]] into a liquid [[metallic hydrogen]] form. This may help researchers better understand [[Gas giant|giant gas planets]], such as [[Jupiter]], [[Saturn]] and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful [[magnetic field]]s.<ref name="NYT-20180816">{{cite news |last=Chang |first=Kenneth |title=Settling Arguments About Hydrogen With 168 Giant Lasers – Scientists at Lawrence Livermore National Laboratory said they were "converging on the truth" in an experiment to understand hydrogen in its liquid metallic state. |url=https://www.nytimes.com/2018/08/16/science/metallic-hydrogen-lasers.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2018/08/16/science/metallic-hydrogen-lasers.html |archive-date=2022-01-01 |url-access=limited |date=16 August 2018 |work=The New York Times |access-date=18 August 2018}}{{cbignore}}</ref><ref name="SCI-20180816">{{cite journal |author=Staff |title=Under pressure, hydrogen offers a reflection of giant planet interiors – Hydrogen is the most-abundant element in the universe and the simplest, but that simplicity is deceptive |url=https://www.sciencedaily.com/releases/2018/08/180816143205.htm |date=16 August 2018 |journal=[[Science Daily]] |access-date=18 August 2018}}</ref>

Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased [[stellar flare]]s and starspots on their host stars, in 2019 this claim was demonstrated to be false in the [[HD 189733]] system. The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.<ref>{{cite journal|last1=Route|first1=Matthew|title=The Rise of ROME. I. A Multiwavelength Analysis of the Star-Planet Interaction in the HD 189733 System|journal=The Astrophysical Journal|date=10 February 2019|volume=872|issue=1|page=79|doi=10.3847/1538-4357/aafc25|arxiv=1901.02048|bibcode=2019ApJ...872...79R|s2cid=119350145 |doi-access=free }}</ref> A later search for radio emissions from eight exoplanets that orbit within 0.1 [[astronomical unit|astronomical units]] of their host stars, conducted by the [[Arecibo_Telescope|Arecibo radio telescope]] also failed to find signs of these magnetic star-planet interactions.<ref>{{cite journal|last1=Route|first1=Matthew|last2=Wolszczan|first2=Alex|title=ROME. III. The Arecibo Search for Star–Planet Interactions at 5 GHz|journal=The Astrophysical Journal|date=1 August 2023|volume=952|issue=2|page=118|doi=10.3847/1538-4357/acd9ad|arxiv=2202.08899|bibcode=2023ApJ...952..118R|doi-access=free }}</ref>

In 2019, the strength of the surface magnetic fields of 4 [[hot Jupiter]]s were estimated and ranged between 20 and 120 [[Gauss (unit)|gauss]] compared to Jupiter's surface magnetic field of 4.3 gauss.<ref>{{Cite web |author1=Rabie |first=Passant |date=2019-07-29 |title=Magnetic Fields of 'Hot Jupiter' Exoplanets Are Much Stronger Than We Thought |url=https://www.space.com/hot-jupiter-magnetic-fields-measured-for-first-time.html |access-date=2022-01-17 |website=Space.com |language=en}}</ref><ref>{{Cite journal|last1=Cauley|first1=P. Wilson|last2=Shkolnik|first2=Evgenya L.|last3=Llama|first3=Joe|last4=Lanza|first4=Antonino F.|date=Dec 2019|title=Magnetic field strengths of hot Jupiters from signals of star-planet interactions|journal=Nature Astronomy|volume=3|issue=12|pages=1128–1134|doi=10.1038/s41550-019-0840-x|arxiv=1907.09068|bibcode=2019NatAs...3.1128C|s2cid=198147426|issn=2397-3366}}</ref>

=== Plate tectonics ===

In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of [[plate tectonics]] on larger [[super-Earth]]s<ref>{{cite journal |doi=10.1016/j.epsl.2009.07.015 |title=Convection scaling and subduction on Earth and super-Earths |date=2009 |last1=Valencia |first1=Diana |last2=O'Connell |first2=Richard J.|journal=Earth and Planetary Science Letters |volume=286 |issue=3–4 |pages=492–502 |bibcode=2009E&PSL.286..492V}}</ref><ref>{{cite journal |doi=10.1016/j.epsl.2011.07.029|title=Plate tectonics on super-Earths: Equally or more likely than on Earth|date=2011 |last1=Van Heck |first1=H.J. |last2=Tackley |first2=P.J. |journal=Earth and Planetary Science Letters |volume=310 |issue=3–4 |pages=252–261 |bibcode=2011E&PSL.310..252V}}</ref> with one team saying that plate tectonics would be episodic or stagnant<ref>{{cite journal |doi=10.1029/2007GL030598 |title=Geological consequences of super-sized Earths|date=2007 |last1=O'Neill |first1=C. |last2=Lenardic |first2=A. |s2cid=41617531|journal=Geophysical Research Letters |volume=34|issue=19|pages=L19204 |bibcode=2007GeoRL..3419204O|doi-access=free }}</ref> and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.<ref>{{Cite journal |first1=Diana |last1=Valencia |first2=Richard J.|last2=O'Connell |first3=Dimitar D |last3=Sasselov |date=November 2007 |title=Inevitability of Plate Tectonics on Super-Earths|journal=Astrophysical Journal Letters |volume=670 |issue=1 |pages=L45–L48 |doi=10.1086/524012 |arxiv=0710.0699|bibcode=2007ApJ...670L..45V|s2cid=9432267}}</ref>

If super-Earths have more than 80 times as much water as Earth, then they become [[ocean planet]]s with all land completely submerged. However, if there is less water than this limit, then the deep water cycle would move enough water between the oceans and mantle to allow continents to exist.<ref>{{Cite web|title=Super Earths Likely To Have Both Oceans and Continents – Astrobiology|url=http://astrobiology.com/2014/01/super-earths-likely-to-have-both-oceans-and-continents.html|access-date=2022-01-17|website=astrobiology.com|date=7 January 2014 }}</ref><ref>{{Cite journal |doi=10.1088/0004-637X/781/1/27 |title=Water Cycling Between Ocean and Mantle: Super-Earths Need Not Be Waterworlds |journal=The Astrophysical Journal |volume = 781| issue=1 |page = 27 |year=2014 |last1=Cowan |first1=N. B. |last2=Abbot |first2=D. S. |bibcode=2014ApJ...781...27C |arxiv=1401.0720 |s2cid=56272100}}</ref>

=== Volcanism ===
{{update section|date=August 2024}}
Large surface temperature variations on [[55 Cancri e]] have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.<ref>{{cite magazine |author=Lemonick |first=Michael D. |date=6 May 2015 |title=Astronomers May Have Found Volcanoes 40 Light-Years From Earth |url=http://news.nationalgeographic.com/2015/05/150506-volcano-planet-space-cancri-astronomy/ |url-status=dead |archive-url=https://web.archive.org/web/20150509051828/http://news.nationalgeographic.com/2015/05/150506-volcano-planet-space-cancri-astronomy/ |archive-date=9 May 2015 |access-date=8 November 2015 |magazine=National Geographic}}</ref><ref>{{cite journal |arxiv=1505.00269 |bibcode=2016MNRAS.455.2018D |doi=10.1093/mnras/stv2239 |volume=455 |issue=2 |title=Variability in the super-Earth 55 Cnc e |journal=Monthly Notices of the Royal Astronomical Society |pages=2018–2027 |year=2015 |last1=Demory |first1=Brice-Olivier |last2=Gillon |first2=Michael |last3=Madhusudhan |first3=Nikku |last4=Queloz |first4=Didier |doi-access=free |s2cid=53662519}}</ref>

=== Rings ===
In 2007, the star [[V1400 Centauri]] was occulted by an object (either a planet or brown dwarf) surrounded by an extensive disc of debris. The object, designated [[J1407b]], was long believed to host a vast planetary [[ring system]] much larger than [[Saturn's rings]].<ref>{{Cite web|title=Scientists Discover a Saturn-like Ring System Eclipsing a Sun-like Star|url=https://www.spacedaily.com/reports/Scientists_Discover_a_Saturn_like_Ring_System_Eclipsing_a_Sun_like_Star_999.html|access-date=2022-01-17|website=www.spacedaily.com}}</ref><ref>{{Cite journal | doi = 10.1088/0004-6256/143/3/72| title = Planetary Construction Zones in Occultation: Discovery of an Extrasolar Ring System Transiting a Young Sun-Like Star and Future Prospects for Detecting Eclipses by Circumsecondary and Circumplanetary Disks| journal = The Astronomical Journal| volume = 143| issue = 3| page = 72| year = 2012| last1 = Mamajek | first1 = E. E. | last2 = Quillen | first2 = A. C. | last3 = Pecaut | first3 = M. J. | last4 = Moolekamp | first4 = F. | last5 = Scott | first5 = E. L. | last6 = Kenworthy | first6 = M. A. | last7 = Cameron | first7 = A. C. | last8 = Parley | first8 = N. R. | bibcode=2012AJ....143...72M|arxiv = 1108.4070 | s2cid = 55818711}}</ref> Follow-up observations found the supposed ring system could instead be a circumplanetary disk.<ref>{{cite journal |last1=Rieder |first1=Steven |last2=Kenworthy |first2=Matthew A. |date=November 2016 |title=Constraints on the size and dynamics of the J1407b ring system |journal=Astronomy & Astrophysics |volume=596 |pages=5 |arxiv=1609.08485 |bibcode=2016A&A...596A...9R |doi=10.1051/0004-6361/201629567 |s2cid=118413749 |id=A9 |doi-access=free}}</ref><ref>{{cite journal |last1=Kenworthy |first1=M. A. |last2=Klaasen |first2=P. D. |last3=Min |first3=M. |last4=van der Marel |first4=N. |last5=Bohn |first5=A. J. |last6=Kama |first6=M. |display-authors=etal |date=January 2020 |title=ALMA and NACO observations towards the young exoring transit system J1407 (V1400 Cen) |journal=Astronomy & Astrophysics |volume=633 |pages=6 |arxiv=1912.03314 |bibcode=2020A&A...633A.115K |doi=10.1051/0004-6361/201936141 |id=A115 |doi-access=free}}</ref>

There is strong evidence of a ring system around [[HIP 41378 f]], given the planet's measured radius is too large for its mass, the radius measurement might have been affected by a ring system around the planet.<ref name="Saillenfest2023">{{Cite journal |last1=Saillenfest |first1=M. |last2=Sulis |first2=S. |last3=Charpentier |first3=P. |last4=Santerne |first4=A. |year=2023 |title=Oblique rings from migrating exomoons: A possible origin for long-period exoplanets with enlarged radii |journal=Astronomy and Astrophysics |volume=675 |doi=10.1051/0004-6361/20234674 |doi-broken-date=1 November 2024}}</ref><ref>{{cite journal |last1=Alam |first1=Munazza K. |last2=Kirk |first2=James |last3=Dressing |first3=Courtney D. |last4=López-Morales |first4=Mercedes |last5=Ohno |first5=Kazumasa |last6=Gao |first6=Peter |last7=Akinsanmi |first7=Babatunde |last8=Santerne |first8=Alexandre |last9=Grouffal |first9=Salomé |last10=Adibekyan |first10=Vardan |last11=Barros |first11=Susana C. C. |last12=Buchhave |first12=Lars A. |last13=Crossfield |first13=Ian J. M. |last14=Dai |first14=Fei |last15=Deleuil |first15=Magali |year=2022 |title=The First Near-infrared Transmission Spectrum of HIP 41378 f, A Low-mass Temperate Jovian World in a Multiplanet System |journal=The Astrophysical Journal Letters |volume=927 |issue=1 |pages=L5 |arxiv=2201.02686 |bibcode=2022ApJ...927L...5A |doi=10.3847/2041-8213/ac559d |s2cid=245837282 |doi-access=free |last16=Giacalone |first16=Steven |last17=Lillo-Box |first17=Jorge |last18=Marley |first18=Mark |last19=Mayo |first19=Andrew W. |last20=Mortier |first20=Annelies |last21=Santos |first21=Nuno C. |last22=Sousa |first22=Sérgio G. |last23=Turtelboom |first23=Emma V. |last24=Wheatley |first24=Peter J. |last25=Vanderburg |first25=Andrew M.}}</ref>

The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a [[axial tilt|tilted rotational axis]], then the different alignments between the inner and outer rings would create a warped ring system.<ref>{{cite journal |arxiv=1104.3863|bibcode = 2011ApJ...734..117S |doi = 10.1088/0004-637X/734/2/117 | volume=734 |issue = 2 |title=Warm Saturns: On the Nature of Rings around Extrasolar Planets That Reside inside the Ice Line |journal=The Astrophysical Journal |page=117|year = 2011 |last1 = Schlichting |first1 = Hilke E. |last2 = Chang |first2=Philip |s2cid=42698264}}</ref>

=== Moons ===
{{Main|Exomoon}}There is evidence that moons around other planets, commonly referred to exomoons, may exist. None has been confirmed so far.

In 2012 a candidate exomoon was detected around WASP-12b via periodic light variations in the planet's [[light curve]].<ref>[http://www.ria.ru/science/20120206/558647431.html Российские астрономы впервые открыли луну возле экзопланеты] (in Russian) - "Studying of a curve of change of shine of WASP-12b has brought to the Russian astronomers unusual result: regular splashes were found out.<...> Though stains on a star surface also can cause similar changes of shine, observable splashes are very similar on duration, a profile and amplitude that testifies for benefit of exomoon existence."</ref> Subsequent observations found this object might actually be a [[trojan planet]].<ref>{{citation |last1=Kislyakova |first1=K. G. |title=On the ultraviolet anomalies of the WASP-12 and HD 189733 systems: Trojan satellites as a plasma source |journal=Monthly Notices of the Royal Astronomical Society |volume=461 |issue=1 |pages=988–999 |year=2016 |arxiv=1605.02507 |bibcode=2016MNRAS.461..988K |doi=10.1093/mnras/stw1110 |s2cid=119205132 |last2=Pilat-Lohinger |first2=E. |last3=Funk |first3=B. |last4=Lammer |first4=H. |last5=Fossati |first5=L. |last6=Eggl |first6=S. |last7=Schwarz |first7=R. |last8=Boudjada |first8=M. Y. |last9=Erkaev |first9=N. V. |doi-access=free}}</ref>

In December 2013, a candidate exomoon was detected in the microlensing event [[MOA-2011-BLG-262L|MOA-2011-BLG-262]], it was believed to be either a {{Earth mass|0.5|link=y}} exomoon around a Jupiter-sized free-floating planet or a Neptune-mass planet around a red dwarf,<ref>{{Cite journal | doi = 10.1088/0004-637X/785/2/155|arxiv=1312.3951| title = MOA-2011-BLG-262Lb: A sub-Earth-mass moon orbiting a gas giant or a high-velocity planetary system in the galactic bulge| journal = The Astrophysical Journal| volume = 785| issue = 2| page = 155| year = 2014| last1 = Bennett | first1 = D. P.| last2 = Batista | first2 = V.| last3 = Bond | first3 = I. A.| last4 = Bennett | first4 = C. S.| last5 = Suzuki | first5 = D.| last6 = Beaulieu | first6 = J. -P. | last7 = Udalski | first7 = A.| last8 = Donatowicz | first8 = J.| last9 = Bozza | first9 = V.| last10 = Abe | first10 = F.| last11 = Botzler | first11 = C. S.| last12 = Freeman | first12 = M.| last13 = Fukunaga | first13 = D.| last14 = Fukui | first14 = A.| last15 = Itow | first15 = Y.| last16 = Koshimoto | first16 = N.| last17 = Ling | first17 = C. H.| last18 = Masuda | first18 = K.| last19 = Matsubara | first19 = Y.| last20 = Muraki | first20 = Y.| last21 = Namba | first21 = S.| last22 = Ohnishi | first22 = K.| last23 = Rattenbury | first23 = N. J.| last24 = Saito | first24 = T. | last25 = Sullivan | first25 = D. J.| last26 = Sumi | first26 = T.| last27 = Sweatman | first27 = W. L.| last28 = Tristram | first28 = P. J.| last29 = Tsurumi | first29 = N.| last30 = Wada | first30 = K.| display-authors = etal|bibcode=2014ApJ...785..155B|s2cid=118327512}}</ref> but follow-up observations confirmed the latter scenario.<ref name="Terry2024">{{Cite journal |arxiv=2410.09147 |first1=Sean K. |last1=Terry |first2=Jean-Philippe |last2=Beaulieu |title=A Candidate High-velocity Exoplanet System in the Galactic Bulge |date=2025 |last3=Bennett |first3=David P. |last4=Bhattacharya |first4=Aparna |last5=Hulberg |first5=Jon |last6=Huston |first6=Macy J. |last7=Koshimoto |first7=Naoki |last8=Blackman |first8=Joshua W. |last9=Bond |first9=Ian A.|journal=The Astronomical Journal |volume=169 |issue=3 |page=131 |doi=10.3847/1538-3881/ad9b0f |doi-access=free |bibcode=2025AJ....169..131T }}</ref> 

On 3 October 2018, evidence suggesting a large exomoon orbiting [[Kepler-1625b]] was reported,<ref>{{Cite journal|last1=Teachey|first1=Alex|last2=Kipping|first2=David M.|date=1 October 2018|title=Evidence for a large exomoon orbiting Kepler-1625b|journal=Science Advances|language=en|volume=4|issue=10|pages=eaav1784|doi=10.1126/sciadv.aav1784|pmid=30306135|pmc=6170104|issn=2375-2548|bibcode=2018SciA....4.1784T|arxiv=1810.02362}}</ref> and in 2021 evidence of an exomoon around Kepler-1708b was also reported.<ref>{{Cite news |title=Scientists think they've found a big, weird moon in a far-off star system |url=https://www.npr.org/2022/01/13/1072570125/kepler-large-exomoon |access-date=2022-03-28 |work=NPR.org |language=en}}</ref> Their existence, however, remain doubtful,<ref>{{cite journal |last1=Heller |first1=René |last2=Hippke |first2=Michael |date=2024 |title=Large exomoons unlikely around Kepler-1625 b and Kepler-1708 B |url=https://ui.adsabs.harvard.edu/abs/2024NatAs...8..193H/abstract |journal=Nature Astronomy |volume=8 |issue=2 |page=193 |arxiv=2312.03786 |bibcode=2024NatAs...8..193H |doi=10.1038/s41550-023-02148-w}}</ref> but follow-up observations may confirm these exomoons.<ref>{{Cite arXiv |eprint=2401.10333 |first1=David |last1=Kipping |first2=Alex |last2=Teachey |title=A Reply to: Large Exomoons unlikely around Kepler-1625 b and Kepler-1708 b |date=2024-01-18 |last3=Yahalomi |first3=Daniel A. |last4=Cassese |first4=Ben |last5=Quarles |first5=Billy |last6=Bryson |first6=Steve |last7=Hansen |first7=Brad |last8=Szulágyi |first8=Judit |last9=Burke |first9=Chri|class=astro-ph.EP }}</ref>

The detection of [[sodium]] in hot Jupiters such as [[WASP-76b]], [[HD 189733 b]] or [[WASP-49b]] is likely due to a [[Io (moon)|Io]]-like exomoon around these planets.<ref>{{Cite journal |last1=Oza |first1=Apurva V. |last2=Johnson |first2=Robert E. |last3=Lellouch |first3=Emmanuel |last4=Schmidt |first4=Carl |last5=Schneider |first5=Nick |last6=Huang |first6=Chenliang |last7=Gamborino |first7=Diana |last8=Gebek |first8=Andrea |last9=Wyttenbach |first9=Aurelien |last10=Demory |first10=Brice-Olivier |last11=Mordasini |first11=Christoph |last12=Saxena |first12=Prabal |last13=Dubois |first13=David |last14=Moullet |first14=Arielle |last15=Thomas |first15=Nicolas |date=2019-08-28 |title=Sodium and Potassium Signatures of Volcanic Satellites Orbiting Close-in Gas Giant Exoplanets |journal=[[The Astrophysical Journal]] |volume=885 |issue=2 |pages=168 |arxiv=1908.10732 |bibcode=2019ApJ...885..168O |doi=10.3847/1538-4357/ab40cc |s2cid=201651224 |doi-access=free}}</ref><ref>{{cite journal |last1=Gebek |first1=Andrea |last2=Oza |first2=Apurva |date=29 July 2020 |title=Alkaline exospheres of exoplanet systems: evaporative transmission spectra |url=https://academic.oup.com/mnras/article-abstract/497/4/5271/5877918?redirectedFrom=fulltext |journal=Monthly Notices of the Royal Astronomical Society |volume=497 |issue=4 |pages=5271–5291 |arxiv=2005.02536 |bibcode=2020MNRAS.497.5271G |doi=10.1093/mnras/staa2193 |s2cid=218516741 |access-date=8 December 2020 |doi-access=free}}</ref>

=== Atmospheres ===
{{main|Exoplanet atmosphere}}
[[File:Cloudy versus clear atmospheres on two exoplanets.jpg|thumb|upright=1.6|Clear versus cloudy atmospheres on two exoplanets.<ref>{{cite web|title=Cloudy versus clear atmospheres on two exoplanets|url=https://www.spacetelescope.org/images/opo1722a/|website=www.spacetelescope.org|access-date=6 June 2017}}</ref>]]

Atmospheres have been detected around several exoplanets. The first to be observed was [[HD 209458 b]] in 2001.<ref>{{cite journal|last=Charbonneau|first=David|display-authors=etal|year=2002|title=Detection of an Extrasolar Planet Atmosphere|journal=The Astrophysical Journal|volume=568|issue=1|pages=377–384|arxiv=astro-ph/0111544|bibcode=2002ApJ...568..377C|doi=10.1086/338770|s2cid=14487268}}</ref>

[[File:PIA18410-TitanSunsetStudies-CassiniSpacecraft-20140527.jpg|thumb|upright=1.4|alt=Artist's concept of the ''Cassini'' spacecraft in front of a sunset on Saturn's moon Titan|Sunset studies on [[Titan (moon)|Titan]] by [[Cassini (spacecraft)|''Cassini'']] help understand exoplanet [[atmosphere]]s (artist's concept).]]
As of February 2014, more than fifty [[Transit method|transiting]] and five [[Direct imaging|directly imaged]] exoplanet atmospheres have been observed,<ref>{{cite book |last1=Madhusudhan|first1=Nikku|pages=739|last2=Knutson|first2=Heather|last3=Fortney|first3=Jonathan|last4=Barman|first4=Travis|title=Protostars and Planets VI| year=2014|  doi=10.2458/azu_uapress_9780816531240-ch032|chapter=Exoplanetary Atmospheres|isbn=978-0-8165-3124-0|arxiv = 1402.1169 |bibcode = 2014prpl.conf..739M |s2cid=118337613}}</ref> resulting in detection of molecular spectral features; observation of day–night temperature gradients; and constraints on vertical atmospheric structure.<ref name="SeagerDeming2010">{{cite journal |arxiv=1005.4037 |last1=Seager |first1=S. |last2=Deming |first2=D. |title=Exoplanet Atmospheres |date=2010|doi = 10.1146/annurev-astro-081309-130837 |bibcode = 2010ARA&A..48..631S |volume=48 |journal=Annual Review of Astronomy and Astrophysics |pages=631–672|s2cid=119269678 }}</ref> Also, an atmosphere has been detected on the non-transiting hot Jupiter [[Tau Boötis b]].<ref name="Rodler2012">{{cite journal | title=Weighing the Non-transiting Hot Jupiter τ Boo b | last1=Rodler | first1=F. | last2=Lopez-Morales | first2=M. | last3=Ribas | first3=I. | journal=The Astrophysical Journal Letters | volume=753 | issue=1 | pages=L25 | id=L25| date=July 2012 | arxiv=1206.6197 | bibcode=2012ApJ...753L..25R | doi=10.1088/2041-8205/753/1/L25 | s2cid=119177983 }}</ref><ref>{{Cite journal | doi = 10.1038/nature11161| pmid = 22739313| title = The signature of orbital motion from the dayside of the planet τ Boötis b| journal = Nature| volume = 486| issue = 7404| pages = 502–504| year = 2012| last1 = Brogi | first1 = M. | last2 = Snellen | first2 = I. A. G. | last3 = De Kok | first3 = R. J. | last4 = Albrecht | first4 = S. | last5 = Birkby | first5 = J. | last6 = De Mooij | first6 = E. J. W. |arxiv = 1206.6109 |bibcode = 2012Natur.486..502B | s2cid = 4368217}}</ref>

In May 2017, glints of light from [[Earth]], seen as twinkling from an orbiting satellite a million miles away, were found to be [[Reflection (physics)|reflected light]] from [[ice crystals]] in the [[Atmosphere of Earth|atmosphere]].<ref name="NYT-20170519">{{cite news |last=St. Fleur |first=Nicholas |title=Spotting Mysterious Twinkles on Earth From a Million Miles Away |url=https://www.nytimes.com/2017/05/19/science/dscovr-satellite-ice-glints-earth-atmosphere.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2017/05/19/science/dscovr-satellite-ice-glints-earth-atmosphere.html |archive-date=2022-01-01 |url-access=limited |date=19 May 2017 |work=The New York Times |access-date=20 May 2017}}{{cbignore}}</ref><ref name="GRL-201760515">{{cite journal |last1=Marshak |first1=Alexander |last2=Várnai |first2=Tamás |last3=Kostinski |first3=Alexander |title=Terrestrial glint seen from deep space: oriented ice crystals detected from the Lagrangian point|date=15 May 2017 |journal=[[Geophysical Research Letters]] |doi=10.1002/2017GL073248 |volume=44 |issue=10 |pages=5197–5202|bibcode = 2017GeoRL..44.5197M |s2cid=109930589 |url=https://zenodo.org/record/1229066|hdl=11603/13118 |hdl-access=free }}</ref> The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.

==== Comet-like tails ====
[[Kepler-1520b]] is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a [[comet]].<ref>{{Cite web|last=University|first=Leiden|title=Evaporating exoplanet stirs up dust|url=https://phys.org/news/2012-08-evaporating-exoplanet.html|access-date=2022-01-17|website=phys.org|language=en}}</ref> The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.<ref>{{Cite web|date=2012-05-18|title=New-found exoplanet is evaporating away|url=https://tgdaily.com/science/space/63469-new-found-exoplanet-is-evaporating-away/|access-date=2022-01-17|website=TGDaily|language=en-US}}</ref>

In June 2015, scientists reported that the atmosphere of [[GJ 436 b]] was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail {{convert|9|e6mi|e6km|order=flip|abbr=unit}} long.<ref name="NYT-20150625">{{cite news |last=Bhanoo |first=Sindya N. |title=A Planet with a Tail Nine Million Miles Long |url=https://www.nytimes.com/interactive/projects/cp/summer-of-science-2015/latest/exoplanet-tail |date=25 June 2015 |work=[[The New York Times]] |access-date=25 June 2015}}</ref>

=== Insolation pattern ===
[[Tidally locked]] planets in a 1:1 [[spin-orbit resonance]] would have their star always shining directly overhead on one spot, which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball, with the hotspot being the pupil.<ref>{{Cite web|last=Raymond|first=Sean|date=2015-02-20|title=Forget "Earth-Like"—We'll First Find Aliens on Eyeball Planets|url=http://nautil.us/blog/forget-earth_likewell-first-find-aliens-on-eyeball-planets|access-date=2022-01-17|website=Nautilus|archive-date=23 June 2017|archive-url=https://web.archive.org/web/20170623082602/http://nautil.us/blog/forget-earth_likewell-first-find-aliens-on-eyeball-planets|url-status=dead}}</ref> Planets with an [[Orbital eccentricity|eccentric orbit]] could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.<ref>{{cite journal |doi=10.1016/j.icarus.2014.12.017|bibcode = 2015Icar..250..395D | volume=250 | title=Insolation patterns on eccentric exoplanets |journal=Icarus |pages=395–399|year = 2015 |last1 = Dobrovolskis |first1 = Anthony R.}}</ref> Planets with both an eccentric orbit and a [[axial tilt|tilted axis of rotation]] would have more complicated insolation patterns.<ref>{{cite journal |doi=10.1016/j.icarus.2013.06.026|bibcode = 2013Icar..226..760D | volume=226 |issue = 1 | title=Insolation on exoplanets with eccentricity and obliquity|journal=Icarus |pages=760–776|year = 2013 |last1 = Dobrovolskis |first1 = Anthony R.}}</ref>