Editing Exoplanet (section)

=== 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>