Hot Jupiter#Puffy planets

Image:Exoplanet Period-Mass Scatter Discovery Method TR.png, indicated with black dots) discovered up to 2 January 2014]]

File:Artist's impression of a Hot Jupiter with hidden water.jpg

Though there is diversity among hot Jupiters, they do share some common properties.

• Their defining characteristics are their large masses and short orbital periods, spanning 0.36–11.8 Jupiter masses and 1.3–111 Earth days.{{Cite journal |last1=Winn |first1=Joshua N. |last2=Fabrycky |first2=Daniel |last3=Albrecht |first3=Simon |last4=Johnson |first4=John Asher |date=2010-01-01 |df=dmy-all |title=Hot stars with hot Jupiters have high obliquities |journal=The Astrophysical Journal Letters |language=en |volume=718 |issue=2 |pages=L145 |doi=10.1088/2041-8205/718/2/L145 |issn=2041-8205 |arxiv=1006.4161 |bibcode=2010ApJ...718L.145W|s2cid=13032700 }} The mass cannot be greater than approximately 13.6 Jupiter masses because then the pressure and temperature inside the planet would be high enough to cause deuterium fusion, and the planet would be a brown dwarf.{{cite journal |doi=10.1051/0004-6361:200500111|title=A companion to AB Pic at the planet/Brown dwarf boundary|year=2005|last1=Chauvin|first1=G.|last2=Lagrange|first2=A.-M.|last3=Zuckerman|first3=B.|last4=Dumas|first4=C.|last5=Mouillet|first5=D.|last6=Song|first6=I.|last7=Beuzit|first7=J.-L.|last8=Lowrance|first8=P.|last9=Bessell|first9=M. S.|journal=Astronomy & Astrophysics|volume=438|issue=3|pages=L29–L32|arxiv=astro-ph/0504658|bibcode=2005A&A...438L..29C|s2cid=119089948}}
• Most have nearly circular orbits (low eccentricities). It is thought that their orbits are circularized by perturbations from nearby stars or tidal forces.{{Cite journal |last1=Fabrycky, D. |last2=Tremaine, S. |title=Shrinking Binary and Planetary Orbits by Kozai Cycles with Tidal Friction |journal=Astrophysical Journal |date=2007-11-10 |df=dmy-all |volume=669 |issue=2 |pages=1298–1315 |doi=10.1086/521702 |bibcode=2007ApJ...669.1298F |arxiv=0705.4285|s2cid=12159532 }} Whether they remain in these circular orbits for long periods of time or collide with their host stars depends on the coupling of their orbital and physical evolution, which are related through the dissipation of energy and tidal deformation.{{cite journal |doi= 10.1093/mnras/stz1081 |last1=Alvarado-Montes J. A. |last2=García-Carmona C. |title=Orbital decay of short-period gas giants under evolving tides |journal=Monthly Notices of the Royal Astronomical Society |volume=486 |year=2019 |issue=3 |pages=3963–3974 |doi-access=free |arxiv=1904.07596 |bibcode=2019MNRAS.486.3963A|s2cid=119313969 }}
• Many have unusually low densities. The lowest one measured thus far is that of TrES-4b at 0.222 g/cm³.{{cite journal |last1=Mandushev |first1=Georgi |last2=O'Donovan |first2=Francis T. |last3=Charbonneau |first3=David |last4=Torres |first4=Guillermo |last5=Latham |first5=David W. |last6=Bakos |first6=Gáspár Á. |last7=Dunham |first7=Edward W. |last8=Sozzetti |first8=Alessandro |last9=Fernández |first9=José M. |date=2007-10-01 |df=dmy-all |title=TrES-4: A Transiting Hot Jupiter of Very Low Density |journal=The Astrophysical Journal |language=en |volume=667 |issue=2 |pages=L195–L198 |doi=10.1086/522115 |bibcode=2007ApJ...667L.195M |arxiv = 0708.0834|s2cid=6087170 }} The large radii of hot Jupiters are not yet fully understood but it is thought that the expanded envelopes can be attributed to high stellar irradiation, high atmospheric opacities, possible internal energy sources, and orbits close enough to their stars for the outer layers of the planets to exceed their Roche limit and be pulled further outward.{{cite journal |last1=Burrows |first1=A. |last2=Hubeny |first2=I. |last3=Budaj |first3=J. |last4=Hubbard |first4=W. B. |date=2007-01-01 |df=dmy-all |title=Possible Solutions to the Radius Anomalies of Transiting Giant Planets |journal=The Astrophysical Journal |language=en |volume=661 |issue=1 |pages=502–514 |doi=10.1086/514326 |issn=0004-637X |bibcode=2007ApJ...661..502B |arxiv=astro-ph/0612703|s2cid=9948700 }}
• Usually they are tidally locked, with one side always facing its host star.{{cite web |url=https://www.sciencealert.com/hot-jupiter-wasp-104b-one-of-the-darkest-planets-ever |website=Science Alert.com |title=Hot Jupiter WASP 104b one of the darkest planets ever|date=23 April 2018 }}
• They are likely to have extreme and exotic atmospheres due to their short periods, relatively long days, and tidal locking.
• Atmospheric dynamics models predict strong vertical stratification with intense winds and super-rotating equatorial jets driven by radiative forcing and the transfer of heat and momentum.{{Cite journal |last1=Cooper |first1=Curtis S. |last2=Showman| first2=Adam P. |date=2005-01-01 |df=dmy-all |title=Dynamic meteorology at the photosphere of HD 209458b |journal=The Astrophysical Journal Letters |language=en |volume=629 |issue=1 |pages=L45 |doi=10.1086/444354 |issn=1538-4357 |bibcode=2005ApJ...629L..45C |arxiv=astro-ph/0502476|s2cid=10022257 }}{{cite journal |last1=Rauscher |first1=Emily |last2=Menou |first2=Kristen |date=2010-01-01 |df=dmy-all |title=Three-dimensional Modeling of Hot Jupiter Atmospheric Flows |journal=The Astrophysical Journal |language=en|volume=714 |issue=2 |pages=1334–1342 |doi=10.1088/0004-637X/714/2/1334 |issn=0004-637X |bibcode=2010ApJ...714.1334R |arxiv=0907.2692|s2cid=17361362 }} Recent models also predict a variety of storms (vortices) that can mix their atmospheres and transport hot and cold regions of gas.{{cite arXiv|last1=Cho|first1=J. Y.-K.|last2=Skinner|first2=J. W.|last3=Thrastarson|first3=H. Th|date=2021-05-26|title=Storms, Variability, and Multiple Equilibria on Hot-Jupiters|class=astro-ph.EP|eprint=2105.12759}}
• The day-night temperature difference at the photosphere is predicted to be substantial, approximately {{cvt|500|K-change|C-change F-change}} for a model based on HD 209458 b.
• They appear to be more common around F- and G-type stars and less so around K-type stars. Hot Jupiters around red dwarfs are very rare.{{cite journal |title=Characterizing the cool KOIs II. The M-dwarf KOI-254 and its hot Jupiter |last1=Johnson, John Asher |last2=Gazak, J. Zachary |last3=Apps, Kevin |last4=Muirhead, Philip S. |last5=Crepp, Justin R. |last6=Crossfield, Ian J.M. |last7=Boyajian, Tabetha |last8=Kaspar von Braun |last9=Barbara Rojas-Ayala |last10=Howard, Andrew W. |last11=Covey, Kevin R. |last12=Schlawin, Everett |last13=Hamren, Katherine |last14=Morton, Timothy D. |last15=Lloyd, James P. |display-authors=3 |arxiv = 1112.0017|doi = 10.1088/0004-6256/143/5/111|year = 2011|journal=The Astronomical Journal|s2cid=25791517 }} Generalizations about the distribution of these planets must take into account the various observational biases, but in general their prevalence decreases exponentially as a function of the absolute stellar magnitude.{{cite journal |last1=Ballesteros, F.J. |last2=Fernandez-Soto, A. |last3=Martinez, V.J. |title=Title: Diving into Exoplanets: are water seas the most common? |year=2019 |doi=10.1089/ast.2017.1720 |pmid=30789285 |journal=Astrobiology |volume=19 |issue=5 |pages=642–654|hdl=10261/213115 |s2cid=73498809 |hdl-access=free }}

There are three schools of thought regarding the possible origin of hot Jupiters. One possibility is that they were formed in situ at the distances at which they are currently observed. Another possibility is that they were formed at a distance but later migrated inward. Such a shift in position might occur due to interactions with gas and dust during the solar nebula phase. It might also occur as a result of a close encounter with another large object destabilizing a Jupiter's orbit.{{cite journal|last1=Dawson |first1=Rebekah I.|last2=Johnson|first2=John Asher|title=Origins of Hot Jupiters|journal=Annual Review of Astronomy and Astrophysics|volume=56|pages=175–221|date=2018|arxiv=1801.06117|doi=10.1146/annurev-astro-081817-051853|bibcode=2018ARA&A..56..175D|s2cid=119332976}}{{cite book|last=D'Angelo|first=G.|last2=Lissauer, J. J.|chapter=Formation of Giant Planets |bibcode=2018haex.bookE.140D| title=Handbook of Exoplanets |publisher=Springer International Publishing| editor=Deeg H., Belmonte J. |pages= 2319–2343|date=2018|arxiv=1806.05649|doi=10.1007/978-3-319-55333-7_140|isbn=978-3-319-55332-0|s2cid=116913980}}

In the migration hypothesis, a hot Jupiter forms beyond the frost line, from rock, ice, and gases via the core accretion method of planetary formation. The planet then migrates inwards to the star where it eventually forms a stable orbit.{{Cite conference

| conference = AAS/Division of Dynamical Astronomy Meeting

| volume = 38

| last = Chambers

| first = John

| title = Planet Formation with Type I and Type II Migration

| date = 2007-07-01

| bibcode=2007DDA....38.0604C

}}{{cite book |chapter=Giant Planet Formation |title=Exoplanets |publisher=University of Arizona Press |first1=Gennaro |last1=D'Angelo |first2=Richard H. |last2=Durisen |first3=Jack J. |last3=Lissauer |editor-first=Sara |editor-last=Seager |pages=319–346 |date=December 2010 |isbn=978-0-8165-2945-2 |bibcode=2010exop.book..319D |arxiv=1006.5486}} The planet may have migrated inward smoothly via type II orbital migration.{{cite journal |last=D'Angelo |first=Gennaro |last2=Lubow |first2=Stephen H. |date=2008 |title=Evolution of Migrating Planets Undergoing Gas Accretion |journal=The Astrophysical Journal |volume=685 |issue=1 |pages=560–583 |arxiv=0806.1771 |bibcode=2008ApJ...685..560D |doi=10.1086/590904 |s2cid=84978}}{{cite book |last=Lubow |first=Stephen H. |title=Exoplanets |last2=Ida |first2=Shigeru |publisher=University of Arizona Press, Tucson, AZ |year=2011 |editor=Seager |editor-first=S. |pages=347–371 |chapter=Planet Migration |bibcode=2010exop.book..347L |arxiv=1004.4137}} Or it may have migrated more suddenly due to gravitational scattering onto eccentric orbits during an encounter with another massive planet, followed by the circularization and shrinking of the orbits due to tidal interactions with the star. A hot Jupiter's orbit could also have been altered via the Kozai mechanism, causing an exchange of inclination for eccentricity resulting in a high eccentricity low perihelion orbit, in combination with tidal friction. This requires a massive body—another planet or a stellar companion—on a more distant and inclined orbit; approximately 50% of hot Jupiters have distant Jupiter-mass or larger companions, which can leave the hot Jupiter with an orbit inclined relative to the star's rotation.{{Cite journal |last1=Knutson |first1=Heather A. |last2=Fulton |first2=Benjamin J. |last3=Montet |first3=Benjamin T. |last4=Kao |first4=Melodie |last5=Ngo |first5=Henry |last6=Howard |first6=Andrew W. |last7=Crepp |first7=Justin R. |last8=Hinkley |first8=Sasha |last9=Bakos |first9=Gaspar A. |last10=Batygin |first10=Konstantin |last11=Johnson |first11=John Asher |last12=Morton |first12=Timothy D. |last13=Muirhead |first13=Philip S. |date=2014-01-01 |title=Friends of Hot Jupiters. I. A Radial Velocity Search for Massive, Long-period Companions to Close-in Gas Giant Planets |journal=The Astrophysical Journal |language=en |volume=785 |issue=2 |pages=126 |arxiv=1312.2954 |bibcode=2014ApJ...785..126K |doi=10.1088/0004-637X/785/2/126 |issn=0004-637X |s2cid=42687848}}

The type II migration happens during the solar nebula phase, i.e. when gas is still present. Energetic stellar photons and strong stellar winds at this time remove most of the remaining nebula.{{cite journal |last1=Heller |first1=René |title=Formation of hot Jupiters through disk migration and evolving stellar tides |journal=Astronomy & Astrophysics |date=1 August 2019 |volume=628 |pages=A42 |doi=10.1051/0004-6361/201833486 |arxiv=1806.06601 |bibcode=2019A&A...628A..42H |s2cid=102352102 |url=https://www.aanda.org/articles/aa/full_html/2019/08/aa33486-18/aa33486-18.html |access-date=5 April 2022 |language=en |issn=0004-6361}} Migration via the other mechanism can happen after the loss of the gas disk.

Instead of being gas giants that migrated inward, in an alternate hypothesis the cores of the hot Jupiters began as more common super-Earths which accreted their gas envelopes at their current locations, becoming gas giants in situ. The super-Earths providing the cores in this hypothesis could have formed either in situ or at greater distances and have undergone migration before acquiring their gas envelopes. Since super-Earths are often found with companions, the hot Jupiters formed in situ could also be expected to have companions. The increase of the mass of the locally growing hot Jupiter has a number of possible effects on neighboring planets. If the hot Jupiter maintains an eccentricity greater than 0.01, sweeping secular resonances can increase the eccentricity of a companion planet, causing it to collide with the hot Jupiter. The core of the hot Jupiter in this case would be unusually large. If the hot Jupiter's eccentricity remains small the sweeping secular resonances could also tilt the orbit of the companion.{{cite journal |last1=Batygin |first1=Konstantin |last2=Bodenheimer |first2=Peter H. |last3=Laughlin |first3=Gregory P. |title=In Situ Formation and Dynamical Evolution of Hot Jupiter Systems |journal=The Astrophysical Journal |year=2016 |volume=829 |issue=2 |page=114 |doi=10.3847/0004-637X/829/2/114 |arxiv=1511.09157 |bibcode=2016ApJ...829..114B|s2cid=25105765 |doi-access=free }} Traditionally, the in situ mode of conglomeration has been disfavored because the assembly of massive cores, which is necessary for the formation of hot Jupiters, requires surface densities of solids ≈ 10⁴ g/cm², or larger.{{Cite journal |last=Rafikov |first=Roman R. |date=2006-01-01 |df=dmy-all |title=Atmospheres of Protoplanetary Cores: Critical Mass for Nucleated Instability |journal=The Astrophysical Journal |language=en |volume=648 |issue=1 |pages=666–682 |doi=10.1086/505695 |issn=0004-637X |bibcode=2006ApJ...648..666R |arxiv=astro-ph/0405507|s2cid=51815430 }}{{Cite journal |last=Hayashi |first=Chushiro |date=1981-01-01 |df=dmy-all |title=Structure of the Solar Nebula, Growth and Decay of Magnetic Fields and Effects of Magnetic and Turbulent Viscosities on the Nebula |journal=Progress of Theoretical Physics Supplement |language=en |volume=70 |pages=35–53 |doi=10.1143/PTPS.70.35 |issn=0375-9687 |bibcode=1981PThPS..70...35H|doi-access=free }}{{cite journal |last1=D'Angelo, G. |last2= Bodenheimer, P. |title=In Situ and Ex Situ Formation Models of Kepler 11 Planets |journal=The Astrophysical Journal |year=2016 |volume=828 |issue=1 |pages=in press |doi=10.3847/0004-637X/828/1/33 |arxiv=1606.08088 |bibcode=2016ApJ...828...33D|s2cid= 119203398 |doi-access= free }} Recent surveys, however, have found that the inner regions of planetary systems are frequently occupied by super-Earth type planets.{{cite arXiv |eprint=1109.2497 |class=astro-ph |first1=M. |last1=Mayor |first2=M. |last2=Marmier |title=The HARPS search for southern extra-solar planets XXXIV. Occurrence, mass distribution and orbital properties of super-Earths and Neptune-mass planets |date=2011-09-12 |df=dmy-all |last3=Lovis |first3=C. |last4=Udry |first4=S. |last5=Ségransan |first5=D. |last6=Pepe |first6=F. |last7=Benz |first7=W. |last8=Bertaux |first8=J.-L. |last9=Bouchy |first9=F. |last10=Santos |first10=N. C. |last11=Queloz |first11=D. |last12=Mordasini |first12=C. |last13=Lo Curto |first13=G. |last14=Dumusque |first14=X.}}{{Cite journal |last1=Batalha |first1=Natalie M. |last2=Rowe |first2=Jason F. |last3=Bryson |first3=Stephen T. |last4=Barclay |first4=Thomas |last5=Burke |first5=Christopher J. |last6=Caldwell |first6=Douglas A. |last7=Christiansen |first7=Jessie L. |last8=Mullally |first8=Fergal |last9=Thompson |first9=Susan E. |date=2013-01-01 |df=dmy-all |title=Planetary Candidates Observed by Kepler. III. Analysis of the First 16 Months of Data |journal=The Astrophysical Journal Supplement Series |language=en |volume=204 |issue=2 |pages=24 |doi=10.1088/0067-0049/204/2/24 |issn=0067-0049 |arxiv=1202.5852 |bibcode=2013ApJS..204...24B|s2cid=19023502 }} If these super-Earths formed at greater distances and migrated closer, the formation of in situ hot Jupiters is not entirely in situ.

If the atmosphere of a hot Jupiter is stripped away via hydrodynamic escape, its core may become a chthonian planet. The amount of gas removed from the outermost layers depends on the planet's size, the gases forming the envelope, the orbital distance from the star, and the star's luminosity. In a typical system, a gas giant orbiting at 0.02 AU around its parent star loses 5–7% of its mass during its lifetime, but orbiting closer than 0.015 AU can mean evaporation of a substantially larger fraction of the planet's mass.{{cite web |url=http://www.astrobio.net/news/article3112.html |title=Exoplanets Exposed to the Core |date=2009-04-25 |access-date=2009-04-25 |df=dmy-all |archive-url=https://web.archive.org/web/20110527183634/http://www.astrobio.net/pressrelease/3112/darwin-in-a-test-tube |archive-date=2011-05-27 |url-status=usurped}} No such objects have been found yet and they are still hypothetical.

File:Clear to cloudy hot Jupiters.jpg, Boinayel, WASP-31b, Bocaprins, HD 189733b, Puli, Ditsö̀, Banksia, HAT-P-1b and HD 209458b]]

Simulations have shown that the migration of a Jupiter-sized planet through the inner protoplanetary disk (the region between 5 and 0.1 AU from the star) is not as destructive as expected. More than 60% of the solid disk materials in that region are scattered outward, including planetesimals and protoplanets, allowing the planet-forming disk to reform in the gas giant's wake.{{cite journal |title=On the formation of terrestrial planets in hot-Jupiter systems |first1=Martyn J. |last1=Fogg |first2=Richard P. |last2=Nelson |journal=Astronomy and Astrophysics |volume=461 |pages=1195–1208 |year=2007 |arxiv=astro-ph/0610314 |bibcode=2007A&A...461.1195F |doi=10.1051/0004-6361:20066171 |issue=3|s2cid=119476713 }} In the simulation, planets up to two Earth masses were able to form in the habitable zone after the hot Jupiter passed through and its orbit stabilized at 0.1 AU. Due to the mixing of inner-planetary-system material with outer-planetary-system material from beyond the frost line, simulations indicated that the terrestrial planets that formed after a hot Jupiter's passage would be particularly water-rich. According to a 2011 study, hot Jupiters may become disrupted planets while migrating inwards; this could explain an abundance of "hot" Earth-sized to Neptune-sized planets within 0.2 AU of their host star.{{cite journal |last=Nayakshin |first=Sergei |title=Hot Super Earths: disrupted young jupiters? |date=20 September 2011 |journal=Monthly Notices of the Royal Astronomical Society |volume=416 |issue=4 |pages=2974–2980 |doi=10.1111/j.1365-2966.2011.19246.x |doi-access=free |arxiv=1103.1846 |bibcode=2011MNRAS.416.2974N|s2cid=53960650 }}

One example of these sorts of systems is that of WASP-47. There are three inner planets and an outer gas giant in the habitable zone. The innermost planet, WASP-47e, is a large terrestrial planet of 6.83 Earth masses and 1.8 Earth radii; the hot Jupiter, b, is little heavier than Jupiter, but about 12.63 Earth radii; a final hot Neptune, c, is 15.2 Earth masses and 3.6 Earth radii.{{cite journal |last1=Becker |first1=Juliette C. |last2=Vanderburg |first2=Andrew |last3=Adams |first3=Fred C. |last4=Rappaport |first4=Saul A. |last5=Schwengeler |first5=Hans Marti |date=10 August 2015 |title=WASP-47: A Hot Jupiter System with Two Additional Planets Discovered by K2 |journal=The Astrophysical Journal Letters |publisher=IOP Publishing |publication-date=October 2015 |volume=812 |issue=2 |pages=L18 |arxiv=1508.02411 |bibcode=2015ApJ...812L..18B |doi=10.1088/2041-8205/812/2/L18 |s2cid=14681933 |quote=The mass of WASP-47d is 15.2±7 M⊕. Only an upper limit can be placed on WASP-47e of < 22 M⊕.}} A similar orbital architecture is also exhibited by the Kepler-30 system.{{Cite web |url=http://kepler.nasa.gov/news/newsaboutplanetfinding/index.cfm?fuseaction=ShowNews&NewsID=218 |archive-url=https://web.archive.org/web/20160814024605/http://kepler.nasa.gov/news/newsaboutplanetfinding/index.cfm?fuseaction=ShowNews&NewsID=218 |url-status=dead |archive-date=14 August 2016 |title=Kepler: A far-off solar system |website=kepler.nasa.gov |date=31 March 2015 |access-date=2016-08-02 |df=dmy-all}}

Several hot Jupiters, such as HD 80606 b, have orbits that are misaligned with their host stars, including several with retrograde orbits such as HAT-P-14b.{{cite journal |last1=Hebrard |first1=G. |last2=Desert |first2=J.-M. |last3=Diaz |first3=R. F. |last4=Boisse |first4=I. |last5=Bouchy |first5=F. |last6=des Etangs |first6=A. Lecavelier |last7=Moutou |first7=C. |last8=Ehrenreich|first8=D. |last9=Arnold |first9=L. |title=Observation of the full 12-hour-long transit of the exoplanet HD80606b. Warm-Spitzer photometry and SOPHIE spectroscopy |arxiv=1004.0790 |journal=Astronomy and Astrophysics |volume=516 |pages=A95 |doi=10.1051/0004-6361/201014327 |issn=0004-6361 |bibcode=2010A&A...516A..95H |year=2010|s2cid=73585455 }}{{Cite journal |last1=Triaud |first1=A. H. M. J. |last2=Queloz |first2=D. |last3=Bouchy |first3=F. |last4=Moutou |first4=C. |last5=Collier Cameron |first5=A. |last6=Claret |first6=A. |last7=Barge |first7=P. |last8=Benz |first8=W. |last9=Deleuil |first9=M. |date=2009-10-01 |df=dmy-all |title=The Rossiter-McLaughlin effect of CoRoT-3b and HD 189733b |journal=Astronomy and Astrophysics |volume=506 |issue=1 |pages=377–384 |doi=10.1051/0004-6361/200911897 |issn=0004-6361 |bibcode=2009A&A...506..377T |arxiv=0907.2956 |s2cid=10454322 |url=https://digital.csic.es/bitstream/10261/24876/1/A.%20Ast.%20506%201%20377-.%20pdf.pdf |access-date=2018-11-04 |archive-url=https://web.archive.org/web/20170820083715/http://digital.csic.es/bitstream/10261/24876/1/A.%20Ast.%20506%201%20377-.%20pdf.pdf |archive-date=2017-08-20 |url-status=dead }}{{cite press release |url=http://www.astro.gla.ac.uk/nam2010/pr10.php |title=Turning planetary theory upside down |date=2010-04-13 |publisher=Royal Astronomical Society |bibcode=2010eso..pres...16. |pages=16 |department=ESO |access-date=2010-04-15 |archive-date=2011-07-16 |archive-url=https://web.archive.org/web/20110716051715/http://www.astro.gla.ac.uk/nam2010/pr10.php |url-status=dead }}{{cite journal | title=Orbital Orientations of Exoplanets: HAT-P-4b is Prograde and HAT-P-14b is Retrograde | last1=Winn | first1=Joshua N. | last2=Howard | first2=Andrew W. | last3=Johnson | first3=John Asher | last4=Marcy | first4=Geoffrey W. | last5=Isaacson | first5=Howard | last6=Shporer | first6=Avi | last7=Bakos | first7=Gáspár Á. | last8=Hartman | first8=Joel D. | last9=Holman | first9=Matthew J. | display-authors=1 | journal=The Astronomical Journal | volume=141 | issue=2 | at=63 | date=2011 | arxiv=1010.1318 | bibcode=2011AJ....141...63W | doi=10.1088/0004-6256/141/2/63 | s2cid=18752702 }} This misalignment may be related to the heat of the photosphere the hot Jupiter is orbiting. There are several proposed hypotheses as to why this might occur. One such hypothesis involves tidal dissipation and suggests there is a single mechanism for producing hot Jupiters and this mechanism yields a range of obliquities. Cooler stars with higher tidal dissipation damps the obliquity (explaining why hot Jupiters orbiting cooler stars are well aligned) while hotter stars do not damp the obliquity (explaining the observed misalignment). Another hypothesis is that the host star sometimes changes rotation early in its evolution, rather than the orbit changing.{{cite magazine |date=1 September 2010 |title=Tilting stars may explain backwards planets |url=https://www.newscientist.com/article/mg20727765.200-tilting-stars-may-explain-backwards-planets.html |url-access=subscription |access-date=19 December 2024 |magazine=New Scientist |issue=2776}} Yet another hypothesis is that hot Jupiters tend to form in dense clusters, where perturbations are more common and gravitational capture of planets by neighboring stars is possible.{{cite web |url=https://phys.org/news/2022-12-spaces-swapping-stars-hot-jupiters.html |title=Trading spaces: How swapping stars create hot Jupiters |last=Paul M. Sutter |agency=Universe Today |date=December 9, 2022 }}

Ultra-hot Jupiters are hot Jupiters with a dayside temperature greater than {{cvt|2200|K|C F}}. In such dayside atmospheres, most molecules dissociate into their constituent atoms and circulate to the nightside where they recombine into molecules again.{{Cite journal |arxiv = 1802.07725|doi = 10.3847/2041-8213/aabcc8|title = Increased Heat Transport in Ultra-hot Jupiter Atmospheres through H 2 Dissociation and Recombination|year = 2018|last1 = Bell|first1 = Taylor J.|last2 = Cowan|first2 = Nicolas B.|journal = The Astrophysical Journal|volume = 857|issue = 2|pages = L20|bibcode = 2018ApJ...857L..20B|s2cid = 119404042 | doi-access=free }}{{Cite journal |doi = 10.1051/0004-6361/201833059|title = From thermal dissociation to condensation in the atmospheres of ultra hot Jupiters: WASP-121b in context|year = 2018|last1 = Parmentier|first1 = Vivien|last2 = Line|first2 = Mike R.|last3 = Bean|first3 = Jacob L.|last4 = Mansfield|first4 = Megan|last5 = Kreidberg|first5 = Laura|last6 = Lupu|first6 = Roxana|last7 = Visscher|first7 = Channon|last8 = Désert|first8 = Jean-Michel|last9 = Fortney|first9 = Jonathan J.|last10 = Deleuil|first10 = Magalie|last11 = Arcangeli|first11 = Jacob|last12 = Showman|first12 = Adam P.|last13 = Marley|first13 = Mark S.|journal = Astronomy & Astrophysics|volume = 617|pages = A110|arxiv = 1805.00096|bibcode = 2018A&A...617A.110P|s2cid = 62895296}}

One example is TOI-1431b, announced by the University of Southern Queensland in April 2021, which has an orbital period of just two and a half days. Its dayside temperature is {{cvt|2700|K|C F}}, making it hotter than 40% of stars in our galaxy.{{cite news |last=Serrels |first=Mark |date=April 27, 2021 |title=Scientists uncover a 'hellish' planet so hot it could vaporize most metals |url=https://www.cnet.com/news/scientists-uncover-a-hellish-planet-so-hot-it-could-vaporize-most-metals/ |access-date=April 27, 2021 |work=CNET}} The nightside temperature is {{cvt|2600|K|C F}}.{{cite news |title='Hellish' new planet discovered |url=https://www.usq.edu.au/news/2021/04/hellish-new-planet |date=April 27, 2021 |work=University of Southern Queensland |access-date=April 27, 2021 |archive-date=April 29, 2021 |archive-url=https://web.archive.org/web/20210429195829/https://www.usq.edu.au/news/2021/04/hellish-new-planet |url-status=dead }}

{{main|Ultra-short period planet}}

Ultra-short period planets (USP) are a class of planets with orbital periods below one day and occur only around stars of less than about 1.25 solar masses.{{cite journal |last=Malavolta, Luca|display-authors=etal |title=An Ultra-short Period Rocky Super-Earth with a Secondary Eclipse and a Neptune-like Companion around K2-141 |date=9 February 2018 |journal=The Astronomical Journal |volume=155 |pages=107 |number=3 |doi=10.3847/1538-3881/aaa5b5 |arxiv=1801.03502|bibcode=2018AJ....155..107M |s2cid=54869937 |doi-access=free }}{{cite journal |title=Transiting extrasolar planetary candidates in the Galactic bulge |journal=Nature |volume=443 |pages=534–540 |year=2006 |doi=10.1038/nature05158 |pmid=17024085 |last1=Sahu |last2=Casertano |first2=S. |last3=Bond |first3=H.E. |last4=Valenti |first4=J. |last5=Smith |first5=T.E. |last6=Minniti |first6=D. |last7=Zoccali |first7=M. |last8=Livio |first8=M. |last9=Panagia |first9=N. |issue=7111 |bibcode=2006Natur.443..534S |arxiv=astro-ph/0610098 |s2cid=4403395 |display-authors=6}}

Confirmed transiting hot Jupiters that have orbital periods of less than one day include WASP-18b, WASP-19b, WASP-43b, WASP-103b, and TOI-1937A b.{{cite web |url=http://wasp-planets.net/wasp-planets/ |title=WASP Planets |date=5 December 2013 |website=wasp-planets.net |access-date=1 April 2018}}

Gas giants with a large radius and very low density are sometimes called "puffy planets"{{cite news |url=https://www.nytimes.com/2006/09/15/science/space/15planet.html |date=2010-11-11 |df=dmy-all |title=Puzzling Puffy Planet, Less Dense Than Cork, Is Discovered | work=The New York Times |first=Kenneth |last=Chang}} or "hot Saturns", due to their density being similar to Saturn's. Puffy planets orbit close to their stars so that the intense heat from the star combined with internal heating within the planet will help inflate the atmosphere. Six large-radius low-density planets have been detected by the transit method. In order of discovery they are: HAT-P-1b,{{cite web |date=2006-09-14 |title=Puffy 'Cork' Planet Would Float on Water |publisher=Space.com |last=Ker Than |url=http://www.space.com/scienceastronomy/060914_cork_planet.html |access-date=2007-08-08 |df=dmy-all}}{{cite news |date=2006-09-15 |df=dmy-all |title=Puffy planet poses pretty puzzle |publisher=BBC News |url=http://news.bbc.co.uk/2/hi/sci/tech/5346998.stm |access-date=2010-03-17}} CoRoT-1b, TrES-4b, WASP-12b, WASP-17b, and Kepler-7b. Some hot Jupiters detected by the radial-velocity method may be puffy planets. Most of these planets are around or below Jupiter mass as more massive planets have stronger gravity keeping them at roughly Jupiter's size. Indeed, hot Jupiters with masses below Jupiter, and temperatures above 1800 Kelvin, are so inflated and puffed out that they are all on unstable evolutionary paths which eventually lead to Roche-Lobe overflow and the evaporation and loss of the planet's atmosphere.

Even when taking surface heating from the star into account, many transiting hot Jupiters have a larger radius than expected. This could be caused by the interaction between atmospheric winds and the planet's magnetosphere creating an electric current through the planet that heats it up, causing it to expand. The hotter the planet, the greater the atmospheric ionization, and thus the greater the magnitude of the interaction and the larger the electric current, leading to more heating and expansion of the planet. This theory matches the observation that planetary temperature is correlated with inflated planetary radii.{{Cite journal |arxiv=1101.3800 |doi=10.1088/0004-637X/738/1/1 |title=Evolution of Ohmically Heated Hot Jupiters |journal=The Astrophysical Journal |volume=738 |issue=1 |page=1 |year=2011 |last1=Batygin |first1=Konstantin |last2=Stevenson |first2=David J. |last3=Bodenheimer |first3=Peter H. |last4=Huang |first4=Xu|bibcode=2011ApJ...738....1B |s2cid=43150278 }}

Theoretical research suggests that hot Jupiters are unlikely to have moons, due to both a small Hill sphere and the tidal forces of the stars they orbit, which would destabilize any satellite's orbit, the latter process being stronger for larger moons. This means that for most hot Jupiters, stable satellites would be small asteroid-sized bodies.{{cite journal |title=Stability of Satellites around Close-in Extrasolar Giant Planets|journal = The Astrophysical Journal|volume = 575|issue = 2|pages = 1087–1093|bibcode = 2002ApJ...575.1087B|last1 = Barnes|first1 = Jason W.|last2 = O'Brien|first2 = D. P.|year = 2002|arxiv = astro-ph/0205035|doi = 10.1086/341477|s2cid = 14508244}} Furthermore, the physical evolution of hot Jupiters can determine the final fate of their moons: stall them in semi-asymptotic semimajor axes, or eject them from the system where they may undergo other unknown processes.{{cite journal |doi= 10.1093/mnras/stx1745 |last1=Alvarado-Montes J. A. |last2=Zuluaga J. |last3=Sucerquia M. |title=The effect of close-in giant planets' evolution on tidal-induced migration of exomoons |journal=Monthly Notices of the Royal Astronomical Society |volume=471 |year=2017 |issue=3 |pages=3019–3027 |doi-access=free |arxiv=1707.02906 |bibcode=2017MNRAS.471.3019A|s2cid=119346461 }} In spite of this, observations of WASP-12b suggest that it is orbited by at least one large exomoon.{{cite web |url=http://www.ria.ru/science/20120206/558647431.html |script-title=ru:Российские астрономы впервые открыли луну возле экзопланеты] |language=ru |quote=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.|date=6 February 2012 }}

It has been proposed that gas giants orbiting red giants at distances similar to that of Jupiter could be hot Jupiters due to the intense irradiation they would receive from their stars. It is very likely that in the Solar System Jupiter will become a hot Jupiter after the transformation of the Sun into a red giant.{{cite journal |last1=Spiegel |first1=David S. |last2=Madhusudhan |first2=Nikku |date=2012-09-01 |df=dmy-all |title=Jupiter will Become a Hot Jupiter: Consequences of Post-main-sequence Stellar Evolution on Gas Giant Planets |journal=The Astrophysical Journal |volume=756 |issue=2 |pages=132 |doi=10.1088/0004-637X/756/2/132 |issn=0004-637X |bibcode=2012ApJ...756..132S |arxiv=1207.2770|s2cid=118416430 }} The recent discovery of particularly low-density gas giants orbiting red giant stars supports this hypothesis.{{Cite journal |last1=Grunblatt |first1=Samuel K. |last2=Huber |first2=Daniel |date=2017-12-01 |df=dmy-all |title=Seeing Double with K2: Testing Re-inflation with Two Remarkably Similar Planets around Red Giant Branch Stars |journal=The Astrophysical Journal |volume=154 |issue=6 |pages=254 |doi=10.3847/1538-3881/aa932d |bibcode=2017AJ....154..254G |arxiv=1706.05865|s2cid=55959801 |doi-access=free }}

Hot Jupiters orbiting red giants would differ from those orbiting main-sequence stars in a number of ways, most notably the possibility of accreting material from the stellar winds of their stars and, assuming a fast rotation (not tidally locked to their stars), a much more evenly distributed heat with many narrow-banded jets. Their detection using the transit method would be much more difficult due to their tiny size compared to the stars they orbit, as well as the long time needed (months or even years) for one to transit their star as well as to be occulted by it.

Theoretical research since 2000 suggested that "hot Jupiters" may cause increased flaring due to the interaction of the magnetic fields of the star and its orbiting exoplanet, or because of tidal forces between them. These effects are called "star–planet interactions" or SPIs. The HD 189733 system is the best-studied exoplanet system where this effect was thought to occur.

In 2008, a team of astronomers first described how as the exoplanet orbiting HD 189733 A reaches a certain place in its orbit, it causes increased stellar flaring. In 2010, a different team found that every time they observe the exoplanet at a certain position in its orbit, they also detected X-ray flares. In 2019, astronomers analyzed data from Arecibo Observatory, MOST, and the Automated Photoelectric Telescope, in addition to historical observations of the star at radio, optical, ultraviolet, and X-ray wavelengths to examine these claims. They found that the previous claims were exaggerated and the host star failed to display many of the brightness and spectral characteristics associated with stellar flaring and solar active regions, including sunspots. Their statistical analysis also found that many stellar flares are seen regardless of the position of the exoplanet, therefore debunking the earlier claims. The magnetic fields of the host star and exoplanet do not interact, and this system is no longer believed to have a "star-planet interaction."{{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=February 10, 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 }} Some researchers had also suggested that HD 189733 accretes, or pulls, material from its orbiting exoplanet at a rate similar to those found around young protostars in T Tauri star systems. Later analysis demonstrated that very little, if any, gas was accreted from the "hot Jupiter" companion.{{cite journal|last1=Route|first1=Matthew|last2=Looney|first2=Leslie|title=ROME (Radio Observations of Magnetized Exoplanets). II. HD 189733 Does Not Accrete Significant Material from Its Exoplanet Like a T Tauri Star from a Disk|journal=The Astrophysical Journal|date=December 20, 2019|volume=887|issue=2|page=229|doi=10.3847/1538-4357/ab594e|arxiv=1911.08357|bibcode=2019ApJ...887..229R|s2cid=208158242 |doi-access=free }}

• Grand tack hypothesis – a model that posits that Jupiter migrated inwards through the early Solar System, before being pulled to its present location as a result of orbital resonance with Saturn
• Hot Neptune
• Lists of planets
• Planetary migration
• Sub-brown dwarf

• [https://arxiv.org/abs/1801.06117 Origins of Hot Jupiters], Rebekah I. Dawson, John Asher Johnson, 18 Jan 2018

{{reflist|25em}}

• {{cite web |url=http://www.space.com/scienceastronomy/060605_mm_pegasids.html |series=Inside Exoplanets |title=Motley Crew of Worlds Share Common Thread |website=space.com|date=5 June 2006 }}
• {{cite web |archive-date=2013-06-18 |df=dmy-all |archive-url=https://web.archive.org/web/20130618082709/http://www.spitzer.caltech.edu/news/256-ssc2007-09-NASA-Finds-Extremely-Hot-Planet-Makes-First-Exoplanet-Weather-Map |url=http://www.spitzer.caltech.edu/news/256-ssc2007-09-NASA-Finds-Extremely-Hot-Planet-Makes-First-Exoplanet-Weather-Map |title=NASA finds extremely hot planet – makes first exoplanet weather map |publisher=California Institute of Technology}}
• {{cite journal |title=First known theoretical prediction regarding existence of hot Jupiters by Otto Struve in 1952|journal = The Observatory|volume = 72|pages = 199|bibcode = 1952Obs....72..199S|last1 = Struve|first1 = O.|year = 1952}}
• {{cite web |url=http://w.astro.berkeley.edu/~gmarcy/struve.html |title=Proposal for a Project of High-Precision Stellar Radial Velocity Work |first=Otto |last=Struve |author-link=Otto Struve |date=24 July 1952}}
• {{cite AV media |url=http://www.astronomycast.com/extrasolar-planets/hot-jupiters-and-pulsar-planets/ |medium=audio |last=Cain, Gay |title=Hot Jupiters and pulsar planets |date=September 2006 |website=astronomycast.com |series=Extrasolar Planets}}

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Category:Types of planet

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