View source for Exocomet

{{distinguish|Interstellar comet}}
{{short description|Comet outside the Solar System}}
[[File:NASA-ExocometsAroundBetaPictoris-ArtistView.jpg|thumb|300px|{{center|Exocomets and various [[planet formation|planet-formation]] processes around [[Beta Pictoris]], a very young [[A-type main-sequence star]]<br />([[NASA]]; artist's conception).}}]]
An '''exocomet''', or '''extrasolar comet''', is a [[comet]] outside the [[Solar System]], which includes [[rogue comet]]s and comets that orbit [[star]]s other than the [[Sun]]. The first exocomets were detected in 1987<ref name="Ferlet et al.1987">{{cite journal | bibcode=1987A&A...185..267F |title=The Beta Pictoris circumstellar disk. V – Time variations of the CA II-K line|author1=Ferlet, R. |author2=Vidal-Madjar, A. |author3= Hobbs, L. M. |name-list-style=amp |year=1987|journal=[[Astronomy and Astrophysics]]|volume=185|pages=267–270}}</ref><ref name="Beust1990">{{cite journal |bibcode=1990A&A...236..202B |title=The Beta Pictoris circumstellar disk. X – Numerical simulations of infalling evaporating bodies |last1=Beust |first1=H. |last2=Lagrange-Henri |first2=A.M. |last3=Vidal-Madjar |first3=A. |last4=Ferlet |first4=R. |volume=236 |year=1990 |pages=202–216 |journal=Astronomy and Astrophysics}}</ref> around [[Beta Pictoris]], a very young [[A-type main-sequence star]]. There are now (as of February 2019) a total of 27 stars around which exocomets have been observed or suspected.<ref name=":2">{{Cite journal|last1=Welsh|first1=Barry Y.|last2=Montgomery|first2=Sharon L.|date=February 2018|title=Further detections of exocomet absorbing gas around Southern hemisphere A-type stars with known debris discs|journal=MNRAS|language=en|volume=474|issue=2|pages=1515–1525|doi=10.1093/mnras/stx2800|issn=0035-8711|bibcode=2018MNRAS.474.1515W|doi-access=free}}</ref><ref name=":3" /><ref name="arXiv:1509.03622">{{cite journal |title=Planet Hunters IX. KIC 8462852 – where's the flux? |journal=[[Monthly Notices of the Royal Astronomical Society]] |last1=Boyajian |author-link1=Tabetha S. Boyajian |first1=T. S. |last2=LaCourse |first2=D. M. |last3=Rappaport |first3=S. A. |last4=Fabrycky |first4=D. |last5=Fischer |first5=D. A. |last6=Gandolfi |first6=D. |last7=Kennedy |first7=G. M. |last8=Liu |first8=M. C. |last9=Moor |first9=A. |last10= Olah |first10=K. |last11=Vida |first11=K. |last12=Wyatt |first12=M. C. |last13=Best |first13=W. M. J. |last14=Ciesla |first14=F. |last15=Csak |first15=B. |last16=Dupuy |first16=T. J. |last17=Handler |first17=G. |last18=Heng |first18=K. |last19=Korhonen |first19=H. |last20= Kovacs |first20=J. |last21=Kozakis |first21=T. |last22=Kriskovics |first22=L. |last23=Schmitt |first23=J. R. |last24=Szabo |first24=Gy. |last25=Szabo |first25=R. |last26=Wang |first26=J. |last27=Goodman |first27=S. |last28=Hoekstra |first28=A. |last29=Jek |first29=K. J. |display-authors=1 |volume=457 |issue=4 |pages=3988–4004 |date=April 2016 |doi=10.1093/mnras/stw218 |doi-access=free |bibcode=2016MNRAS.457.3988B |arxiv=1509.03622|s2cid=54859232 }}</ref><ref name=":0" /><ref name=":4" />
The majority of discovered exocometary systems ([[Beta Pictoris]], [[HR 10]],<ref name="Lagrange-Henri et al.1990">{{cite journal|author1=Lagrange-Henri, A. M.|author2=Beust, H.|author3=Ferlet, R.|author4=Vidal-Madjar, A.|author5=Hobbs, L. M.|name-list-style=amp|year=1990|title=HR 10 – A new Beta Pictoris-like star?|journal=[[Astronomy and Astrophysics]]|volume=227|pages=L13–L16|bibcode=1990A&A...227L..13L}}</ref> [[51 Ophiuchi]], [[HR 2174]],<ref name="Lecavelier Des Etangs et al.1997">{{cite journal|author=Lecavelier Des Etangs, A.|display-authors=etal|year=1997|title=HST-GHRS observations of candidate β Pictoris-like circumstellar gaseous disks.|journal=[[Astronomy and Astrophysics]]|volume=325|pages=228–236|bibcode=1997A&A...325..228L}}</ref> [[HD 85905]],<ref>{{Cite journal|last1=Welsh|first1=B. Y.|last2=Craig|first2=N.|last3=Crawford|first3=I. A.|last4=Price|first4=R. J.|date=1998-10-01|title=Beta Pic-like circumstellar disk gas surrounding HR 10 and HD 85905|url=http://adsabs.harvard.edu/abs/1998A%26A...338..674W|journal=Astronomy and Astrophysics|volume=338|pages=674–682|bibcode=1998A&A...338..674W|issn=0004-6361}}</ref> [[49 Ceti]], [[5 Vulpeculae]], [[2 Andromedae]], [[HD 21620]], [[Rho Virginis]], [[HD 145964]],<ref name="Welsh et al.2013">{{cite journal|author1=Welsh, B. Y.|author2=Montgomery, S.|year=2013|title=Circumstellar Gas-Disk Variability Around A-Type Stars: The Detection of Exocomets?|journal=[[Publications of the Astronomical Society of the Pacific]]|volume=125|issue=929|pages=759–774|bibcode=2013PASP..125..759W|doi=10.1086/671757|doi-access=free}}</ref><ref name="Space-20130107">{{cite web|title='Exocomets' Common Across Milky Way Galaxy |url=http://www.space.com/19156-exocomets-alien-solar-systems.html |date=7 January 2013 |publisher=[[Space.com]] |access-date=8 January 2013 |url-status=dead |archive-url=https://web.archive.org/web/20140916085824/http://www.space.com/19156-exocomets-alien-solar-systems.html |archive-date=16 September 2014 }}</ref> [[HD 172555]],<ref name="Kiefer et al.2014">{{cite journal|author1=Kiefer, F.|author2=Lecavelier Des Etangs, A.|display-authors=etal|year=2014|title=Exocomets in the circumstellar gas disk of HD 172555|journal=[[Astronomy and Astrophysics]]|volume=561|pages=L10|arxiv=1401.1365|bibcode=2014A&A...561L..10K|doi=10.1051/0004-6361/201323128|s2cid=118533377}}</ref> [[Lambda Geminorum]], [[HD 58647]], [[Phi Geminorum]], [[Delta Corvi]], [[HR 4796|HD 109573]],<ref name=":5">{{Cite journal|last1=Welsh|first1=Barry Y.|last2=Montgomery|first2=Sharon L.|date=2015|title=The Appearance and Disappearance of Exocomet Gas Absorption|journal=Advances in Astronomy|volume=2015|pages=980323|doi=10.1155/2015/980323|bibcode=2015AdAst2015E..26W|doi-access=free}}</ref> [[Phi Leonis]],<ref name=":6" /> [[35 Aquilae]],<ref>{{Cite journal|last1=Montgomery|first1=Sharon L.|last2=Welsh|first2=Barry Y.|date=2017-06-01|title=Unusually high circumstellar absorption variability around the delta Scuti /lambda Boötis star HD 183324|journal=Monthly Notices of the Royal Astronomical Society|volume=468|issue=1 |pages=L55–L58|doi=10.1093/mnrasl/slx016|doi-access=free |bibcode=2017MNRAS.468L..55M|issn=0035-8711}}</ref> [[HD 24966]], [[HD 38056]], [[HD 79469]] and [[HD 225200]]<ref name=":2" />) are around very young [[Stellar classification#Class A|A-type stars]]. The relatively old shell star [[Phi Leonis]] shows evidence of exocomets in the spectrum<ref name=":6">{{Cite journal|last1=Eiroa|first1=C.|last2=Rebollido|first2=I.|last3=Montesinos|first3=B.|last4=Villaver|first4=E.|last5=Absil|first5=O.|last6=Henning|first6=Th|last7=Bayo|first7=A.|last8=Canovas|first8=H.|last9=Carmona|first9=A.|last10=Chen|first10=Ch|last11=Ertel|first11=S.|date=2016-10-01|title=Exocomet signatures around the A-shell star φ Leonis?|journal=Astronomy & Astrophysics|language=en|volume=594|pages=L1|arxiv=1609.04263|doi=10.1051/0004-6361/201629514|bibcode=2016A&A...594L...1E|s2cid=41231308|issn=0004-6361}}</ref> and comet-like activity was detected around the old F2V-type star [[Eta Corvi]].<ref name=":3">{{Cite journal|last1=Welsh|first1=Barry|last2=Montgomery|first2=Sharon L.|date=January 2019|title=Comet-like activity in the circumstellar debris disk surrounding the 1.4 Gyr-old F2V star HD 109085|journal=AAS|language=en|volume=233|pages=340.06|bibcode=2019AAS...23334006W}}</ref> In 2018 transiting exocomets were discovered around [[F-type star]]s, using data from the [[Kepler space telescope]].<ref name=":0">{{Cite journal|last1=Rappaport|first1=S.|last2=Vanderburg|first2=A.|last3=Jacobs|first3=T.|last4=LaCourse|first4=D.|last5=Jenkins|first5=J.|last6=Kraus|first6=A.|last7=Rizzuto|first7=A.|last8=Latham|first8=D. W.|last9=Bieryla|first9=A.|last10=Lazarevic|first10=M.|last11=Schmitt|first11=A.|date=2018-02-21|title=Likely transiting exocomets detected by Kepler|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=474|issue=2|pages=1453–1468|arxiv=1708.06069|doi=10.1093/mnras/stx2735|doi-access=free |pmid=29755143|pmc=5943639|issn=0035-8711|bibcode=2018MNRAS.474.1453R}}</ref> Some late B-type star (e.g. 51 Ophiuchi, HD 58647) are known to host exocomets.<ref name=":5" /><ref name="Lecavelier Des Etangs et al.1997" />
Observations of comets, and especially exocomets, improve our understanding of [[planet formation]]. Indeed, in the standard model of planet formation by accretion, [[planet]]s are the result of the agglomeration of [[planetesimal]]s, themselves formed by the coalescence of dust from the [[protoplanetary disk]] surrounding the star shortly after its formation. Thus, comets are the residuals of the volatile-rich planetesimals that remained in the planetary system without having been incorporated into the planets. They are considered fossil bodies that have seen the physical and chemical conditions prevailing at the time of planet formation.{{citation needed|date=October 2014}}
Researching exocomets might provide answers to fundamental questions of the past of the solar system and the development of a life-supporting environment. Researchers can investigate the transport of [[water]], [[cyanide]]s, [[sulfide]]s and [[Abiogenesis|pre-biotic molecules]] onto Earth-mass exoplanets with the help of exocomets.<ref>{{Cite journal|last1=Cuntz|first1=Manfred|last2=Loibnegger|first2=Birgit|last3=Dvorak|first3=Rudolf|date=2018-11-30|title=Exocomets in the 47 UMa System: Theoretical Simulations Including Water Transport|journal=The Astronomical Journal|language=en|volume=156|issue=6|pages=290|arxiv=1811.09579|doi=10.3847/1538-3881/aaeac7|issn=1538-3881|bibcode=2018AJ....156..290C|s2cid=118921188 |doi-access=free }}</ref><ref>{{cite journal|last1=Matrà|first1=Luca|last2=Kral|first2=Quentin|last3=Su|first3=Kate|last4=Brandeker|first4=Alexis|last5=Dent|first5=William|last6=Gaspar|first6=Andras|last7=Kennedy|first7=Grant|last8=Marino|first8=Sebastian|last9=Öberg|first9=Karin|last10=Roberge|first10=Aki|last11=Wilner|first11=David|date=2019-04-04|title=Exocometary Science|journal=Bulletin of the American Astronomical Society|volume=51|issue=3|pages=391|arxiv=1904.02715|bibcode=2019BAAS...51c.391M}}</ref>
== Nomenclature ==
The scientific term of an exocomet is Falling Evaporating Body (FEB).<ref name=":0" /> The term Evaporating Infalling Bodies (EIBs) was first used,<ref>{{Cite journal|last1=Lagrange-Henri|first1=A. M.|last2=Gosset|first2=E.|last3=Beust|first3=H.|last4=Ferlet|first4=R.|last5=Vidal-Madjar|first5=A.|date=October 1992|title=The beta Pictoris circumstellar disk. XIII. Survey of the variable CA II lines.|journal= Astronomy and Astrophysics|language=en|volume=264|pages=637–653|issn=0004-6361|bibcode=1992A&A...264..637L}}</ref> but eventually the term FEBs was adopted from the "Falling Evaporating Bodies" model<ref>{{Cite journal|last=Beust|first=H.|date=1994|title=β Pictoris: The "Falling Evaporating Bodies" Model|journal=CDDP|language=en|volume=10|pages=35|bibcode=1994cddp.conf...35B}}</ref> or Falling Evaporating Body (FEB) scenario.<ref>{{Cite journal|last1=Vidal-Madjar|first1=A.|last2=Lagrange-Henri|first2=A.-M.|last3=Feldman|first3=P. D.|last4=Beust|first4=H.|last5=Lissauer|first5=J. J.|last6=Deleuil|first6=M.|last7=Ferlet|first7=R.|last8=Gry|first8=C.|last9=Hobbs|first9=L. M.|last10=McGrath|first10=M. A.|last11=McPhate|first11=J. B.|date=October 1994|title=HST-GHRS observations of β Pictoris: additional evidence for infalling comets|journal= Astronomy and Astrophysics|language=en|volume=290|pages=245–258|issn=0004-6361|bibcode=1994A&A...290..245V}}</ref>
== Observation ==
The exocomets can be detected by [[spectroscopy]] as they transit their host stars. The transits of exocomets, like the transits of [[exoplanet]]s, produce variations in the [[light]] received from the [[star]]. Changes are observed in the [[Absorption spectroscopy|absorption lines]] of the stellar spectrum: the occultation of the star by the gas cloud coming from the exocomet produces additional absorption features beyond those normally seen in that star, like those observed in the ionized [[calcium]] lines. As the comet comes close enough to the star, cometary gas is evolved from the evaporation of volatile ices and dust with it. The absorption lines of a star hosting exocomets represent, beside a stable component, one or several variable [[redshift]]ed components. The variable components change on short-time scales of one hour. The variable component represent the exocomets. The exocomet falls towards the star and any absorption line produced by the evaporation of the exocomet is redshifted compared to the absorption line of the star.<ref name="Lagrange-Henri et al.1990" />
Observations of [[HD 256|HR 10]] with the [[PIONIER (VLTI)]] and 32 years of radial velocity observations revealed that this exocomet host candidate turned out to be a [[binary star]] with each star being surrounded by a circumstellar shell. This new result can explain the variable spectral lines without exocomets. The study points out that 50% of the A-type stars could be resolved into binaries in the future and more systems with variable spectral lines attributed to exocomets could turn out to be binaries.<ref>{{Cite journal|last1=Montesinos|first1=B.|last2=Eiroa|first2=C.|last3=Lillo-Box|first3=J.|last4=Rebollido|first4=I.|last5=Djupvik|first5=A. A.|last6=Absil|first6=O.|last7=Ertel|first7=S.|last8=Marion|first8=L.|last9=Kajava|first9=J. J. E.|last10=Redfield|first10=S.|last11=Isaacson|first11=H.|last12=Cánovas|first12=H.|last13=Meeus|first13=G.|last14=Mendigutía|first14=I.|last15=Mora|first15=A.|last16=Rivière-Marichalar|first16=P.|last17=Villaver|first17=E.|last18=Maldonado|first18=J.|last19=Henning|first19=T.|date=September 2019|title=HR 10: A main-sequence binary with circumstellar envelopes around both components. Discovery and analysis|journal=Astronomy & Astrophysics|volume=629|pages=A19|doi=10.1051/0004-6361/201936180|issn=0004-6361|arxiv=1907.12441|bibcode=2019A&A...629A..19M|s2cid=198967613}}</ref>
[[Transit (astronomy)|Transiting]] exocomets were first detected around [[KIC 3542116]] and possibly [[KIC 11084727]] by a group of [[Citizen science|citizen scientists]] and professional [[astronomer]]s. The [[Kepler space telescope|Kepler mission]] detected asymmetrical dips around KIC 3542116, a F2V-type star that are consistent with models of transiting exocomets. The dips were found by one of the authors, a [[Planet Hunters]] participant, in a visual search over 5 months of the complete Q1-Q17 ''Kepler'' light curve archive spanning 201250 target stars.<ref name=":0" /><ref>{{Cite web|url=https://www.newsweek.com/comets-detected-outside-our-solar-system-first-time-696446|title=Astronomers have detected comets outside our solar system for the first time ever|last=EDT|first=Meghan Bartels On 10/30/17 at 2:24 PM|date=2017-10-30|website=Newsweek|language=en|access-date=2019-11-12}}</ref> [[Transiting Exoplanet Survey Satellite|TESS]] did observe transits of exocomets around Beta Pictoris.<ref name=":1">{{Cite journal|last1=Zieba|first1=S.|last2=Zwintz|first2=K.|last3=Kenworthy|first3=M. A.|last4=Kennedy|first4=G. M.|date=2019-05-01|title=Transiting exocomets detected in broadband light by TESS in the β Pictoris system|journal=Astronomy & Astrophysics|language=en|volume=625|pages=L13|arxiv=1903.11071|doi=10.1051/0004-6361/201935552|issn=0004-6361|bibcode=2019A&A...625L..13Z|s2cid=85529617}}</ref> The shape of a dip caused by a transiting exocomet is modelled as a very specific "rounded triangular" shape and can be distinguished from most [[transiting exoplanet]]s.<ref>{{Cite journal|last1=Lecavelier Des Etangs|first1=A.|last2=Vidal-Madjar|first2=A.|last3=Burki|first3=G.|last4=Lamers|first4=H. J. G. L. M.|last5=Ferlet|first5=R.|last6=Nitschelm|first6=C.|last7=Sevre|first7=F.|date=December 1997|title=Beta Pictoris light variations. I. The planetary hypothesis|journal=Astronomy and Astrophysics|language=en|volume=328|pages=311|bibcode=1997A&A...328..311L|issn=0004-6361}}</ref><ref>{{Cite journal|last1=Lecavelier Des Etangs|first1=A.|last2=Vidal-Madjar|first2=A.|last3=Ferlet|first3=R.|date=21 Dec 1998|title=Photometric stellar variation due to extra-solar comets|journal=Astronomy and Astrophysics|language=en|volume=343|pages=916|arxiv=astro-ph/9812381|bibcode=1999A&A...343..916L|issn=0004-6361}}</ref> A transiting exocomet around [[HD 182952]] (KIC 8027456) is the first exocomet found in an automated search for transiting exocomets.<ref name=":4">{{Cite journal|last1=Kennedy|first1=Grant M.|last2=Hope|first2=Greg|last3=Hodgkin|first3=Simon T.|last4=Wyatt|first4=Mark C.|date=2019-02-01|title=An automated search for transiting exocomets|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=482|issue=4|pages=5587–5596|arxiv=1811.03102|doi=10.1093/mnras/sty3049|doi-access=free |issn=0035-8711|bibcode=2019MNRAS.482.5587K|s2cid=53691133}}</ref> Irregular dimming events around [[Tabby's Star|KIC 8462852]]<ref name="arXiv:1509.03622" /> have been interpreted as exocomets, but the shape of the dips are different from discovered exocomet transits.<ref name=":1" />
During formation of the Oort Cloud through planetary perturbations, stellar encounters, and the galactic tide, a comet can be ejected and leave the solar system.<ref>{{Cite journal|last1=Duncan|first1=M.|last2=Quinn|first2=T.|last3=Tremaine|first3=S.|date=November 1987|title=The Formation and Extent of the Solar System Comet Cloud|journal=The Astronomical Journal|language=en|volume=94|pages=1330|doi=10.1086/114571|issn=0004-6256|bibcode=1987AJ.....94.1330D|doi-access=free}}</ref> Binary systems are another possible source of ejected exocomets.<ref>{{Cite journal|last1=Jackson|first1=Alan P.|last2=Tamayo|first2=Daniel|last3=Hammond|first3=Noah|last4=Ali-Dib|first4=Mohamad|last5=Rein|first5=Hanno|date=2018-07-21|title=Ejection of rocky and icy material from binary star systems: implications for the origin and composition of 1I/'Oumuamua|journal=Monthly Notices of the Royal Astronomical Society: Letters|language=en|volume=478|issue=1|pages=L49–L53|arxiv=1712.04435|doi=10.1093/mnrasl/sly033|doi-access=free |bibcode=2018MNRAS.478L..49J|s2cid=102489895|issn=1745-3925}}</ref> These ejected exocomets belong to the [[interstellar comets]] and can be observed directly if they enter the solar system.<ref>{{cite journal|last1=Hallatt|first1=Tim|last2=Wiegert|first2=Paul|title=The Dynamics of Interstellar Asteroids and Comets within the Galaxy: an Assessment of Local Candidate Source Regions for 1I/'Oumuamua and 2I/Borisov|journal=The Astronomical Journal|year=2020|volume=159|issue=4|page=147|doi=10.3847/1538-3881/ab7336|arxiv=1911.02473|bibcode=2020AJ....159..147H|s2cid=207772669 |doi-access=free }}</ref><ref>{{cite journal|last1=Bolin|first1=Bryce T.|last2=Lisse|first2=Carey M.|last3=Kasliwal|first3=Mansi M.|last4=Quimby|first4=Robert|last5=Tan|first5=Hanjie|last6=Copperwheat|first6=Chris|last7=Lin|first7=Zhong-Yi|last8=Morbidelli|first8=Alessandro|last9=Bauer|first9=James|last10=Burdge|first10=Kevin B.|last11=Coughlin|first11=Michael|title=Characterization of the Nucleus, Morphology and Activity of Interstellar Comet 2I/Borisov by Optical and Near-Infrared GROWTH, Apache Point, IRTF, ZTF and Keck Observations|journal=The Astronomical Journal|year=2020|volume=160|issue=1 |page=26|doi=10.3847/1538-3881/ab9305|arxiv=1910.14004|bibcode=2020AJ....160...26B |s2cid=204960829 |doi-access=free }}</ref>
Observations of [[β Pictoris]] with [[TESS]] in 2022 led to the discovery of 30 new exocomets.<ref>{{cite web |last1=Maginiot |first1=François |last2=Lecavelier des Etangs |first2=Alain |title=Discovery of 30 exocomets in a young planetary system |url=https://www.cnrs.fr/en/discovery-30-exocomets-young-planetary-system |website=CNRS |date=28 April 2022 |access-date=14 May 2022}}</ref>
=== Indirect evidence of exocomets ===
Exocomets are suggested as one source of [[White dwarf#Debris disks and planets|white dwarf pollution]]. After a star from the main sequence becomes a giant star, it loses mass. Planetesimals in an analog of the solar [[Oort cloud|Oort Cloud]] can be directed toward the inner stellar system. This is a consequence of the mass-loss during the [[Asymptotic giant branch|AGB stage]].<ref>{{Cite journal|last1=Caiazzo|first1=Ilaria|last2=Heyl|first2=Jeremy S.|date=2017-08-11|title=Polluting white dwarfs with perturbed exo-comets|url=https://academic.oup.com/mnras/article/469/3/2750/3786428|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=469|issue=3|pages=2750–2759|arxiv=1702.07682|doi=10.1093/mnras/stx1036|doi-access=free |issn=0035-8711|bibcode=2017MNRAS.469.2750C|s2cid=119482670}}</ref> The giant star will eventually become a white dwarf and an exocomet that gets too close to the white dwarf will sublimate or [[Roche limit|tidal disrupted]] by the gravity of the white dwarf. This will produce dusty debris around the white dwarf, which is measurable in infrared wavelengths.<ref>{{Cite journal|last1=Stone|first1=Nicholas|last2=Metzger|first2=Brian D.|last3=Loeb|first3=Abraham|date=2015-03-21|title=Evaporation and accretion of extrasolar comets following white dwarf kicks|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=448|issue=1|pages=188–206|arxiv=1404.3213|doi=10.1093/mnras/stu2718|doi-access=free |issn=0035-8711|bibcode=2015MNRAS.448..188S|s2cid=118616060}}</ref> The material can be accreted by the white dwarf and pollute the atmosphere. This pollution appears in the spectra of a white dwarf as [[White dwarf#Metal-rich white dwarfs|metal lines]].<ref>{{Cite journal|last1=Zuckerman|first1=B.|last2=Koester|first2=D.|last3=Reid|first3=I. N.|last4=Hünsch|first4=M.|date=2003-09-10|title=Metal Lines in DA White Dwarfs*|journal=The Astrophysical Journal|language=en|volume=596|issue=1|pages=477|doi=10.1086/377492|issn=0004-637X|bibcode=2003ApJ...596..477Z|doi-access=free}}</ref> In 2017 a study concluded that spectral lines in the white dwarf [[WD 1425+540]] are attributed to an accretion of a [[Kuiper belt|Kuiper-Belt]] analog. Kuiper-Belt objects are icy bodies in the solar system that sometimes become comets.<ref name="ESO white dwarf">{{Cite web|url=https://www.spacetelescope.org/news/heic1703/|title=Hubble finds big brother of Halley's Comet ripped apart by white dwarf|last=|website=www.spacetelescope.org|language=en|access-date=2019-12-27}}</ref><ref>{{Cite journal|last1=Xu (许偲艺)|first1=S.|last2=Zuckerman|first2=B.|last3=Dufour|first3=P.|last4=Young|first4=E. D.|last5=Klein|first5=B.|last6=Jura|first6=M.|date=2017-02-09|title=The Chemical Composition of an Extrasolar Kuiper-Belt-Object|journal=The Astrophysical Journal|language=en|volume=836|issue=1|pages=L7|doi=10.3847/2041-8213/836/1/l7|issn=2041-8213|bibcode=2017ApJ...836L...7X|arxiv=1702.02868|s2cid=39461293 |doi-access=free }}</ref> Dusty material around the white dwarf [[G 29-38]]<ref>{{Cite web|url=http://www.jpl.nasa.gov/news/news.php?feature=1011|title=NASA's Spitzer Finds Possible Comet Dust Around Dead Star|website=NASA/JPL|access-date=2019-12-27}}</ref> and [[WD 1337+705]]<ref>{{Cite journal |last1=Johnson |first1=Ted M. |last2=Klein |first2=Beth L. |last3=Koester |first3=D. |last4=Melis |first4=Carl |last5=Zuckerman |first5=B. |last6=Jura |first6=M. |date=2022-12-01 |title=Unusual Abundances from Planetary System Material Polluting the White Dwarf G238-44 |journal=The Astrophysical Journal |volume=941 |issue=2 |pages=113 |arxiv=2211.02673 |bibcode=2022ApJ...941..113J |doi=10.3847/1538-4357/aca089 |issn=0004-637X |doi-access=free}}</ref> also has been attributed to an exocomet.
[[Carbon monoxide]] gas was found in [[Debris disk|debris disks]] around mostly A-type stars with an age between 10 and 50 Myrs, but in some cases in older systems (e.g. Eta Corvi 1-2 Gyrs) and in colder systems (TWA 7). It is not clear if this gas is primordial or secondary produced by collision of exocomets. Around 30 such systems exist.<ref>{{Cite journal |last1=Cataldi |first1=Gianni |last2=Aikawa |first2=Yuri |last3=Iwasaki |first3=Kazunari |last4=Marino |first4=Sebastian |last5=Brandeker |first5=Alexis |last6=Hales |first6=Antonio |last7=Henning |first7=Thomas |last8=Higuchi |first8=Aya E. |last9=Hughes |first9=A. Meredith |last10=Janson |first10=Markus |last11=Kral |first11=Quentin |last12=Matrà |first12=Luca |last13=Moór |first13=Attila |last14=Olofsson |first14=Göran |last15=Redfield |first15=Seth |date=2023-07-01 |title=Primordial or Secondary? Testing Models of Debris Disk Gas with ALMA |journal=The Astrophysical Journal |volume=951 |issue=2 |pages=111 |arxiv=2305.12093 |bibcode=2023ApJ...951..111C |doi=10.3847/1538-4357/acd6f3 |doi-access=free |issn=0004-637X}}</ref> Carbon monoxide gas around [[49 Ceti]] has been attributed to the collisions of comets in that [[planetary system]].<ref>{{cite journal|last1=Zuckerman|first1=B.|last2=Song|first2=Inseok|year=2012|title=A 40 Myr Old Gaseous Circumstellar Disk at 49 Ceti: Massive CO-Rich Comet Clouds at Young A-Type Stars|journal=The Astrophysical Journal|volume=758|issue=2|pages=77|arxiv=1207.1747|doi=10.1088/0004-637X/758/2/77|bibcode=2012ApJ...758...77Z|s2cid=119198485}}</ref>
== Gallery ==
<gallery>
PIA22357-InterstellarObject-'Oumuamua-ExitsSolarSystem.jpg|[[Interstellar object]] [['Oumuamua]] exiting the [[Solar System]] (artist concept) ([https://photojournal.jpl.nasa.gov/archive/PIA22357_JPL-20180620-ASTRDSf-0007-Interstellar%20Asteroid%20animation-720p.mp4 animation])
Exocomets plunging into a young star (artist’s impression).jpg|Artist's impression of exocomets plunging into young star [[HD 172555]].<ref>{{cite web|title=Exocomets plunging into a young star (artist's impression)|url=https://www.spacetelescope.org/images/opo1702a/|website=www.spacetelescope.org|access-date=12 January 2017}}</ref>
Artist’s impression of exocomets around Beta Pictoris.ogv|Video – artist's impression of exocomets orbiting the star [[Beta Pictoris]].
Comet falling into white dwarf.jpg|Artist's impression of an exocomet falling into white dwarf [[WD 1425+540]].<ref name="ESO white dwarf" />
PIA20053-PossibleCometSwarmAroundKIC8462852-ArtistConcept-20151124.jpg|Artist's concept of a cloud of disintegrating exocomets around KIC 8462852 ([[Tabby's Star]]).
</gallery>
== See also ==
{{Commons category|Exocomets}}
{{div col|colwidth=25em}}
* {{annotated link|Exomoon}}
* {{annotated link|Exoplanet}}
* {{annotated link|Interstellar object}}
* {{annotated link|Kepler space telescope}}
* {{annotated link|List of stars that dim oddly}}
* {{annotated link|Rogue planet}}
* {{annotated link|2I/Borisov}}
{{div col end}}
== References ==
{{Reflist|colwidth=30em}}
== External links ==
{{Wiktionary|Exocomet}}
* [https://nasasearch.nasa.gov/search?query=extrasolar+comet&affiliate=nasa&utf8=%E2%9C%93 Extrasolar Comets – NASA]
{{Comets|nonobject=yes}}
{{Asteroids}}
{{Exoplanets}}
{{portal bar|Astronomy|Space}}
[[Category:Articles containing video clips]]
[[Category:Comets]]
[[Category:Exoplanetology]]