Observations of comets, and especially exocomets, improve our understanding of
planet formation. Indeed, in the standard model of planet formation by accretion,
planets are the result of the agglomeration of
planetesimals, 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]
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,
cyanides,
sulfides and
pre-biotic molecules onto Earth-mass exoplanets with the help of exocomets.[17][18]
Nomenclature
The scientific term of an exocomet is Falling Evaporating Body (FEB).[6] The term Evaporating Infalling Bodies (EIBs) was first used,[19] but eventually the term FEBs was adopted from the "Falling Evaporating Bodies" model[20] or Falling Evaporating Body (FEB) scenario.[21]
Observation
The exocomets can be detected by
spectroscopy as they transit their host stars. The transits of exocomets, like the transits of
exoplanets, produce variations in the
light received from the
star. Changes are observed in the
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
redshifted 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.[8]
Observations of
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.[22]
Transiting exocomets were first detected around
KIC 3542116 and possibly
KIC 11084727 by a group of
citizen scientists and professional
astronomers. The
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.[6][23]TESS did observe transits of exocomets around Beta Pictoris.[24] 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 exoplanets.[25][26] A transiting exocomet around
HD 182952 (KIC 8027456) is the first exocomet found in an automated search for transiting exocomets.[7] Irregular dimming events around
KIC 8462852[5] have been interpreted as exocomets, but the shape of the dips are different from discovered exocomet transits.[24]
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.[27] Binary systems are another possible source of ejected exocomets.[28] These ejected exocomets belong to the
interstellar comets and can be observed directly if they enter the solar system.[29][30]
Observations of
β Pictoris with
TESS in 2022 led to the discovery of 30 new exocomets.[31]
Indirect evidence of exocomets
Exocomets are suggested as one source of
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 can be directed toward the inner stellar system. This is a consequence of the mass-loss during the
AGB stage.[32] The giant star will eventually become a white dwarf and an exocomet that gets too close to the white dwarf will sublimate or
tidal disrupted by the gravity of the white dwarf. This will produce dusty debris around the white dwarf, which is measurable in infrared wavelengths.[33] The material can be accreted by the white dwarf and pollute the atmosphere. This pollution appears in the spectra of a white dwarf as
metal lines.[34] In 2017 a study concluded that spectral lines in the white dwarf
WD 1425+540 are attributed to an accretion of a
Kuiper-Belt analog. Kuiper-Belt objects are icy bodies in the solar system that sometimes become comets.[35][36] Dusty material around the white dwarf
G 29-38 also has been attributed to an exocomet.[37]
A gaseous cloud around
49 Ceti has been attributed to the collisions of comets in that
planetary system.[38]
^
abWelsh, Barry; Montgomery, Sharon L. (January 2019). "Comet-like activity in the circumstellar debris disk surrounding the 1.4 Gyr-old F2V star HD 109085". AAS. 233: 340.06.
Bibcode:
2019AAS...23334006W.
^Matrà, Luca; Kral, Quentin; Su, Kate; Brandeker, Alexis; Dent, William; Gaspar, Andras; Kennedy, Grant; Marino, Sebastian; Öberg, Karin; Roberge, Aki; Wilner, David (2019-04-04). "Exocometary Science". Bulletin of the American Astronomical Society. 51 (3): 391.
arXiv:1904.02715.
Bibcode:
2019BAAS...51c.391M.
^Lagrange-Henri, A. M.; Gosset, E.; Beust, H.; Ferlet, R.; Vidal-Madjar, A. (October 1992). "The beta Pictoris circumstellar disk. XIII. Survey of the variable CA II lines". Astronomy and Astrophysics. 264: 637–653.
Bibcode:
1992A&A...264..637L.
ISSN0004-6361.
^Beust, H. (1994). "β Pictoris: The "Falling Evaporating Bodies" Model". CDDP. 10: 35.
Bibcode:
1994cddp.conf...35B.
^Vidal-Madjar, A.; Lagrange-Henri, A.-M.; Feldman, P. D.; Beust, H.; Lissauer, J. J.; Deleuil, M.; Ferlet, R.; Gry, C.; Hobbs, L. M.; McGrath, M. A.; McPhate, J. B. (October 1994). "HST-GHRS observations of β Pictoris: additional evidence for infalling comets". Astronomy and Astrophysics. 290: 245–258.
Bibcode:
1994A&A...290..245V.
ISSN0004-6361.
^Montesinos, B.; Eiroa, C.; Lillo-Box, J.; Rebollido, I.; Djupvik, A. A.; Absil, O.; Ertel, S.; Marion, L.; Kajava, J. J. E.; Redfield, S.; Isaacson, H.; Cánovas, H.; Meeus, G.; Mendigutía, I.; Mora, A.; Rivière-Marichalar, P.; Villaver, E.; Maldonado, J.; Henning, T. (September 2019). "HR 10: A main-sequence binary with circumstellar envelopes around both components. Discovery and analysis". Astronomy & Astrophysics. 629: A19.
arXiv:1907.12441.
Bibcode:
2019A&A...629A..19M.
doi:
10.1051/0004-6361/201936180.
ISSN0004-6361.
S2CID198967613.
^Lecavelier Des Etangs, A.; Vidal-Madjar, A.; Burki, G.; Lamers, H. J. G. L. M.; Ferlet, R.; Nitschelm, C.; Sevre, F. (December 1997). "Beta Pictoris light variations. I. The planetary hypothesis". Astronomy and Astrophysics. 328: 311.
Bibcode:
1997A&A...328..311L.
ISSN0004-6361.
^Jackson, Alan P.; Tamayo, Daniel; Hammond, Noah; Ali-Dib, Mohamad; Rein, Hanno (2018-07-21). "Ejection of rocky and icy material from binary star systems: implications for the origin and composition of 1I/'Oumuamua". Monthly Notices of the Royal Astronomical Society: Letters. 478 (1): L49–L53.
arXiv:1712.04435.
Bibcode:
2018MNRAS.478L..49J.
doi:
10.1093/mnrasl/sly033.
ISSN1745-3925.
S2CID102489895.