Protoplanet
{{Short description|Large planetary embryo}}
A protoplanet is a large planetary embryo that originated within a protoplanetary disk and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitationally perturb each other's orbits and collide, gradually coalescing into the dominant planets.
The planetesimal hypothesis
A planetesimal is an object formed from dust, rock, and other materials, measuring from meters to hundreds of kilometers in size.
According to the Chamberlin–Moulton planetesimal hypothesis and the theories of Viktor Safronov, a protoplanetary disk of materials such as gas and dust would orbit a star early in the formation of a planetary system. The action of gravity on such materials form larger and larger chunks until some reach the size of planetesimals.{{cite web |last1=Cessna |first1=Abby |title=Planetesimals |url=https://www.universetoday.com/35974/planetesimals/ |website=Universe Today |access-date=5 April 2022 |date=26 July 2009}}{{cite journal |last1=Ahrens |first1=T J |title=Impact Erosion of Terrestrial Planetary Atmospheres |journal=Annual Review of Earth and Planetary Sciences |date=1 May 1993 |volume=21 |issue=1 |pages=525–555 |doi=10.1146/annurev.ea.21.050193.002521 |bibcode=1993AREPS..21..525A |url=https://doi.org/10.1146/annurev.ea.21.050193.002521 |access-date=5 April 2022 |issn=0084-6597|hdl=2060/19920021677 |hdl-access=free }}
It is thought that the collisions of planetesimals created a few hundred larger planetary embryos. Over the course of hundreds of millions of years, they collided with one another. The exact sequence whereby planetary embryos collided to assemble the planets is not known, but it is thought that initial collisions would have replaced the first "generation" of embryos with a second generation consisting of fewer but larger embryos. These in their turn would have collided to create a third generation of fewer but even larger embryos. Eventually, only a handful of embryos were left, which collided to complete the assembly of the planets proper.{{cite book|title=An Introduction to the Solar System|pages=56|first=Neil|last=McBride|author2=Iain Gilmour |author3=Philip A. Bland |author4=Elaine A. Moore |author5=Mike Widdowson |author6=Ian Wright |publisher=Cambridge University Press|location=Cambridge|year=2004|isbn=9780521837354}}
Early protoplanets had more radioactive elements,{{cite web|url=https://www.universetoday.com/37053/protoplanets/|title=Protoplanets
|first=Abby|last=Cessna|year=2009|publisher=Universe Today}} the quantity of which has been reduced over time due to radioactive decay. Heating due to radioactivity, impact, and gravitational pressure melted parts of protoplanets as they grew toward being planets. In melted zones their heavier elements sank to the center, whereas lighter elements rose to the surface. Such a process is known as planetary differentiation. The composition of some meteorites show that differentiation took place in some asteroids.
Evidence in the Solar System - surviving remnant protoplanets
In the case of the Solar System, it is thought that the collisions of planetesimals created a few hundred planetary embryos. Such embryos were similar to Ceres and Pluto with masses of about 1022 to 1023 kg and were a few thousand kilometers in diameter.{{fact|date=October 2022}}
According to the giant impact hypothesis, the Moon formed from a colossal impact of a hypothetical protoplanet called Theia with Earth, early in the Solar System's history.{{cite web
|last1=Nace |first1=Trevor
|title = New Evidence For 4.5 Billion Year Old Impact Formed Our Moon
|work = Forbes
|url = https://www.forbes.com/sites/trevornace/2016/01/30/new-evidence-4-5-billion-year-old-impact-formed-moon/
|date = 2016-01-30
|access-date = 2016-01-30
}}{{cite journal |last1=Young |first1=E. D. |last2=Kohl |first2=I. E. |last3=Warren |first3=P. H. |last4=Rubie |first4=D. C. |last5=Jacobson |first5=S. A. |last6=Morbidelli |first6=A. |title=Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact |journal=Science |date=28 January 2016 |volume=351 |issue=6272 |pages=493–496 |doi=10.1126/science.aad0525|pmid=26823426 |arxiv=1603.04536 |bibcode=2016Sci...351..493Y |s2cid=6548599 }}{{cite web|last1=Wolpert|first1=Stuart|title=Moon was produced by a head-on collision between Earth and a forming planet|date=January 28, 2016|url=http://newsroom.ucla.edu/releases/moon-was-produced-by-a-head-on-collision-between-earth-and-a-forming-planet|website=UCLA newsroom|publisher=UCLA}}
In the inner Solar System, the three protoplanets to survive more-or-less intact are the asteroids Ceres, Pallas, and Vesta. Psyche is likely the survivor of a violent hit-and-run with another object that stripped off the outer, rocky layers of a protoplanet.{{cite web|url=http://www.nasa.gov/press-release/nasa-selects-investigations-for-future-key-planetary-mission|title=NASA Selects Investigations for Future Key Planetary Mission|date=30 September 2015 }} The asteroid Metis may also have a similar origin history to that of Psyche.{{cite journal|last=Kelley |first=Michael S|author2=Michael J. Gaffey
|title=9 Metis and 113 Amalthea: A Genetic Asteroid Pair|journal=Icarus|volume=144 |issue=1 |pages=27–38 |date=2000|doi=10.1006/icar.1999.6266 |bibcode=2000Icar..144...27K}} The asteroid Lutetia also has characteristics that resemble a protoplanet.{{cite web
|title = BIG PIC: 2 Pallas, the Asteroid with Protoplanetary Attitude|work = Discovery Space| publisher = Discovery Communications| date = 2009-10-08| url = http://dsc.discovery.com/space/big-pic/hubble-pallas-asteroid-protoplanet.html| access-date = 2009-10-08}}{{cite web| last = Klotz| first = Irene| title = ASTEROID FAILS TO MAKE IT BIG: A newly studied asteroid is actually a planetary building block that stopped growing.| work = Discovery News| publisher = Discovery Communications| date = 2011-10-27|url=http://news.discovery.com/space/asteroid-protoplanet-lutetia-rosetta-111027.html| access-date = 2011-10-27}} Kuiper-belt dwarf planets have also been referred to as protoplanets.{{cite web|date=2009-10-08|title=Protoplanet frozen in time|publisher=MSNBC|author=Alan Boyle|url=http://cosmiclog.msnbc.msn.com/archive/2009/10/08/2092402.aspx|archive-url=https://web.archive.org/web/20091010023833/http://cosmiclog.msnbc.msn.com/archive/2009/10/08/2092402.aspx|url-status=dead|archive-date=2009-10-10|access-date=2009-09-12}} Because iron meteorites have been found on Earth, it is deemed likely that there once were other metal-cored protoplanets in the asteroid belt that since have been disrupted and that are the source of these meteorites.{{fact|date=October 2022}}
Extrasolar protoplanets - observed protoplanets
The first directly imaged exoplanet candidates were confirmed in 2005. Several of them are very young, DH Tauri b, GQ Lupi b, 2M1207b and show signs of accretion. However, all these candidates either lack in confirmation of a planetary mass or in confirmation that they formed within the protoplanetary disk of the host object.
In January 2012 astronomers made the first direct observation of a candidate protoplanet forming in a disk of gas and dust around a distant star, LkCa 15. Subsequent observations, however, refuted the existence of this candidate.
In February 2013 astronomers made the first direct observation of a candidate protoplanet, that is still a candidate, forming in a disk of gas and dust around a distant star, HD 100546.{{cite web|url=http://www.eso.org/public/news/eso1310/|title=The Birth of a Giant Planet?|publisher=European Southern Observatory|date=28 February 2013|access-date=2 March 2013}}{{cite journal |author=Quanz, Sascha P. |author2=Amara, Adam |author3=Meyer, Michael P. |author4=Kenworthy, Matthew P. |author5=Kasper, Markus |author6=Girard, Julien H. |display-authors=4 |date=2013 |title=A young protoplanet candidate embedded in the circumstellar disk of HD 100546 |journal=Astrophysical Journal |volume=766 |issue=1 |at=L1 |arxiv=1302.7122 |bibcode=2013ApJ...766L...1Q |doi=10.1088/2041-8205/766/1/l1 |s2cid=56140977}} Subsequent observations suggest that several protoplanets may be present in the gas disk.{{cite journal |last1=Garufi |first1=A. |last2=Quanz |first2=S. P. |last3=Schmid |first3=H. M. |last4=Mulders |first4=G. D. |last5=Avenhaus |first5=H. |last6=Boccaletti |first6=A. |last7=Ginski |first7=C. |last8=Langlois |first8=M. |last9=Stolker |first9=T. |last10=Augereau |first10=J.-C. |last11=Benisty |first11=M. |last12=Lopez |first12=B. |last13=Dominik |first13=C. |last14=Gratton |first14=R. |last15=Henning |first15=T. |last16=Janson |first16=M. |last17=Ménard |first17=F. |last18=Meyer |first18=M. R. |last19=Pinte |first19=C. |last20=Sissa |first20=E. |last21=Vigan |first21=A. |last22=Zurlo |first22=A. |last23=Bazzon |first23=A. |last24=Buenzli |first24=E. |last25=Bonnefoy |first25=M. |last26=Brandner |first26=W. |last27=Chauvin |first27=G. |last28=Cheetham |first28=A. |last29=Cudel |first29=M. |last30=Desidera |first30=S. |last31=Feldt |first31=M. |last32=Galicher |first32=R. |last33=Kasper |first33=M. |last34=Lagrange |first34=A.-M. |last35=Lannier |first35=J. |last36=Maire |first36=A. L. |last37=Mesa |first37=D. |last38=Mouillet |first38=D. |last39=Peretti |first39=S. |last40=Perrot |first40=C. |last41=Salter |first41=G. |last42=Wildi |first42=F. |title=The SPHERE view of the planet-forming disk around HD 100546 |journal=Astronomy & Astrophysics |date=April 2016 |volume=588 |pages=A8 |doi=10.1051/0004-6361/201527940 |url=https://www.aanda.org/articles/aa/full_html/2016/04/aa27940-15/aa27940-15.html |access-date=5 April 2022 |language=en |issn=0004-6361|doi-access=free |arxiv=1601.04983 |bibcode=2016A&A...588A...8G }}
Another protoplanet, AB Aur b, may be in the earliest observed stage of formation for a gas giant. It is located in the gas disk of the star AB Aurigae. AB Aur b is among the largest exoplanets identified, and has a distant orbit, three times as far as Neptune is from the Earth's sun. Observations of AB Aur b may challenge conventional thinking about how planets are formed. It was viewed by the Subaru Telescope and the Hubble Space Telescope.{{cite news |title=Gigantic Jupiter-like alien planet observed still 'in the womb' |url=https://www.cbc.ca/news/science/planet-in-the-womb-1.6408539 |access-date=5 April 2022 |work=CBC News |date=April 5, 2022}}
Rings, gaps, spirals, dust concentrations and shadows in protoplanetary disks could be caused by protoplanets. These structures are not completely understood and are therefore not seen as a proof for the presence of a protoplanet.{{Cite journal |last1=Pinte |first1=Christophe |last2=Teague |first2=Richard |last3=Flaherty |first3=Kevin |last4=Hall |first4=Cassandra |last5=Facchini |first5=Stefano |last6=Casassus |first6=Simon |date=2022-03-01 |title=Kinematic Structures in Planet-Forming Disks |journal=Protostars and Planets VII |volume=534 |page=645 |arxiv=2203.09528 |bibcode=2023ASPC..534..645P |url=https://ui.adsabs.harvard.edu/abs/2022arXiv220309528P}} One new emerging way to study the effect of protoplanets on the disk are molecular line observations of protoplanetary disks in the form of gas velocity maps. HD 97048 b is the first protoplanet detected by disk kinematics in the form of a kink in the gas velocity map.{{Cite journal |last1=Pinte |first1=C. |last2=van der Plas |first2=G. |last3=Ménard |first3=F. |last4=Price |first4=D. J. |last5=Christiaens |first5=V. |last6=Hill |first6=T. |last7=Mentiplay |first7=D. |last8=Ginski |first8=C. |last9=Choquet |first9=E. |last10=Boehler |first10=Y. |last11=Duchêne |first11=G. |last12=Perez |first12=S. |last13=Casassus |first13=S. |date=2019-08-01 |title=Kinematic detection of a planet carving a gap in a protoplanetary disk |url=https://ui.adsabs.harvard.edu/abs/2019NatAs...3.1109P |journal=Nature Astronomy |volume=3 |issue=12 |pages=1109–1114 |doi=10.1038/s41550-019-0852-6 |arxiv=1907.02538 |bibcode=2019NatAs...3.1109P |s2cid=195820690 |issn=2397-3366}}
= Unconfirmed protoplanets =
The confident detection of protoplanets is difficult. Protoplanets usually exist in gas-rich protoplanetary disks. Such disks can produce over-densities by a process called disk fragmentation. Such fragments can be small enough to be unresolved and mimic the appearance of a protoplanet.{{Cite journal |last1=Teague |first1=Richard |last2=Jankovic |first2=Marija R. |last3=Haworth |first3=Thomas J. |last4=Qi |first4=Chunhua |last5=Ilee |first5=John D. |date=2020-06-01 |title=A three-dimensional view of Gomez's hamburger |journal=Monthly Notices of the Royal Astronomical Society |volume=495 |issue=1 |pages=451–459 |doi=10.1093/mnras/staa1167 |doi-access=free |arxiv=2003.02061 |bibcode=2020MNRAS.495..451T |issn=0035-8711}} A number of unconfirmed protoplanet candidates are known and some detections were later questioned.
See also
References
{{reflist}}
External links
- [https://archive.today/20130105085824/http://tech.groups.yahoo.com/group/mpml/message/25780 Thread on the definition of a protoplanet] (Minor Planet Mailing List : July 15, 2011)
{{Exoplanet}}
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