Iron planet
{{Short description|Type of terrestrial planet}}
An iron planet is a type of planet that consists primarily of an iron-rich core with little or no mantle. Mercury is the largest celestial body of this type in the Solar System (as the other terrestrial planets are silicate planets), but larger iron-rich exoplanets are called super-Mercuries.
Iron is the sixth most abundant element in the universe by mass after hydrogen, helium, oxygen, carbon, and neon.
Origin
Iron-rich planets may be the remnants of normal metal/silicate rocky planets whose rocky mantles were stripped away by giant impacts. Some are thought to consist of diamond fields. Current planet formation models predict iron-rich planets will form in close-in orbits or orbiting massive stars where the
protoplanetary disk presumably consists of iron-rich material."Characteristics of Terrestrial Planets" by John Chambers, from "The Great Planet Debate: Science as Process", August 14–16, 2008, The Johns Hopkins University Applied Physics Laboratory
Kossiakoff Center, Laurel, MD. http://gpd.jhuapl.edu/abstracts/abstractFiles/chambers_abstract.pdf {{Webarchive|url=https://web.archive.org/web/20110817105036/http://gpd.jhuapl.edu/abstracts/abstractFiles/chambers_abstract.pdf |date=2011-08-17 }}
Characteristics
Iron-rich planets are smaller and denser than other types of planets of comparable mass.{{cite web|url=http://www.astrobio.net/pressrelease/2476/all-planets-possible|title=All Planets Possible - Astrobiology Magazine|date=30 September 2007|website=astrobio.net|access-date=19 April 2018 |url-status=usurped |archive-url=https://web.archive.org/web/20160305134113/http://www.astrobio.net/topic/solar-system/meteoritescomets-and-asteroids/all-planets-possible/ |archive-date=2016-03-05}}
Such planets would have no plate tectonics or strong magnetic field as they cool rapidly after formation. These planets are not like Earth. Since water and iron are unstable over geological timescales, wet iron planets in the goldilocks zone may be covered by lakes of iron carbonyl and other exotic volatiles rather than water."Big Planets: Super-Earths in Science Fiction" by Stephen Baxter, JBIS Vol 67, No 03 (March 2014), p.108
In science fiction, such a planet has been called a "Cannonball".{{cite book|last=Gillett|first=Stephen L.|title=World-Building|url=https://archive.org/details/worldbuildingsci00gill|url-access=limited|year=1996|publisher=Writer's Digest Books|location=Cincinnati, Ohio|isbn=158297134X|page=[https://archive.org/details/worldbuildingsci00gill/page/n175 173]|editor=Ben Bova}}
Candidates
An extrasolar planet candidate that may be composed mainly of iron is Kepler-974b.{{cite journal|url=http://stacks.iop.org/2041-8205/773/i=1/a=L15|title=The Roche Limit for Close-orbiting Planets: Minimum Density, Composition Constraints, and Application to the 4.2 hr Planet KOI 1843.03|first1=Saul|last1=Rappaport|first2=Roberto|last2=Sanchis-Ojeda|first3=Leslie A.|last3=Rogers|first4=Alan|last4=Levine|first5=Joshua N.|last5=Winn|date=19 April 2018|journal=The Astrophysical Journal Letters|volume=773|issue=1|pages=L15|access-date=19 April 2018|via=Institute of Physics|doi=10.1088/2041-8205/773/1/L15|arxiv=1307.4080|bibcode=2013ApJ...773L..15R|s2cid=35253735 }}
A super-Mercury candidate is GJ 367b.{{cite journal | arxiv=2307.09181 | doi=10.3847/2041-8213/ace0c7 | doi-access=free | title=Company for the Ultra-high Density, Ultra-short Period Sub-Earth GJ 367 b: Discovery of Two Additional Low-mass Planets at 11.5 and 34 Days | date=2023 | last1=Goffo | first1=Elisa | last2=Gandolfi | first2=Davide | last3=Egger | first3=Jo Ann | last4=Mustill | first4=Alexander J. | last5=Albrecht | first5=Simon H. | last6=Hirano | first6=Teruyuki | last7=Kochukhov | first7=Oleg | last8=Astudillo-Defru | first8=Nicola | last9=Barragan | first9=Oscar | last10=Serrano | first10=Luisa M. | last11=Hatzes | first11=Artie P. | last12=Alibert | first12=Yann | last13=Guenther | first13=Eike | last14=Dai | first14=Fei | last15=Lam | first15=Kristine W. F. | last16=Csizmadia | first16=Szilárd | last17=Smith | first17=Alexis M. S. | last18=Fossati | first18=Luca | last19=Luque | first19=Rafael | last20=Rodler | first20=Florian | last21=Winther | first21=Mark L. | last22=Rørsted | first22=Jakob L. | last23=Alarcon | first23=Javier | last24=Bonfils | first24=Xavier | last25=Cochran | first25=William D. | last26=Deeg | first26=Hans J. | last27=Jenkins | first27=Jon M. | last28=Korth | first28=Judith | last29=Livingston | first29=John H. | last30=Meech | first30=Annabella | journal=The Astrophysical Journal Letters | volume=955 | issue=1 | pages=L3 | bibcode=2023ApJ...955L...3G | display-authors=1 }}
The star HD 23472 is orbited by two super-Mercuries.{{cite journal | arxiv=2209.13345 | doi=10.1051/0004-6361/202244293 | title=HD 23472: A multi-planetary system with three super-Earths and two potential super-Mercuries | date=2022 | last1=Barros | first1=S. C. C. | last2=Demangeon | first2=O. D. S. | last3=Alibert | first3=Y. | last4=Leleu | first4=A. | last5=Adibekyan | first5=V. | last6=Lovis | first6=C. | last7=Bossini | first7=D. | last8=Sousa | first8=S. G. | last9=Hara | first9=N. | last10=Bouchy | first10=F. | last11=Lavie | first11=B. | last12=Rodrigues | first12=J. | last13=Da Silva | first13=J. Gomes | last14=Lillo-Box | first14=J. | last15=Pepe | first15=F. A. | last16=Tabernero | first16=H. M. | last17=Osorio | first17=M. R. Zapatero | last18=Sozzetti | first18=A. | last19=Mascareño | first19=A. Suárez | last20=Micela | first20=G. | last21=Prieto | first21=C. Allende | last22=Cristiani | first22=S. | last23=Damasso | first23=M. | last24=Di Marcantonio | first24=P. | last25=Ehrenreich | first25=D. | last26=Faria | first26=J. | last27=Figueira | first27=P. | last28=Hernández | first28=J. I. González | last29=Jenkins | first29=J. | last30=Lo Curto | first30=G. | journal=Astronomy & Astrophysics | volume=665 | pages=A154 | bibcode=2022A&A...665A.154B | display-authors=1 }}
HD 137496 b is a dense hot super-Mercury.{{cite journal | arxiv=2111.08764 | doi=10.1051/0004-6361/202141520 | title=The HD 137496 system: A dense, hot super-Mercury and a cold Jupiter | date=2022 | last1=Azevedo Silva | first1=T. | last2=Demangeon | first2=O. D. S. | last3=Barros | first3=S. C. C. | last4=Armstrong | first4=D. J. | last5=Otegi | first5=J. F. | last6=Bossini | first6=D. | last7=Delgado Mena | first7=E. | last8=Sousa | first8=S. G. | last9=Adibekyan | first9=V. | last10=Nielsen | first10=L. D. | last11=Dorn | first11=C. | last12=Lillo-Box | first12=J. | last13=Santos | first13=N. C. | last14=Hoyer | first14=S. | last15=Stassun | first15=K. G. | last16=Almenara | first16=J. M. | last17=Bayliss | first17=D. | last18=Barrado | first18=D. | last19=Boisse | first19=I. | last20=Brown | first20=D. J. A. | last21=Díaz | first21=R. F. | last22=Dumusque | first22=X. | last23=Figueira | first23=P. | last24=Hadjigeorghiou | first24=A. | last25=Hojjatpanah | first25=S. | last26=Mousis | first26=O. | last27=Osborn | first27=A. | last28=Santerne | first28=A. | last29=Strøm | first29=P. A. | last30=Udry | first30=S. | journal=Astronomy & Astrophysics | volume=657 | pages=A68 | bibcode=2022A&A...657A..68A | display-authors=1 }}
LHS 3844 b is potentially an Fe-rich super-Mercury.{{cite journal | arxiv=2007.14493 | doi=10.3847/PSJ/abaab5 | doi-access=free | title=A Volatile-poor Formation of LHS 3844b Based on Its Lack of Significant Atmosphere | date=2020 | last1=Kane | first1=Stephen R. | last2=Roettenbacher | first2=Rachael M. | last3=Unterborn | first3=Cayman T. | last4=Foley | first4=Bradford J. | last5=Hill | first5=Michelle L. | journal=The Planetary Science Journal | volume=1 | issue=2 | page=36 | bibcode=2020PSJ.....1...36K }}
See also
References
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