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Planetary core
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{{short description|Innermost layer(s) of a planet}} {{for|the Earth's core|Structure of the Earth#Core}} {{for|core body of planetary formation|Accretion (astrophysics)}} [[Image:Terrestial Planets internal en.jpg|thumb|The internal structure of the inner planets.]] [[File:Gas Giant Interiors.jpg|thumb|280px|The internal structure of the outer planets.]] A '''planetary core''' consists of the innermost layers of a [[planet]].<ref name="sci.1112328">{{cite journal |last=Solomon |first=S.C. |title=Hot News on Mercury's core |journal=Science |pages=702β3 |date=2007 |volume=316 |issue=5825 |doi=10.1126/science.1142328 |pmid=17478710 |s2cid=129291662 }}</ref> Cores may be entirely liquid, or a mixture of solid and liquid layers as is the case in the Earth.<ref name="Williams and Nimmo 2004">{{cite journal |last1=Williams |first1=Jean-Pierre |last2=Nimmo |first2=Francis |s2cid=40968487 |title=Thermal evolution of the Martian core: Implications for an early dynamo |journal=Geology |pages=97β100 |date=2004 |volume=32 |issue=2 |doi=10.1130/g19975.1|bibcode = 2004Geo....32...97W }}</ref> In the [[Solar System]], core sizes range from about 20% (the [[Moon]]) to 85% of a planet's radius ([[Mercury (planet)|Mercury]]). [[Gas giants]] also have cores, though the composition of these are still a matter of debate and range in possible composition from traditional stony/iron, to ice or to [[Metallic hydrogen#Liquid metallic hydrogen|fluid metallic hydrogen]].<ref name="Pollack, et al. 1977">{{cite journal |last1=Pollack |first1=James B. |last2=Grossman |first2=Allen S. |last3=Moore |first3=Ronald |last4=Graboske |first4=Harold C. Jr. |title=A Calculation of Saturn's Gravitational Contraction History |journal=Icarus |publisher=Academic Press, Inc |date=1977 |volume=30 |issue=1 |pages=111β128 |doi=10.1016/0019-1035(77)90126-9 |bibcode=1977Icar...30..111P}}</ref><ref name="Fortney and Hubbard 2003">{{cite journal |last1=Fortney |first1=Jonathan J. |last2=Hubbard |first2=William B. |title=Phase separation in giant planets: inhomogeneous evolution of Saturn |journal=Icarus |volume=164 |issue=1 |date=2003 |pages=228β243 |doi=10.1016/s0019-1035(03)00130-1|arxiv = astro-ph/0305031 |bibcode = 2003Icar..164..228F |s2cid=54961173 }}</ref><ref name="Stevenson 1982">{{cite journal |last=Stevenson |first=D. J. |title=Formation of the Giant Planets |journal=Planet. Space Sci. |publisher=Pergamon Press Ltd. |volume=30 |issue=8 |date=1982 |pages=755β764 |doi=10.1016/0032-0633(82)90108-8|bibcode = 1982P&SS...30..755S }}</ref> Gas giant cores are proportionally much smaller than those of terrestrial planets, though they can be considerably larger than the Earth's nevertheless; [[Jupiter]]'s is 10β30 times heavier than Earth,<ref name="Stevenson 1982" /> and [[exoplanet]] [[HD149026 b]] may have a core 100 times the mass of the Earth.<ref name="Sato, et al. 2005">{{cite journal |last1=Sato |first1=Bun'ei |last2=al. |first2=et |title=The N2K Consortium. II. A Transiting Hot Saturn around HD 149026 with a Large Dense Core |journal=The Astrophysical Journal |volume=633 |issue=1 |date=November 2005 |pages=465β473 |doi=10.1086/449306 |bibcode=2005ApJ...633..465S|arxiv = astro-ph/0507009 |s2cid=119026159 }}</ref> Planetary cores are challenging to study because they are impossible to reach by drill and there are almost no samples that are definitively from the core. Thus, they are studied via indirect techniques such as seismology, mineral physics, and planetary dynamics.
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