Editing Astronomy (section)

=== Planetary science ===
[[File:dust.devil.mars.arp.750pix.jpg|thumb|The black spot at the top is a [[dust devil]] climbing a crater wall on [[Mars]]. This moving, swirling column of [[Atmosphere of Mars|Martian atmosphere]] (comparable to a terrestrial [[tornado]]) created the long, dark streak.]]
{{Main|Planetary science|Planetary geology}}
Planetary science is the study of the assemblage of [[planet]]s, [[natural satellite|moons]], [[dwarf planet]]s, [[comet]]s, [[asteroid]]s, and other bodies orbiting the Sun, as well as extrasolar planets. The [[Solar System]] has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of the Sun's planetary system, although many new discoveries are still being made.<ref name="geology">{{cite book|url=http://marswatch.tn.cornell.edu/rsm.html|title=Remote Sensing for the Earth Sciences: Manual of Remote Sensing|date=2004|publisher=John Wiley & Sons|edition=3rd|author=Bell III, J. F.|author2=Campbell, B.A.|author3=Robinson, M.S.|access-date=17 November 2016|archive-url=https://web.archive.org/web/20060811220029/http://marswatch.tn.cornell.edu/rsm.html|archive-date=11 August 2006 }}</ref>

The Solar System is divided into the [[inner Solar System]] (subdivided into the inner planets and the [[asteroid belt]]), the [[outer Solar System]] (subdivided into the outer planets and [[Centaurs (minor planets)|centaurs]]), comets, the trans-Neptunian region (subdivided into the [[Kuiper belt]], and the [[scattered disc]]) and the farthest regions (e.g., boundaries of the [[heliosphere]], and the [[Oort Cloud]], which may extend as far as a light-year). The inner [[terrestrial planet]]s consist of [[Mercury (planet)|Mercury]], [[Venus]], Earth, and [[Mars]]. The outer [[giant planet]]s are the [[gas giant]]s ([[Jupiter]] and [[Saturn]]) and the [[ice giant]]s ([[Uranus]] and [[Neptune]]).<ref name="planets">{{cite web|author = Grayzeck, E.|author2 = Williams, D.R.| date = 11 May 2006|url=http://nssdc.gsfc.nasa.gov/planetary/|title = Lunar and Planetary Science|publisher = NASA|access-date =21 August 2006| archive-url= https://web.archive.org/web/20060820173205/http://nssdc.gsfc.nasa.gov/planetary/| archive-date= 20 August 2006 | url-status= live}}</ref>

The planets were formed 4.6 billion years ago in the [[protoplanetary disk]] that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that, with time, became protoplanets. The [[radiation pressure]] of the [[solar wind]] then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up, or eject, the remaining matter during a period of intense bombardment, evidenced by the many [[impact crater]]s on the Moon. During this period, some of the protoplanets may have collided and one such collision may have [[giant impact hypothesis|formed the Moon]].<ref name=Montmerle2006>{{cite journal|last=Montmerle|first=Thierry|author2=Augereau, Jean-Charles|author3= Chaussidon, Marc|title=Solar System Formation and Early Evolution: the First 100 Million Years|journal=Earth, Moon, and Planets|volume=98|issue=1–4|pages=39–95|date=2006|doi=10.1007/s11038-006-9087-5| bibcode=2006EM&P...98...39M|s2cid=120504344|display-authors=etal}}</ref>

Once a planet reaches sufficient mass, the materials of different densities segregate within, during [[planetary differentiation]]. This process can form a stony or metallic core, surrounded by a mantle and an outer crust. The core may include solid and liquid regions, and some planetary cores generate their own [[magnetic field]], which can protect their atmospheres from solar wind stripping.<ref>Montmerle, 2006, pp. 87–90</ref>

A planet or moon's interior heat is produced from the collisions that created the body, by the decay of radioactive materials (''e.g.'' [[uranium]], [[thorium]], and [[26Al|<sup>26</sup>Al]]), or [[tidal acceleration|tidal heating]] caused by interactions with other bodies. Some planets and moons accumulate enough heat to drive geologic processes such as [[volcanism]] and tectonics. Those that accumulate or retain an [[atmosphere]] can also undergo surface [[erosion]] from wind or water. Smaller bodies, without tidal heating, cool more quickly; and their geological activity ceases with the exception of impact cratering.<ref name="new solar system">{{cite book|editor=Beatty, J.K.|editor2=Petersen, C.C.|editor3=Chaikin, A.|title=The New Solar System|publisher=Cambridge press|url=https://books.google.com/books?id=iOezyHMVAMcC&pg=PA70|page=70edition = 4th|date=1999|isbn=978-0-521-64587-4|access-date=26 August 2020|archive-date=30 March 2015|archive-url=https://web.archive.org/web/20150330114739/http://books.google.com/books?id=iOezyHMVAMcC&pg=PA70|url-status=live}}</ref>