Planet | Definition, Characteristics, & Facts

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border-left-sm open"> <div class="drawer d-flex flex-column open"> <div class="left-rail-section-content"> <div class="topic-left-rail-header text-truncate bg-gray-50 position-relative text-right d-flex align-items-center"> <div class="tlr-title px-20 py-15 text-left"> <em class="material-icons text-gray-400 d-lg-none" data-icon="toc"></em> <a class="font-serif font-weight-bold text-black link-blue" href="https://www.britannica.com/science/planet">planet</a> </div> <button aria-label="Close" class="js-sections-close-button btn-link btn-sm btn d-lg-none position-absolute top-0 p-10 right-0" > <em class="material-icons font-26" data-icon="close"></em> </button> </div> <div class="section-content pl-10 pr-20 pl-sm-50 pr-sm-60 pl-lg-5 pr-lg-10 pt-10 pt-lg-0 bg-gray-50 clear-catfish-ad"> <div class="toc mb-20"> <div class="font-serif font-14 font-weight-bold mx-15 mb-15 mt-20"> Table of Contents </div> <ul class="list-unstyled my-0" data-level="h1"><li data-target="#ref1"><div class="pl-25"><a class="link-gray-900 w-100" href="/science/planet">Introduction</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref256233"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/planet#ref256233">Planets of the solar system</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref256234"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/planet#ref256234">Planets of other stars</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li></ul> <a class="toc-extra-link link-gray-900" href="https://www.britannica.com/science/planet/additional-info">References & Edit History</a> <a class="toc-extra-link link-gray-900" href="/facts/planet">Related Topics</a> </div> <div class="tlr-media-slider pb-10 mb-30"> <a class="section-header link-gray-900 font-serif font-14 font-weight-bold mb-10 mx-10" 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Cupid gathers a basket of red hearts from a pine tree which, in the language of flowers represents daring. Valentine's Day St. Valentine's Day February 14 love romance history and society heart In Roman mythology Cupid was the son of Venus, goddess of love (Eros and Aphrodite in the Greek Pantheon)." width="200" height="200" /> </a> </div> <div class="card-body ilf-content"> <a class="font-weight-semi-bold d-block mb-5 font-16 ilf-title" href="/story/why-do-we-give-valentine-cards" >Why Do We Give Valentine Cards?</a> </div> </div> <div class="imagelink-with-image-on-the-side card card-horizontal tlr-img-with-side-link ml-15 link-gray-900 mb-10" > <div class="position-relative card-media" style="flex: 0;"> <a class="ilf-image position-relative" href="/list/10-best-hockey-players-of-all-time"> <img loading="lazy" src="https://cdn.britannica.com/13/134913-131-D9520745/Ice-Sledge-Hockey-Canada-Cup-USA-ice-2010.jpg?w=200&h=200&c=crop" alt="Ice Sledge Hockey, Hockey Canada Cup, USA (left) vs Canada, 2009. 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Bearing an adult head and holding an axe, George gazes blankly up at his father, Augustine, who questions his son while holding a partially cut cherry tree. All of this takes place in front of a 20th-century brick building, which Wood modeled after his own home in Iowa. The scene illustrates the fable of Washington and the cherry tree, which Weems invented in 1806 for the first president's biography. Wood playfully suggests the artifice of the tale here, presenting the story in the manner of a costumed stage play rather than a real event. Amid the action, two Black figures pick cherries in the background, a reference to Washington' slaveholding past and a reminder of of what often gets left out in historical mythmaking." width="200" height="200" /> </a> </div> <div class="card-body ilf-content"> <a class="font-weight-semi-bold d-block mb-5 font-16 ilf-title" href="/story/did-george-washington-really-say-i-cant-tell-a-lie" >Did George Washington Really Say, “I Can’t Tell a Lie”? </a> </div> </div> </div> </div> </div> </div> <button class="drawerToggle btn position-sticky border btn-xs btn-white btn-circle rounded-sm d-none d-lg-flex " type="button" aria-label="Toggle Drawer"> <em class="material-icons font-18 text-blue" data-icon="keyboard_arrow_left"></em> </button> </div> </div> <div class="col"> <div class="h-100 ml-0 pr-lg-0 "> <div class="h-100 grid gx-0 gx-lg-20"> <div class="h-100 col-sm"> <div class="h-100 infinite-pagination-container d-flex flex-column position-relative"> <div class="position-absolute top-0 h-100 w-100"> <div class="toc-sticky-header d-none d-lg-none bg-gray-50 px-10 px-sm-30 position-sticky w-100 "> <div class="toc-sticky-header-inner-container align-items-center d-flex mx-auto h-100 w-100"> <button class="d-flex d-lg-none btn btn-sm btn-white text-blue border-2 border-gray-100 gtm-mobile-toc-header-button js-sections-button d-lg-none p-10"> <em class="material-icons my-n5 md-icon" data-icon="toc"></em> Contents </button> <div class="header-ai-ask-button-placeholder"></div> <div class="header-ai-summarize-button-placeholder"></div> </div> </div> </div> <div class="grey-box w-100 grey-box-top grey-box-bottom"> <div class="grey-box-content mx-auto w-100"> <script type="application/ld+json"> { "@context" : "https://schema.org", "@type" : "BreadcrumbList", "itemListElement" : [ { "@type" : "ListItem", "position" : 1, "item" : { "@id" : "https://www.britannica.com/browse/Science", "name": "Science" } } , { "@type" : "ListItem", "position" : 2, "item" : { "@id" : "https://www.britannica.com/browse/Astronomy", "name": "Astronomy" } } ] } </script> <nav class="breadcrumb mt-20"> <span class="breadcrumb-item "> <a class="link-gray-600" href="/browse/Science">Science</a> </span> <span class="breadcrumb-item "> <a class="link-gray-600" href="/browse/Astronomy">Astronomy</a> </span> </nav> <div class="page2ref-true topic-content topic-type-REGULAR" data-student-article="true"> <script class="page-description-json" type="application/json"> { "url": "/science/planet", "shareUrl": "https://www.britannica.com/science/planet", "browserTitle": "Planet | Definition, Characteristics, & Facts", "firstTopicPage": true, "topicId":463008 } </script> <div class="reading-channel"> <div class="desktop-header-image module-spacing"> <figure class="md-assembly m-0 mb-20 mb-md-0 card card-borderless print-false" data-assembly-id="13684" data-asm-type="image"> <div class="md-assembly-wrapper card-media " data-type="image"> <a style="min-height: 160px;" href="https://cdn.britannica.com/01/81401-050-8EB87030/planets-solar-system-Pluto-montage-images-Sun.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/463008/74702"> <picture> <source media="(min-width: 680px)" srcset="https://cdn.britannica.com/01/81401-050-8EB87030/planets-solar-system-Pluto-montage-images-Sun.jpg?w=300"> <img src="https://cdn.britannica.com/01/81401-050-8EB87030/planets-solar-system-Pluto-montage-images-Sun.jpg?w=400&h=300&c=crop" alt="solar system to scale" loading="eager" /> </picture> <button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"> <em class="material-icons link-blue" data-icon="zoom_in"></em> </button> </a> </div> <figcaption class="card-body"> <div class="md-assembly-caption text-muted font-14 font-serif line-clamp"> <span> <a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/01/81401-050-8EB87030/planets-solar-system-Pluto-montage-images-Sun.jpg" data-href="/media/1/463008/74702">solar system to scale</a> <span><span>The eight planets of the solar system and Pluto, in a montage of images scaled to show the approximate sizes of the bodies relative to one another. 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Please refer to the appropriate style manual or other sources if you have any questions. </div> <div class="label mt-20 mb-10">Select Citation Style</div> <select class="js-citation-format-select form-select"> <option selected value="mla">MLA</option> <option value="apa">APA</option> <option value="chicago">Chicago Manual of Style</option> </select> <div class="citation font-serif border rounded p-15 mt-20" data-authors="The Editors of Encyclopaedia Britannica" data-authors-initial="The Editors of Encyclopaedia Britannica" data-title="planet" data-published-date="8 Feb. 2025" data-url="https://www.britannica.com/science/planet" > <div class="citation-text"></div> </div> <button class="js-copy-citation-button mt-20 btn btn-xs btn-outline-blue border shadow-sm pr-10" > <em class="material-icons md-icon ml-n10 my-n5 mr-5" data-icon="file_copy"></em> <span class="js-citation-status-text">Copy Citation</span> </button> </div> </div> </div> <div> <button class="js-share-modal-button js-modal btn btn-sm btn-link gtm-topic-tool font-weight-bold btn-link" data-modal="[data-topic-id=463008] .md-share-modal"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="share"></em> Share </button> <div class="md-share-modal size-lg d-none qa-share-modal"> <div class="md-modal-body"> <div class="h2 font-serif d-flex align-items-center pb-15 border-bottom"> <em class="material-icons text-blue mr-10" data-icon="share"></em> Share </div> <div class="label my-20">Share to social media</div> <div class="md-social-toolbar-circle d-flex align-items-start inverted" data-value="share" title="planet" data-url="https://www.britannica.com/science/planet" > <a class="social-icon facebook justify-content-center d-flex align-items-center align-self-center" data-provider="facebook" href="https://www.facebook.com/BRITANNICA/" target="_blank" rel="noopener"><span>Facebook</span></a> <a class="social-icon x justify-content-center d-flex align-items-center align-self-center" data-provider="x" href="https://x.com/britannica" target="_blank" rel="noopener"><span>X</span></a> </div> <div class="label pt-20 mt-20 mb-5 border-top">URL</div> <a class="font-serif text-truncate d-inline-block" href="https://www.britannica.com/science/planet">https://www.britannica.com/science/planet</a> </div> </div> </div> <div> <button class="js-feedback-modal-button js-modal btn btn-sm btn-link gtm-topic-tool font-weight-bold btn-link" data-modal=".md-feedback-modal"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="message"></em> Feedback </button> </div> <div> <button class="qa-external-website-modal-button js-modal btn btn-sm btn-link gtm-topic-tool font-weight-bold btn-link" data-modal="[data-topic-id=463008] .md-websites-modal"> <em class="material-icons md-icon ml-n10 mr-5" data-icon="link"></em> External Websites </button> </div> </div> <div class="md-feedback-modal size-lg d-none"> <div class="md-modal-body"> <div class="h2 font-serif pb-15 border-bottom"> Feedback </div> <form method="post" action="/submission/feedback/463008"> <div class="my-20"> Corrections? 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btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button js-sections-inline-button module-spacing"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <button class="ai-ask-button btn border-2 ai-ask-button btn border-2 module-spacing btn-sm js-inline-ai-ask-button btn-outline-red-400 border-red-400 p-10 ml-5"> Ask the Chatbot a Question </button> </div> <div class="js-qf-module qf-module px-40 px-sm-20 py-15 mx-auto module-spacing font-14 bg-gray-50 rounded"> <div class="facts-list mt-10"> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Key People: </dt> <dd><a href="/biography/Isaac-Newton" topicid="413189">Isaac Newton</a></dd> <dd><a href="/biography/Johannes-Kepler" topicid="315225">Johannes Kepler</a></dd> <dd><a href="/biography/Galileo-Galilei" topicid="224058">Galileo</a></dd> <dd><a href="/biography/Michel-Mayor" topicid="1583764">Michel Mayor</a></dd> <dd><a href="/biography/Didier-Queloz" topicid="1676539">Didier Queloz</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> </div> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Related Topics: </dt> <dd><a href="/place/Mars-planet" topicid="366330">Mars</a></dd> <dd><a href="/place/Saturn-planet" topicid="525169">Saturn</a></dd> <dd><a href="/place/Venus-planet" topicid="625665">Venus</a></dd> <dd><a href="/place/Jupiter-planet" topicid="308403">Jupiter</a></dd> <dd><a href="/place/Uranus-planet" topicid="619284">Uranus</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> <div class="text-center"> <a class="btn btn-sm btn-link p-0" href="/facts/planet"> See all related content </a> </div> </div> </div> </div><div class="bg-gray-50 p-15 rounded module-spacing recent-news d-flex flex-column float-false"> <div> <h2 class="font-weight-bold font-14 m-0 d-inline"> News <span class="text-gray-600">•</span> </h2> <div class="recent-news-item first-recent-news-item d-inline"> <a class="font-14 gtm-ap-news-link" href="https://spacenews.com/sky-perfect-jsat-is-the-customer-behind-planets-230-million-leo-order/" rel="nofollow">SKY Perfect JSAT is the customer behind Planet’s $230 million LEO order</a> <span class="font-14 text-gray-600"> <span>•</span> Feb. 5, 2025, 5:48 AM ET (SpaceNews) <button class="btn btn-link d-inline p-0 font-12 js-toggle-recent-news"> <span class="text-gray-500">...</span><span>(Show more)</span> </button> </span> </div> </div> <div class="rest-of-recent-news-items"> <div class="recent-news-item mt-5"> <a class="font-14 gtm-ap-news-link" href="https://www.cnbc.com/amp/2025/01/29/planet-lands-230-million-contract-for-pelican-imagery-satellites.html" rel="nofollow">Planet lands $230 million contract for Pelican imagery satellites</a> <span class="font-14 text-gray-600"> <span>•</span> Jan. 29, 2025, 6:13 AM ET (CNBC) </span> </div> <button class="js-toggle-recent-news d-flex btn btn-unstyled font-14 pr-10 rounded-sm mt-10" aria-label="Toggle additional news items"> Show less <em class="material-icons" data-icon="expand_less"></em> </button> </div> </div><span class="marker before-article"></span><section data-level="1" id="ref1"><span class="marker PREMOD1 mod-inline"></span><p class="topic-paragraph"><strong><span id="ref100731"></span>planet</strong>, (from Greek <em>planētes</em>, “wanderers”), broadly, any relatively large natural body that revolves in an <a href="https://www.britannica.com/science/orbit-astronomy" class="md-crosslink autoxref " data-show-preview="true">orbit</a> around the <a href="https://www.britannica.com/place/Sun" class="md-crosslink " data-show-preview="true">Sun</a> or around some other <a href="https://www.britannica.com/science/star-astronomy" class="md-crosslink " data-show-preview="true">star</a> and that is not radiating <a href="https://www.britannica.com/science/energy" class="md-crosslink autoxref " data-show-preview="true">energy</a> from internal <a href="https://www.britannica.com/science/nuclear-fusion" class="md-crosslink " data-show-preview="true">nuclear fusion</a> reactions. In addition to the above description, some scientists impose additional constraints regarding characteristics such as size (e.g., the object should be more than about 1,000 km [600 miles] across, or a little larger than the largest known <a href="https://www.britannica.com/science/asteroid" class="md-crosslink " data-show-preview="true">asteroid</a>, <a href="https://www.britannica.com/place/Ceres-dwarf-planet" class="md-crosslink " data-show-preview="true">Ceres</a>), shape (it should be large enough to have been squeezed by its own <a href="https://www.britannica.com/science/gravity-physics" class="md-crosslink autoxref " data-show-preview="true">gravity</a> into a sphere—i.e., roughly 700 km [435 miles] across, depending on its density), or mass (it must have a mass insufficient for its core to have experienced even temporary nuclear fusion). As the term is applied to bodies in Earth’s solar system, the <a href="https://www.britannica.com/topic/International-Astronomical-Union" class="md-crosslink autoxref " data-show-preview="true">International Astronomical Union</a> (IAU), which is charged by the scientific <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="community" href="https://www.merriam-webster.com/dictionary/community" data-type="MW">community</a> with classifying astronomical objects, lists eight planets orbiting the Sun; in order of increasing distance, they are <a href="https://www.britannica.com/place/Mercury-planet" class="md-crosslink " data-show-preview="true">Mercury</a>, <a href="https://www.britannica.com/place/Venus-planet" class="md-crosslink " data-show-preview="true">Venus</a>, <a href="https://www.britannica.com/place/Earth" class="md-crosslink " data-show-preview="true">Earth</a>, <a href="https://www.britannica.com/place/Mars-planet" class="md-crosslink " data-show-preview="true">Mars</a>, <a href="https://www.britannica.com/place/Jupiter-planet" class="md-crosslink " data-show-preview="true">Jupiter</a>, <a href="https://www.britannica.com/place/Saturn-planet" class="md-crosslink " data-show-preview="true">Saturn</a>, <a href="https://www.britannica.com/place/Uranus-planet" class="md-crosslink " data-show-preview="true">Uranus</a>, and <a href="https://www.britannica.com/place/Neptune-planet" class="md-crosslink " data-show-preview="true">Neptune</a>. <a href="https://www.britannica.com/place/Pluto-dwarf-planet" class="md-crosslink " data-show-preview="true">Pluto</a> also was listed as a planet until 2006. Until the close of the 20th century, the only planets to be recognized were components of Earth’s <a href="https://www.britannica.com/science/solar-system" class="md-crosslink " data-show-preview="true">solar system</a>. At that time astronomers confirmed that other stars have objects that appear to be planets in orbit around them.</p><span class="marker MOD1 mod-inline"></span></section> <span class="marker h2"></span><section data-level="1" id="ref256233"><h2 class="h1">Planets of the solar system</h2> <span class="marker PREMOD2 mod-inline"></span><p class="topic-paragraph">The idea of what exactly <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="constitutes" href="https://www.merriam-webster.com/dictionary/constitutes" data-type="MW">constitutes</a> a planet of the solar system has been traditionally the product of historical and cultural <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="consensus" href="https://www.merriam-webster.com/dictionary/consensus" data-type="MW">consensus</a>. Ancient sky gazers applied the term <em>planet</em> to the seven celestial bodies that were observed to move appreciably against the background of the apparently fixed stars. These included the Sun and Earth’s Moon, as well as the five planets in the modern sense—Mercury, Venus, Mars, Jupiter, and Saturn—that were readily visible as celestial wanderers before the invention of the <a href="https://www.britannica.com/science/optical-telescope" class="md-crosslink autoxref " data-show-preview="true">telescope</a>. After the idea of an Earth-centred cosmos was dispelled (<em>see</em> <a href="https://www.britannica.com/science/Copernican-system" class="md-crosslink " data-show-preview="true">Copernican system</a>) and more distinctions were made about the nature and movement of objects in the sky, the term <em>planet</em> was reserved only for those larger bodies that orbited the Sun. When the giant bodies Uranus and Neptune were discovered in 1781 and 1846, respectively, their obvious kinship with the other known planets left little question regarding their addition to the planetary ranks. So also, at first, appeared to be the case for Pluto when, during a concerted search for a <a href="https://www.britannica.com/science/Planet-Nine" class="md-crosslink " data-show-preview="true">ninth planet</a>, it was observed in 1930 as a seemingly lone object beyond the orbit of Neptune. In later decades, however, Pluto’s planetary status became increasingly questioned by astronomers who noted that its tiny size, unusual orbital characteristics, and <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="composition" href="https://www.merriam-webster.com/dictionary/composition" data-type="MW">composition</a> of ice and <a href="https://www.britannica.com/science/rock-geology" class="md-crosslink autoxref " data-show-preview="true">rock</a> made it an <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="anomaly" href="https://www.merriam-webster.com/dictionary/anomaly" data-type="MW">anomaly</a> among the other recognized planets. After many more Pluto-sized and smaller icy objects were found orbiting beyond Neptune beginning in the 1990s, astronomers recognized that Pluto, far from being unique in its part of the solar system, is almost undoubtedly one of the larger and nearer pieces of this debris, known collectively as the <a href="https://www.britannica.com/place/Kuiper-belt" class="md-crosslink " data-show-preview="true">Kuiper belt</a>, that is left over from the formation of the planets. (<em>See also</em> <a href="https://www.britannica.com/science/planetesimal" class="md-crosslink " data-show-preview="true">planetesimal</a>.)</p><span class="marker MOD2 mod-inline"></span> <span class="marker PREMOD3 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="242877" data-asm-type="video"><div class="md-assembly-wrapper card-media" data-type="video" video-id="212435"><a data-id="212435" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/Is-Pluto-Planet-Demystified-video/-242877"><img src="https://cdn.britannica.com/35/212435-138-F20FFC58/Is-Pluto-Planet-Demystified-video.jpg?w=800&h=450&c=crop" alt="Why isn't Pluto a planet?" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/hcpdVo46.m3u8" } ], "image": "https://cdn.britannica.com/35/212435-138-F20FFC58/Is-Pluto-Planet-Demystified-video.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/1WCRf4Ed", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/I4cej2UR.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="212435">Why isn't Pluto a planet?</span><span>Being a dwarf planet isn't really a downgrade.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/planet/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">In August 2006, after intense debate over the question of Pluto’s planetary status, the general assembly of the IAU approved a definition for a solar system planet that <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="excluded" href="https://www.britannica.com/dictionary/excluded" data-type="EB">excluded</a> Pluto. At the same time, it defined a new distinct class of objects called <a href="https://www.britannica.com/science/dwarf-planet" class="md-crosslink " data-show-preview="true">dwarf planets</a>, for which Pluto qualified. Following the IAU proclamations, many scientists protested the definitions, considering them flawed and unscientific and calling for their reconsideration.</p><a class="link-module shadow-sm d-block qa-quiz-module" href="/quiz/planets-and-the-earths-moon" data-link-module-iframe-link=""> <img loading="lazy" src="https://cdn.britannica.com/74/166074-131-677298A0/Saturn-rings-planet-solar-system-Space-Art.jpg" alt="Saturn. Saturn and its rings. Second largest planet of the solar system. Space Art" class="rounded-sm mr-15" width="70" /> <div class="line-clamp clamp-5"> <div class="module-title bg-green">Britannica Quiz</div> <div class="font-weight-semi-bold mt-5">Planets and the Earth’s Moon</div> </div> </a><span class="marker MOD3 mod-inline"></span> <span class="marker PREMOD4 mod-inline"></span><p class="topic-paragraph">According to the 2006 IAU decision, for a celestial body to be a planet of the solar system, it must meet three conditions: it must be in orbit around the Sun, have been molded by its own gravity into a round or nearly round shape, and have “cleared the neighbourhood around its orbit,” meaning that its mass must be large enough for its gravity to have removed rocky and icy <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="debris" href="https://www.britannica.com/dictionary/debris" data-type="EB">debris</a> from its orbital vicinity. Pluto failed on the third requirement because it orbits partially within, and is considered to be part of, the Kuiper belt.</p><span class="marker MOD4 mod-inline"></span> <span class="marker PREMOD5 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="229605" data-asm-type="video"><div class="md-assembly-wrapper card-media" data-type="video" video-id="196688"><a data-id="196688" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/dwarf-planets/-229605"><img src="https://cdn.britannica.com/88/196688-138-FD6DA6C2/dwarf-planets.jpg?w=800&h=450&c=crop" alt="Learn about the dwarf planets within our solar system" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/sFp9CTky.m3u8" } ], "image": "https://cdn.britannica.com/88/196688-138-FD6DA6C2/dwarf-planets.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/kKxhJ6N0", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/hwXdq5qO.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="196688">Learn about the dwarf planets within our solar system</span><span>Learn about dwarf planets.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/planet/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">To be a dwarf planet under the IAU definition, the object must meet the first two conditions described above; in addition, it must not have cleared its neighbourhood, and it must not be a <a href="https://www.britannica.com/science/moon-natural-satellite" class="md-crosslink autoxref " data-show-preview="true">moon</a> of another body. Pluto falls into this category, as do the asteroid Ceres and the large Kuiper belt object <a href="https://www.britannica.com/place/Eris-astronomy" class="md-crosslink " data-show-preview="true">Eris</a>, which was discovered in 2005 beyond the orbit of Pluto. By contrast, <a href="https://www.britannica.com/place/Charon-astronomy" class="md-crosslink " data-show-preview="true">Charon</a>, by virtue of its being a moon of Pluto, is not a dwarf planet, even though its <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="diameter" href="https://www.britannica.com/dictionary/diameter" data-type="EB">diameter</a> is more than half that of Pluto. The ranks of dwarf planets will likely be expanded as other objects known or yet to be discovered are determined to meet the conditions of the definition.</p><span class="marker MOD5 mod-inline"></span> <span class="marker PREMOD6 mod-inline"></span><p class="topic-paragraph">In June 2008 the IAU created a new category, <span id="ref993771"></span><a href="https://www.britannica.com/science/plutoid" class="md-crosslink ">plutoids</a>, within the dwarf planet category. Plutoids are dwarf planets that are farther from the Sun than Neptune; that is, they are the largest objects in the Kuiper belt. Two of the dwarf planets, Pluto and Eris, are plutoids; Ceres, because of its location in the asteroid belt, is not.</p><span class="marker MOD6 mod-inline"></span> <span class="marker PREMOD7 mod-inline"></span><p class="topic-paragraph">Of the eight currently recognized planets of the solar system, the inner four, from Mercury to <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="Mars" href="https://www.britannica.com/dictionary/Mars" data-type="EB">Mars</a>, are called <span id="ref883570"></span><a href="https://www.britannica.com/science/terrestrial-planet" class="md-crosslink ">terrestrial planets</a>; those from Jupiter to Neptune are called <span id="ref883576"></span><a href="https://www.britannica.com/science/giant-planet" class="md-crosslink ">giant planets</a> or Jovian planets. Between these two main groups is a belt of numerous <a href="https://www.britannica.com/science/small-body" class="md-crosslink " data-show-preview="true">small bodies</a> called <span id="ref883581"></span><a href="https://www.britannica.com/science/asteroid" class="md-crosslink " data-show-preview="true">asteroids</a>. After Ceres and other larger asteroids were discovered in the early 19th century, the bodies in this class were also referred to as minor planets or planetoids, but the term <em>asteroid</em> is now used most widely.</p><div class="module-spacing"> <DIV class="marketing-INLINE_SUBSCRIPTION marketing-content" data-marketing-id="INLINE_SUBSCRIPTION"><style> .student-promo-banner-wrapper { container-type: inline-size; margin-bottom: 15px; } @container (min-width: 475px) { .student-promo-banner { flex-direction: row; } .student-promo-banner-img-wrapper { margin-bottom: 0; margin-right: 10px; justify-content: flex-start; } .student-promo-banner-text-wrapper { text-align: left; margin-bottom: 0px; margin-left: 10px; } .student-promo-banner-button-wrapper { margin-right: 0; } }</style> <div class="student-promo-banner-wrapper"> <div class="student-promo-banner d-flex flex-column align-items-center bg-blue rounded p-20"> <div class="student-promo-banner-img-wrapper mb-20 mr-0 d-flex justify-content-center"> <img class="rounded" style="max-width: 100px; min-width: 80px" src="https://cdn.britannica.com/marketing/BlueThistle.webp" /> </div> <div class="student-promo-banner-text-wrapper ml-0 mb-10 text-center text-white"> <div class="h2 mb-10">Get Unlimited Access</div> <div class="h4 font-weight-semi-bold">Try Britannica Premium for free and discover more.</div> </div> <div class="student-promo-banner-button-wrapper d-flex justify-content-center align-items-center ml-auto mr-auto"> <a class="btn btn-m btn-orange" href="https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=inline-cta&utm_campaign=august-2024">Subscribe</a> </div> </div> </div> </DIV></div><div class="one-good-fact-module"> </div><span class="marker MOD7 mod-inline"></span></section> <span class="marker h3"></span><section data-level="1" id="ref256234"><h2 class="h1">Planets of other stars</h2> <span class="marker PREMOD8 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="204480" data-asm-type="video"><div class="md-assembly-wrapper card-media" data-type="video" video-id="186389"><a data-id="186389" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/Overview-planets/-204480"><img src="https://cdn.britannica.com/89/186389-138-90CF88EC/Overview-planets.jpg?w=800&h=450&c=crop" alt="Explore exoplanets and the techniques used to detect them" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/Tz5GgKsl.m3u8" } ], "image": "https://cdn.britannica.com/89/186389-138-90CF88EC/Overview-planets.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/V2N3H8kP", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/c6zPnXJi.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="186389">Explore exoplanets and the techniques used to detect them</span><span> Learn about extrasolar planets (exoplanets) and the techniques used to detect them.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/planet/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">The planets and other objects that circle the Sun are thought to have formed when part of an interstellar cloud of gas and dust collapsed under its own gravitational attraction and formed a disk-shaped <a href="https://www.britannica.com/science/nebula" class="md-crosslink autoxref " data-show-preview="true">nebula</a>. Further compression of the disk’s central region formed the Sun, while the gas and dust left behind in the midplane of the surrounding disk eventually <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="coalesced" href="https://www.britannica.com/dictionary/coalesced" data-type="EB">coalesced</a> to form ever-larger objects and, ultimately, the planets. (<em>See</em> <a href="https://www.britannica.com/science/solar-system/Origin-of-the-solar-system#ref54374" class="md-crosslink " data-show-preview="true">solar system: Origin of the solar system</a>.) Astronomers have long wondered if this process of planetary formation could have accompanied the birth of stars other than the Sun. In the glare of their parent stars, however, such small, dim objects would not be easy to detect directly in images made with telescopes from Earth’s vicinity. Instead, astronomers concentrated on attempting to observe them indirectly through the gravitational effects they exert on their parent stars. After decades of searching for such <span id="ref883585"></span><a href="https://www.britannica.com/science/extrasolar-planet" class="md-crosslink " data-show-preview="true">extrasolar planets</a>, astronomers in the early 1990s indirectly identified three planets circling a <a href="https://www.britannica.com/science/pulsar" class="md-crosslink autoxref " data-show-preview="true">pulsar</a> (i.e., a rapidly spinning neutron star) called <span id="ref883588"></span><a href="https://www.britannica.com/place/PSR-1257-12" class="md-crosslink " data-show-preview="true">PSR B1257+12</a>. The first discovery of a planet revolving around a star more like the Sun came in 1995 with the announcement of the existence of a massive planet orbiting the star <a href="https://www.britannica.com/place/51-Pegasi" class="md-crosslink autoxref " data-show-preview="true">51 Pegasi</a>. More than 5,000 planets around other stars are known, and in 2005 astronomers obtained the first direct infrared images of what were interpreted to be extrasolar planets. In size these objects range from about the size of Earth’s <a href="https://www.britannica.com/place/Moon" class="md-crosslink " data-show-preview="true">Moon</a> to more than a dozen times the mass of Jupiter. Astronomers have <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="yet" href="https://www.britannica.com/dictionary/yet" data-type="EB">yet</a> to develop a rigorous, generally accepted definition of planet that will successfully accommodate extrasolar planets and distinguish them from bodies that are more starlike in character (e.g., <a href="https://www.britannica.com/science/brown-dwarf" class="md-crosslink " data-show-preview="true">brown dwarfs</a>).</p><span class="marker MOD8 mod-inline"></span><span class="md-signature"><a href="/editor/The-Editors-of-Encyclopaedia-Britannica/4419">The Editors of Encyclopaedia Britannica</a></span> </section><span class="md-signature">This article was most recently revised and updated by <a href="/editor/Erik-Gregersen/6723">Erik Gregersen</a>.</span><span class="marker end-of-content"></span><span class="marker after-article"></span></div> <div id="chatbot-root"></div> </div> </div> </div> <div class="ai-dialog-placeholder"></div> </div> </div> <aside class="col-md-da-320"></aside> </div> </div> </div> </div> </article> </div> </div> </div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript> <script type="text/javascript" id="_informizely_script_tag"> var IzWidget = IzWidget || {}; (function (d) { var scriptElement = d.createElement('script'); scriptElement.type = 'text/javascript'; scriptElement.async = true; scriptElement.src = "https://insitez.blob.core.windows.net/site/f780f33e-a610-4ac2-af81-3eb184037547.js"; var node = d.getElementById('_informizely_script_tag'); node.parentNode.insertBefore(scriptElement, node); } )(document); </script>  <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Topic","templateName":"DESKTOP","pageNumber":1,"pagesTotal":1,"pageId":463008,"pageLength":1217,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-133/js/libs/jquery-3.5.0.min.js?v=3.133.9"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-133/dist/topic-page.js?v=3.133.9' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"A","adLeg":"A","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"LARGE","gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":true} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>

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