Why Are Planets Round?

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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="Erik Gregersen" data-title="Why Are Planets Round?" data-published-date="28 Sep. 2016" data-url="https://www.britannica.com/story/why-are-planets-round" > <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 gtm-topic-tool btn btn-xs btn-outline-blue border shadow-sm pr-10" data-modal=".md-share-modal"> <em class="material-icons md-icon ml-n10 my-n5 mr-5" 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="Why Are Planets Round?" data-url="https://www.britannica.com/story/why-are-planets-round" > <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/story/why-are-planets-round">https://www.britannica.com/story/why-are-planets-round</a> </div> </div> </div> </div> <div class="md-byline module-spacing font-12"> <div class="font-serif font-12"> <span class="written-by text-gray-700"> Written by </span> <div class="editor-popover popover p-0"> <a class="d-block p-20 gtm-byline font-12 byline-contributor" href="/editor/Erik-Gregersen/6723" > <div class="editor-title font-16 font-weight-bold">Erik Gregersen</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Erik Gregersen is a senior editor at Encyclopaedia Britannica, specializing in the physical sciences and technology. Before joining Britannica in 2007, he worked at the University of Chicago Press on the...</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> Erik Gregersen</span></div> <div class="font-serif font-12 text-gray-700"> <span class="qa-fact-checked-by">Fact-checked by</span> <div class="editor-popover popover p-0"> <a class="d-block p-20 font-12" href="/editor/The-Editors-of-Encyclopaedia-Britannica/4419" > <div class="editor-title font-16 font-weight-bold">The Editors of Encyclopaedia Britannica</div> <div class="editor-description font-12 font-serif mt-5 text-black">Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link font-12 "> The Editors of Encyclopaedia Britannica</span></div></div> </div> <article class="topic-content content mt-20"><figure class="md-assembly md-feature-assembly print-false" data-type="1" data-descid="1"><div class="md-assembly-wrapper card-media " data-type="image"><a href="https://cdn.britannica.com/17/157617-050-7F520316/Solar-system-illustration.jpg" target="_blank" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center " data-href="*"><img src="https://cdn.britannica.com/17/157617-050-7F520316/Solar-system-illustration.jpg?w=600&q=60" alt="Solar system illustration. (planets; sun)" data-width="1600" data-height="800" loading="eager"><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=""><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><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><cite class="credit d-block mt-5">漏 Andrea Danti/Fotolia</cite></figcaption></figure><p class="topic-paragraph">The <a href="https://www.britannica.com/place/Sun" class="md-crosslink " data-show-preview="true">Sun</a> and all eight <a href="https://www.britannica.com/science/planet" class="md-crosslink " data-show-preview="true">planets</a> of the <a href="https://www.britannica.com/science/solar-system" class="md-crosslink " data-show-preview="true">solar system</a> are round. Why? <strong>The gravitational force of a planet鈥檚 mass pulls all of its material toward the center, smoothing out any jarring non-roundness.</strong> Many of the smaller bodies of the solar system are not round because their <a href="https://www.britannica.com/science/gravity-physics" class="md-crosslink " data-show-preview="true">gravity</a> is not enough to smooth out their shape.</p><span class="MOD1"></span><p class="topic-paragraph">We can see this from the <a href="https://www.britannica.com/science/escape-velocity" class="md-crosslink " data-show-preview="true">escape velocity</a> of various bodies. To escape <a href="https://www.britannica.com/place/Earth" class="md-crosslink " data-show-preview="true">Earth鈥檚</a> gravity, you need to travel at about 11 km/second, or about 40,000 km/hour. Such speeds require the largest <a href="https://www.britannica.com/technology/rocket-jet-propulsion-device-and-vehicle" class="md-crosslink " data-show-preview="true">rockets</a>. Earth has a mass of 6 x 10^24 kg and is quite round. To escape the gravity of <a href="https://www.britannica.com/science/comet-astronomy" class="md-crosslink " data-show-preview="true">Comet</a> 67P, which the European <a href="https://www.britannica.com/topic/Rosetta-European-Space-Agency-spacecraft" class="md-crosslink " data-show-preview="true">Rosetta and Philae</a> probes visited, you need to travel at about 1 meter/second. You could jump faster than that. Comet 67P is not round at all; it has a mass of 10^13 kg, nearly one trillion times lighter than Earth, and is shaped like a rubber <a href="https://www.britannica.com/animal/duck" class="md-crosslink " data-show-preview="true">duck</a>.</p><span class="MOD2"></span><p class="topic-paragraph">Once a body becomes more than a few hundred kilometers across, it becomes more round than not. In our example, Earth鈥檚 diameter is about 12,700 km across; Comet 67P is about 4 km across.</p><span class="MOD3"></span><p class="topic-paragraph">Despite its improbability, some scientists have wondered about what a cubical planet would be like. Assuming that the rocky part of the planet would somehow maintain its cubeness, the <a href="https://www.britannica.com/science/air" class="md-crosslink " data-show-preview="true">air</a> and <a href="https://www.britannica.com/science/water" class="md-crosslink " data-show-preview="true">water</a> would not have any such magical property and would pool in the center of each cube side. Life would be confined to the shores of the central <a href="https://www.britannica.com/science/lake" class="md-crosslink " data-show-preview="true">lakes</a>, with the cube edges and corners being giant impassable <a href="https://www.britannica.com/science/mountain-landform" class="md-crosslink " data-show-preview="true">mountains</a>.</p><span class="MOD4"></span><span class="AM-END-OF-ARTICLE am-inline"></span></article> </div> </div> </div> <aside class="cm-rightrail d-none d-md-block ad-rail"></aside> </div> </div> </div> </div> </main> <div class="grid gx-0"> <div class="col"> <div class="md-loading hidden display-4 font-weight-normal"></div> <div class="text-center my-20 mb-100"> <button class="md-infinite-load-next btn btn-blue" data-value="next-page">Load More</button> </div> </div> <div class="col-md-da-320 hidden-xs"> </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":"Story","templateName":"DEMYSTIFIED","pageNumber":1,"pagesTotal":11,"pageId":5913,"pageLength":null,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-130/js/libs/jquery-3.5.0.min.js?v=3.130.14"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-130/dist/story-page.js?v=3.130.14' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"A","adLeg":"A","userType":"ANONYMOUS","pageType":"Story","pageSubtype":"DEMYSTIFIED","articleTemplateType":null,"gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":false} </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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