How old is the Earth, and how do scientists know?

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data-featured-image="GettyImages-1313642868-min.jpg"> <img width="1200" height="675" src="data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHZpZXdCb3g9IjAgMCAxMjAwIDY3NSIgd2lkdGg9IjEyMDAiIGhlaWdodD0iNjc1IiBkYXRhLXU9IiUyRndwLWNvbnRlbnQlMkZ1cGxvYWRzJTJGMjAyMiUyRjAyJTJGR2V0dHlJbWFnZXMtMTMxMzY0Mjg2OC1taW4uanBnIiBkYXRhLXc9IjEyMDAiIGRhdGEtaD0iNjc1IiBkYXRhLWJpcD0iIj48L3N2Zz4=" data-spai="1" class="feature-image wp caption" alt="image of the earth from space at night" decoding="async" loading="eager" fetchpriority="high" /><noscript data-spai="1"><img width="1200" height="675" src="https://cdn.shortpixel.ai/spai/q_lossy+ret_img+to_auto/cosmosmagazine.com/wp-content/uploads/2022/02/GettyImages-1313642868-min.jpg" data-spai-egr="1" class="feature-image wp caption" alt="image of the earth from space at night" decoding="async" loading="eager" fetchpriority="high" srcset="https://cdn.shortpixel.ai/spai/q_lossy+ret_img+to_auto/cosmosmagazine.com/wp-content/uploads/2022/02/GettyImages-1313642868-min.jpg 1200w, https://cdn.shortpixel.ai/spai/q_lossy+ret_img+to_auto/cosmosmagazine.com/wp-content/uploads/2022/02/GettyImages-1313642868-min-600x338.jpg 600w, https://cdn.shortpixel.ai/spai/q_lossy+ret_img+to_auto/cosmosmagazine.com/wp-content/uploads/2022/02/GettyImages-1313642868-min-768x432.jpg 768w, https://cdn.shortpixel.ai/spai/q_lossy+ret_img+to_auto/cosmosmagazine.com/wp-content/uploads/2022/02/GettyImages-1313642868-min-300x169.jpg 300w" sizes="(max-width: 1200px) 100vw, 1200px" /></noscript> <div class="caption feature">How do we date something as old as the Earth? Image credit: Bernt Ove Moss/EyeEm/Getty Images</div> </div> </header> <div class="entry-content cols-6 md-cols-1 cols-gap-1 margin-top-1"> <div class="content-left-sidebar cols-1 col"> <section class="contributors md-cols-1 rows-gap"> <div class="row post-date"> <span class="published-date">April 14, 2022</span> </div> <a href="https://cosmosmagazine.com/contributor/amalyah-hart/" class="author row md-hide"> <img src="data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHZpZXdCb3g9IjAgMCA1MTIgNTEyIiB3aWR0aD0iNTEyIiBoZWlnaHQ9IjUxMiIgZGF0YS11PSIlMkZ3cC1jb250ZW50JTJGdXBsb2FkcyUyRjIwMjElMkYwMyUyRmFtYWx5YWguanBnIiBkYXRhLXc9IjUxMiIgZGF0YS1oPSI1MTIiIGRhdGEtYmlwPSIiPjwvc3ZnPg==" data-spai="1" alt="Amalyah Hart" width="200" /><noscript data-spai="1"><img src="https://cdn.shortpixel.ai/spai/q_lossy+ret_img+to_auto/cosmosmagazine.com/wp-content/uploads/2021/03/amalyah.jpg" data-spai-egr="1" alt="Amalyah Hart" width="200" /></noscript> <div class="bio"> <h3>Amalyah Hart</h3> <p>Amalyah Hart has a BA (Hons) in Archaeology and Anthropology from the University of Oxford and an MA in Journalism from the University of Melbourne.</p> </div> </a> <div id="author-name" class="hide"> <a href="https://cosmosmagazine.com/contributor/amalyah-hart/" class="bio"> By Amalyah Hart </a> </div> <div class="favourite__wrapper row cols"> <button class="showDialog simplefavorite-button" aria-label="Favourite button"> </button> </div> </section> </div> <div class="primary-content cols-1 col-span-5 col"> <div class="flexible-content "> <div class="copy"> <blockquote class="wp-block-quote"> <p>How do you know that it was 4.6 billion years ago that the Earth formed? That seems a very specific number to be confident about! (As compared with saying “estimated to be around 5 billion years.”)</p> <cite>– David </cite></blockquote> <p>It’s a question humans have grappled with for centuries – how old is the Earth?</p> <p>For a long time in the Western world, the history of our planet and species was derived word-for-word from the Christian <em>Bible</em>.</p> <p>Back in 1642, John Lightfoot, an English clergyman, calculated what he thought to be the exact date of the creation of the universe: September 17, 3928 BC. He arrived at this conclusion by counting backwards through the epic genealogies recorded in the <em>Bible</em>. </p> <p>According to science, however, the Earth is a much older rock. It’s been labouring around the sun for 4.54 billion years, in fact, which we often round down to 4.5 billion. </p> <p>Now, it may seem a very specific number, but that age actually has an error margin of around 50 million years each way – no small sum of years, but a pretty tight margin in the grand scheme of things.</p> <p>So how can scientists peer back into the misty past to actually know how old our planet is? And how have they narrowed the number down so much?</p> <div class="advert ad-in-content ad-mobile auh-250"><div id="adn-00000000001fc2ca-i0p0" style="display: none"></div></div> <p>We can credit our increasingly sophisticated dating techniques to the development of <a href="https://cosmosmagazine.com/earth/earth-sciences/what-is-radiometric-dating/" target="_blank" rel="noreferrer noopener">radiometric dating</a>. Radiometric dating establishes the age of a material based on the presence of a radioactive <a href="https://cosmosmagazine.com/science/chemistry/what-is-an-isotope-isotopic-analysis-explainer/" target="_blank" rel="noreferrer noopener" data-type="URL" data-id="https://cosmosmagazine.com/science/chemistry/what-is-an-isotope-isotopic-analysis-explainer/">isotope</a> within it. Radioactive isotopes are isotopes that decay over time (radioactivity), and radioactive decay is the process by which an unstable atomic nucleus slowly leeches energy over that time period. </p> <hr class="wp-block-separator" /> <p><iframe title="Huh? Science Explained" src="https://omny.fm/shows/huh-science-explained/playlists/podcast/embed?selectedClip=d81cb928-df42-495f-9cf8-ae76000afd26&style=cover&autoplay=0&list=0" width="100%" height="180" frameborder="0"></iframe></p> <hr class="wp-block-separator" /> <p>The basic logic behind it is that if you compare the presence of a radioactive isotope in an item or material with its known abundance, or with the presence of the stable isotope it’s known to decay into, you can figure out how long that material has been around. This works for isotopes of potassium, uranium, carbon and a few other elements, though carbon-dating might be the one you’ve heard most about.</p> <p>Carbon dating, however, can only date a material that once belonged to a living creature, such as an animal or a plant, which takes in carbon as part of its life-cycle. In geology, scientists use radioactive isotopes with a much longer half-life than carbon (a half-life being the length of time it takes for half the isotopes in a sample to have decayed). These include potassium-40 (with a half-life of 1.248 billion years), uranium-238 (with a half-life of 4.468 billion years), and rubidium-87 (with a half-life of 47 billion years).</p> <a href="https://link.cosmosmagazine.com/OP59" class="newsletter-cta" data-device="mobile"> <img decoding="async" src="data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHZpZXdCb3g9IjAgMCAzMDAgMjUwIiB3aWR0aD0iMzAwIiBoZWlnaHQ9IjI1MCIgZGF0YS11PSIlMkZ3cC1jb250ZW50JTJGdXBsb2FkcyUyRjIwMjQlMkYwMyUyRldlZWtseS1DYXRjaHVwLTMwMHgyNTAtMS5wbmciIGRhdGEtdz0iMzAwIiBkYXRhLWg9IjI1MCIgZGF0YS1iaXA9IiI+PC9zdmc+" data-spai="1" alt="Newsletter" title="how do scientists know how old the earth is? 1"><noscript data-spai="1"><img decoding="async" src="https://cdn.shortpixel.ai/spai/q_lossy+ret_img+to_auto/cosmosmagazine.com/wp-content/uploads/2024/03/Weekly-Catchup-300x250-1.png" data-spai-egr="1" alt="Newsletter" title="how do scientists know how old the earth is? 1"></noscript> </a> <p>The oldest rocks on our planet and in our cosmic neighbourhood solidified from red-hot metals. These rocks have small amounts of uranium-238. We know that when a rock solidifies, the isotopes within it become locked-in and start to decay. By knowing the half-life of uranium-238, and measuring its abundance in relation to lead, the stable isotope it decays into, we can estimate how long it is since that rock was formed. For example, if the half-life of U-238 is 4.47 billion years, and a rock sample has a 50:50 ratio of U-238 to lead inside it, we could say that rock is 4.47 billion years old.</p> <div class="in-content-area related-box-wrapper content-third content-right margin-bottom-1"> <div class="gradient-title">Read More</div> <ul class="related-box article-listing"> <li class="cols"> <div class="article article-post"> <a href="https://cosmosmagazine.com/people/society/summer-of-science-questions-acm/" class="thumbnail ar-16-9" aria-label="Article link"> <span data-spai-bg-on="1" class="image" style="background-image: url(data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHZpZXdCb3g9IjAgMCAxMjAwIDY3NCIgd2lkdGg9IjEyMDAiIGhlaWdodD0iNjc0IiBkYXRhLXU9Imh0dHBzJTNBJTJGJTJGY29zbW9zbWFnYXppbmUuY29tJTJGd3AtY29udGVudCUyRnVwbG9hZHMlMkYyMDIyJTJGMDElMkZTdW1tZXJPZlNjaWVuY2VfTmV3SGVhZGVyLTEyMDAuanBnIiBkYXRhLXc9IjEyMDAiIGRhdGEtaD0iNjc0IiBkYXRhLWJpcD0iIj48L3N2Zz4=)"></span> </a> <div class="article-content has-very-light-grey-background-color"> <a href="/people/society/"> <span class="category people-color"> Society </span> </a> <a href="https://cosmosmagazine.com/people/society/summer-of-science-questions-acm/"> <h3>Cosmos: solving science stumpers for summer</h3> </a> </div> </div> </li> </ul> </div> <p>Armed with this toolkit, scientists have spent decades scouring the planet for its oldest rocks. Some of these are in our own backyard – there are <a href="https://earthobservatory.nasa.gov/images/6331/western-australias-jack-hills" target="_blank" rel="noreferrer noopener">zircon crystals in the Jack Hills</a> of Western Australia that formed 4.4 billion years ago, not long after the formation of Earth itself, and remain the oldest known materials on Earth. </p> <div class="advert ad-in-content ad-mobile auh-250"><div id="adn-00000000001fc2ca-i1p0" style="display: none"></div></div> <p>But there’s a problem: we know that rocks are continuously recycling, being formed, reformed and destroyed on an epic geological timescale, thanks to patterns of volcanism and erosion on our dynamic, atmosphere-shrouded planet. So how can we be sure that our oldest rocks <em>really are </em>our oldest rocks? How do we know that an even <em>older </em>rock hasn’t simply ground down to dust somewhere, or been subducted beneath a neighbouring continent?</p> <p>Well, to get around this problem, scientists have also dated lumps of space rock – specifically meteorites and lunar rocks – believed to have formed from the same disc of material that birthed our own planet, at the same time. The results are strikingly aligned; our neighbourly space debris can be dated to the same 4.5-billion-year-old window.</p> <p>So, we can date Earth with such a degree of precision because we have multiple lines of evidence pointing to the same window of time.</p> <hr class="wp-block-separator" /> <p><strong><em>Why is the sky blue? What actually is carbon capture and storage? Why does my vacuum cleaner make that noise? How does bitcoin work? And could Yoda really force push Palpatine?</em></strong></p> <p><strong><em>There’s no such thing as a stupid science question, but sometimes the answers can be tricky to find.</em></strong></p> <div class="advert ad-in-content ad-mobile auh-250"><div id="adn-00000000001fc2ca-i2p0" style="display: none"></div></div> <p><strong><em>This summer we’ve partnered with ACM for the <a href="https://cosmosmagazine.com/people/society/summer-of-science-questions-acm/" target="_blank" rel="noreferrer noopener">Summer of science</a>: Ask us anything! <a href="https://form.jotform.com/212998415734062?howDid=cosmos" target="_blank" rel="noreferrer noopener">Send us your curliest chemistry conundrum, perplexing physics problem or any science question at all</a> and we’ll get our journalists onto the case.</em></strong></p>  <img id="cosmos-post-tracker" width='1' height='1' style="opacity:0; height:1px!important; width:1px!important; border: 0!important; position: absolute!important; z-index: 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How old is the Earth, and how do scientists know?