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When the first asteroids were discovered over 200 years ago, they were celebrated as new planets, and the story goes that the rapid and overwhelming discovery of hundreds and then thousands more quickly convinced scientists that asteroids shouldn't be planets - but that's totally wrong: the real reason asteroids were demoted from "planet" status came nearly 100 years after the hundredth asteroid was discovered and decades after the thousandth, and had nothing to do with their quantity, but in fact, their quality. Of course, up until 1800, only seven primary
planets - objects that directly orbit the Sun - only seven primary planets had been discovered: Mercury, Venus, Earth, Mars, Jupiter, Saturn and Uranus. But telescopes got better and we found more: a small 8th primary planet, Ceres, was discovered in 1801 between Jupiter and Mars, and then there was a surge of new planets discovered with almost identical orbits to Ceres: Pallas, Juno, Vesta, and more, and more, and more! It soon became clear that there was a band of small planets orbiting the Sun between Mars and Jupiter, and so, when describing the solar system,
astronomers found it useful to group them together as "the small planets", " the minor planets", "the planetoids", "the asteroids" or "the numbered planets", because eventually so many were being discovered so quickly it was easier to keep track of them all right away with numbers rather than coming up with fancy names. But there wasn't yet any scientific reason to stop thinking of the asteroids as planets. I mean, "having too many" isn't by itself a good scientific criteria (think about all the thousands of known planets we've now discovered
orbiting other stars - their sheer number isn't a reason not to call them planets!). So despite some embarrassment over the rapidly growing list of planets 19th and early 20th century astronomers still considered all the asteroids to be planets. Well, except one astronomer, who argued that the asteroids should be their own category, but his reasoning was based on incorrect data and theoretical misunderstandings and was ultimately rejected by the wider astronomical communidy. Even today, the International Astronomical Union's commission on asteroids is STILL technically named
the "Minor Planet Center" (even though, as we are about to discover, it's a poor name for them). So why don't we include the asteroids in our list of planets today? Look at this analysis of scientific papers in the last 200 years: the terms "small" and "minor" planets had consistent use relative to "asteroid" right up until 1953, when, boom! their use plummeted. This change wasn't because of a fear of a huge list of planets, nor from a vote by a bureaucracy, but instead from a scientific discovery: Asteroids, it turns out, are not just miniature
version of the larger planets, but are in fact the fragments from the collisions and disintegration of fully formed planets (or protoplanets), rather than miniature versions of the larger planets. This distinction matters because when planets form, the release of gravitational potential energy partially melts their interiors, causing heavy elements to sink to the core and lighter materials to float upwards, sorting into layers. If asteroids were formed like planets, but smaller, they would each be made of a mix of materials, and would be too small to have
enough energy to sort the materials into layers. If instead asteroids were relics from collisions between planets, each one would be a mostly homogenous chunk of material from a single layer. And what astronomers figured out was that asteroids are definitely broken planet pieces - in fact, asteroids are literally classified into groups based on what materials they're made of. Out of the ridiculous number of objects in the asteroid belt, there are only three or four that we think formed on their own as a planet rather than being broken planet fragments: these three or four are the biggest: Ceres, Vesta, Pallas, and maybe Hygia.
So. The asteroids were demoted to non-planet status, not by a vote, nor from a dislike of a long list of planets, but due to a scientific discovery that asteroids are, in fact, physically different from everything else that we call planets. The discovery meant they were split off (in the minds of scientists) into their own group, and those scientists organically stopped calling them planets. In principle, the taxonomic split could have gone the other way - researchers could have kept calling asteroids planets, and come up with a
new name for the remaining non-fragmented objects - but given the fact that asteroids were already previously considered a subcategory of planet (minor planets), it makes much more taxonomic sense to split them off and keep all the rest of the planets, as planets. By this logic, we should also probably stop calling Ceres, Vesta and Pallas asteroids since they formed like planets do. So, even though there are lots of asteroids, for the question of whether or not asteroids count as planets, it turns out that quality matters more than quantity.
Seeing the innumerable fascinating - but completely inhospitable - worlds in our solar system is a reminder of just how diverse, unique and delicate our home planet is. But to keep planet Earth as our home, we have to look after it. That's why we working with the nature-protection community called Planet Wild, our sponser for this video. Every month, the planet wild community funds a mission to restore the oceans, forests and life here on planet Earth. All of this is documented through videos here on YouTube, just like this mission restoring lost peatland ecosystems. You can join in supporting solution-oriented missions globally
by giving whatever amount you want. The first 100 people to sign up using our code MINUTEPHYICS1 will get their first month covered. Just scan this QR code or click the link in the description.
