The REAL Reason Asteroids Aren't Planets

0:04 When the first asteroids were discovered over 200 years ago,

0:07 they were celebrated as new planets, and the story goes that the rapid

0:10 and overwhelming discovery of hundreds and then

0:12 thousands more quickly convinced scientists that asteroids

0:14 shouldn't be planets- but that’s totally wrong:

0:16 the real reason asteroids were demoted from “planet” status came nearly 100

0:20 years after the hundredth asteroid was

0:22 discovered and decades after the thousandth,

0:23 and had nothing to do with their quantity, but in fact, their quality.

0:27 Of course, up until 1800,

0:29 only seven primary planets– objects that directly orbit

0:31 the Sun– only seven primary planets had been discovered:

0:34 Mercury, Venus, Earth, Mars, Jupiter, Saturn and Uranus.

0:36 But telescopes got better and we found more: a small 8th primary planet,

0:40 Ceres, was discovered in 1801 between Jupiter and Mars,

0:43 and then there was a surge of new

0:44 planets discovered with almost identical orbits to Ceres:

0:46 Pallas, Juno, Vesta, and more, and more, and more!

0:49 It soon became clear that there was a band of small

0:52 planets orbiting the Sun between Mars and Jupiter, and so,

0:54 when describing the solar system, astronomers found it useful to group them

0:57 together as “the small planets”,“ the minor planets”,

0:59 “the planetoids”, “the asteroids” or “the numbered planets”,

1:02 because eventually so many were being discovered

1:03 so quickly it was easier to keep track of them all right away with numbers

1:06 rather than coming up with fancy names.

1:07 But there wasn’t yet any scientific reason

1:09 to stop thinking of the asteroids as planets.

1:11 I mean, “having too many” isn’t by itself a good

1:14 scientific criteria (think about all the thousands of known

1:16 planets we’ve now discovered orbiting other stars– their sheer

1:20 number isn’t a reason not to call them planets!).

1:24 So despite some embarrassment over the rapidly growing list of planets 19th

1:28 and early 20th century astronomers still

1:30 considered all the asteroids to be planets.

1:31 Well, except one astronomer,

1:32 who argued that the asteroids should be their own category,

1:35 but his reasoning was based on incorrect data and theoretical

1:37 misunderstandings and was ultimately rejected

1:38 by the wider astronomical communidy.

1:40 Even today, the International Astronomical Union’s commission on asteroids

1:43 is STILL technically named the “Minor Planet Center” (even though,

1:46 as we are about to discover, it’s a poor name for them).

1:49 So why don’t we include the asteroids in our list of planets today?

1:52 Look at this analysis of scientific papers in the last 200 years:

1:55 the terms “small” and “minor” planets had consistent use

1:58 relative to “asteroid” right up until 1953, when, boom!

2:01 their use plummeted.

2:02 This change wasn’t because of a fear of a huge list of planets,

2:05 nor from a vote by a bureaucracy,

2:06 but instead from a scientific discovery: Asteroids, it turns out,

2:09 are not just miniature version of the larger planets,

2:12 but are in fact the fragments from the collisions

2:14 and disintegration of fully formed planets (or protoplanets),

2:15 rather than miniature versions of the larger planets.

2:15 This distinction matters because when planets form,

2:17 the release of gravitational potential energy partially melts their interiors,

2:20 causing heavy elements to sink to the core

2:22 and lighter materials to float upwards, sorting into layers.

2:25 If asteroids were formed like planets, but smaller,

2:27 they would each be made of a mix of materials,

2:29 and would be too small to have enough energy to sort the materials into layers.

2:32 If instead asteroids were relics from collisions between planets,

2:35 each one would be a mostly homogenous chunk of material from a single layer.

2:38 And what astronomers figured out was that asteroids

2:40 are definitely broken planet pieces- in fact,

2:43 asteroids are literally classified into groups

2:44 based on what materials they’re made of.

2:46 Out of the ridiculous number of objects in the asteroid belt,

2:48 there are only three or four that we think formed

2:50 on their own as a planet rather than being broken planet fragments:

2:53 these three or four are the biggest: Ceres, Vesta, Pallas, and maybe Hygia.

2:58 So.

2:58 The asteroids were demoted to non-planet status, not by a vote,

3:02 nor from a dislike of a long list of planets,

3:04 but due to a scientific discovery that asteroids are,

3:06 in fact, physically different from everything else that we call planets.

3:09 The discovery meant they were split off

3:10 (in the minds of scientists) into their own group,

3:12 and those scientists organically stopped calling them planets.

3:15 In principle, the taxonomic split could have gone

3:17 the other way– researchers could have kept calling asteroids planets,

3:19 and come up with a new name for the remaining non-fragmented objects- but given

3:23 the fact that asteroids were already previously

3:25 considered a subcategory of planet (minor planets),

3:27 it makes much more taxonomic sense to split them off

3:30 and keep all the rest of the planets, as planets.

3:32 By this logic, we should also probably stop calling Ceres,

3:35 Vesta and Pallas asteroids since they formed like planets do.

3:38 So, even though there are lots of asteroids,

3:40 for the question of whether or not asteroids count as planets,

3:43 it turns out that quality matters more than quantity.

3:47 Seeing the innumerable fascinating- but completely inhospitable- worlds

3:50 in our solar system is a reminder of just how diverse,

3:53 unique and delicate our home planet is.

3:54 But to keep planet Earth as our home, we have to look after it.

3:57 That’s why we working with the nature-protection community called Planet Wild,

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