That's Why Pluto Is Not a Planet Anymore

Summary

This script delves into the historical and scientific debate over Pluto's planetary status. It recounts the mnemonic 'My very eager mother just served us nine pizzas' used to remember the solar system's planets, including Pluto, before its demotion. The script explores the discovery of Pluto in 1930, its size revisions, and the International Astronomical Union's 2006 definition that led to Pluto's exclusion due to not 'clearing its neighborhood.' It also touches on the controversy and alternative definitions that could reinstate Pluto as a planet, highlighting the complexities in defining celestial bodies.

Takeaways

• 📚 The mnemonic 'My very eager mother just served us nine pizzas' was used to help children remember the order of the planets before Pluto's demotion.
• 🪐 Pluto was discovered in 1930 and was initially thought to be as large as Uranus or Neptune, but its size estimates were repeatedly revised down.
• 🌌 The discovery of Ceres in 1801 led to a reevaluation of what constitutes a planet, as it was similar in size to other objects in the asteroid belt.
• 🌕 The International Astronomical Union (IAU) defined a planet in 2006 as an object that orbits the Sun, is nearly spherical, and has 'cleared the neighborhood' around its orbit.
• 🚫 Pluto failed to meet the third criterion of having 'cleared the neighborhood', as it shares its orbit with other objects in the Kuiper Belt.
• 🌟 The IAU's definition of a planet excludes objects like rogue planets not orbiting a star and places emphasis on the environment around a celestial body rather than its intrinsic properties.
• 🌙 Some scientists propose a different definition that would include Pluto as a planet, but this would also elevate other moons and celestial bodies to planet status.
• 📉 Pluto's demotion to a 'dwarf planet' sparked debate and some controversy, with a few scientists disagreeing with the IAU's ruling.
• 🔍 The script suggests that the current definition of a planet may not be perfect and hints at the possibility of revisiting the criteria in the future.
• 🐶 An amusing aside in the script mentions the myth that Pluto was named after a Disney dog, which is not true.

Q & A

What mnemonic device was traditionally used to remember the order of the planets in our solar system?

The mnemonic device used was 'My very eager mother just served us nine pizzas,' which stood for Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

Why might students today question the number of planets mentioned in the traditional mnemonic?

Students might question the number of planets because Pluto was reclassified and is no longer considered a planet by the International Astronomical Union (IAU).

What significant change occurred in 2006 regarding Pluto's status in the solar system?

In 2006, the IAU redefined the criteria for a celestial body to be considered a planet, and as a result, Pluto was reclassified as a 'dwarf planet' rather than a full-fledged planet.

What was the historical definition of a planet from the age of Galileo to the nineteenth century?

From the age of Galileo to the nineteenth century, a planet was defined as any object orbiting the Sun.

What discovery in 1801 led to a reevaluation of the definition of a planet?

The discovery of Ceres in 1801, which was initially considered a planet but later recognized as part of the asteroid belt due to its small size and the discovery of similar objects, led to a reevaluation of the definition of a planet.

How did the discovery of Pluto's moon, Charon, affect its status as a planet?

The discovery of Charon, which is relatively large compared to Pluto, caused Pluto's orbit to wobble, which is atypical for a planet, leading some astronomers to question its planetary status.

What were the three qualifications set by the IAU in 2006 for an object to be considered a planet?

The three qualifications set by the IAU in 2006 are: 1) the object must orbit the Sun, 2) the object must be a sphere or nearly so, and 3) the object must have 'cleared the neighborhood' around its orbit, meaning there are no other objects of comparable size nearby.

Why did the discovery of Eris in 2005 pose a challenge to Pluto's status as a planet?

Eris, although slightly smaller than Pluto, was initially believed to be more massive, which added to the debate about Pluto's status as a planet and contributed to the IAU's decision to redefine what constitutes a planet.

What alternative definition of a planet has been proposed by some scientists, and what would be the consequence of adopting it?

Some scientists propose that only the second qualification (being a sphere or nearly so) should be required for an object to be considered a planet. The consequence of adopting this definition would be an increase in the number of recognized planets, including our Moon and several moons of other planets, as well as many asteroids.

What are some of the criticisms of the current definition of a planet as set by the IAU?

Some criticisms of the current definition include the exclusion of rogue planets not orbiting any star, an emphasis on the surroundings of a planet rather than the planet itself, and the potential for confusion with celestial bodies like Proxima Centauri, which fulfills all the requirements but is unquestionably a star.

Outlines

00:00

🌞 The Demise of Pluto's Planet Status

This paragraph delves into the historical mnemonic for remembering the order of the planets and how Pluto's demotion from planetary status came about. It discusses the discovery of Pluto in 1930 and how initial misconceptions about its size contributed to its initial classification as a planet. The paragraph also touches on the discovery of Pluto's moon, Charon, and how it complicated Pluto's status due to their similar sizes and the gravitational influence on Pluto's orbit. The narrative concludes with the International Astronomical Union's 2006 decision to redefine what constitutes a planet, leading to Pluto's exclusion based on its failure to 'clear its neighborhood' of other objects.

05:05

🌌 Redefining Planets: The Pluto Debate

The second paragraph continues the discussion on Pluto's planetary status by detailing the International Astronomical Union's new criteria for defining a planet. It explains the three qualifications an object must meet to be considered a planet: orbiting the Sun, being spherical, and having cleared its orbit of other objects. Pluto's inability to meet the third criterion led to its demotion. The paragraph also explores alternative definitions proposed by some scientists that would include Pluto as a planet but would significantly expand the number of recognized planets. It concludes with a critique of the current definition, highlighting its imperfections and the complexities involved in defining celestial bodies, and suggests that a reevaluation of the definition might be necessary.

Keywords

💡Mnemonic device

A mnemonic device is a learning technique that aids in memory retention through the use of patterns, words, or associations. In the video, the 'My very eager mother just served us nine pizzas' sentence is an example of a mnemonic used to remember the order of the planets. It's a tool that helps children to recall information in a structured and memorable way, which is central to the educational theme of the video.

💡Pluto

Pluto is a dwarf planet located in the Kuiper Belt, a region of space beyond Neptune. The video discusses the historical and scientific debate surrounding Pluto's status as a planet, which is a central theme. It was once considered the ninth planet in our solar system, but its demotion to a dwarf planet in 2006 is a key point of discussion.

💡Astronomical Union

The International Astronomical Union (IAU) is the global authority responsible for defining astronomical terminology and standards. In the video, the IAU's 2006 decision to redefine what constitutes a planet is pivotal, as it led to Pluto's reclassification. This decision is a critical part of the narrative, illustrating how scientific consensus can evolve over time.

💡Planetary definition

The term 'planetary definition' refers to the criteria used to determine whether an object in space is classified as a planet. The video explains how the IAU's definition includes three key qualifications: orbiting the Sun, being nearly spherical, and having 'cleared the neighborhood' around its orbit. This definition is central to the video's exploration of Pluto's status.

💡Ceres

Ceres is the largest object in the asteroid belt and was once considered a planet before the definition was refined. The video uses Ceres as a historical precedent for Pluto's situation, highlighting how scientific understanding and classification can change.

💡Asteroid belt

The asteroid belt is a region of space located between Mars and Jupiter, containing numerous small rocky bodies. The video mentions the asteroid belt in the context of Ceres and the reevaluation of what constitutes a planet, emphasizing the challenges in defining celestial bodies.

💡Charon

Charon is Pluto's largest moon and is significant because of its relative size to Pluto, which complicates the classification of Pluto as a planet. The video discusses how Charon's size and gravitational influence on Pluto contribute to the debate over Pluto's planetary status.

💡Eris

Eris is a dwarf planet in the scattered disc, a region beyond the Kuiper Belt. The video notes the discovery of Eris as a catalyst for the IAU's reevaluation of Pluto's status, as Eris is similar in size and composition to Pluto, which added to the complexity of defining 'planet'.

💡Dwarf planet

A dwarf planet is a celestial body that orbits the Sun, is not a moon, has sufficient mass for self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and has not cleared the neighborhood around its orbit of other debris. The video explains that Pluto is now classified as a dwarf planet, which is central to the discussion about its planetary status.

💡Neighborhood clearance

The term 'neighborhood clearance' refers to the criterion that a planet must have cleared its orbit of other debris to be considered a planet. The video explains that Pluto fails this test because of the presence of other objects in its orbit, which is a key reason for its demotion from planetary status.

💡Rogue planet

A rogue planet is a hypothetical type of planet that does not orbit a star. The video touches on the idea that the current definition of a planet might exclude such objects, even though they fulfill other criteria, suggesting that the definition of a planet could be further refined.

Highlights

Mnemonic devices like 'My very eager mother just served us nine pizzas' were used to teach the order of the planets before 2006.

Pluto's status as a planet was reevaluated due to advancements in our understanding of space.

Pluto was discovered in 1930, and its size estimates have been revised multiple times, finally determining it to be smaller than Earth's moon.

The discovery of Pluto's moon Charon in 1978 raised questions about Pluto's classification as a planet.

Pluto's orbit is affected by Charon, indicating it has not 'cleared the neighborhood' around its orbit.

The discovery of Eris in 2005 further complicated Pluto's status, as it is similar in size and composition.

The International Astronomical Union revised the definition of a planet in 2006, excluding Pluto.

A planet must now orbit the Sun, be spherical, and have cleared its orbit of other objects.

Pluto fails the third criterion of having cleared its orbit, leading to its reclassification as a dwarf planet.

Some scientists propose a different definition that would include Pluto as a planet.

The proposed alternative definition could increase the number of recognized planets to 115.

The current definition of a planet has its flaws, such as excluding rogue planets.

The debate over Pluto's status raises questions about the definitions of celestial bodies.

The naming of Pluto was not inspired by Disney's dog, contrary to a common misconception.

The video encourages viewers to like and share if they learned something new.

Transcripts

Now listen up. My very eager mother just served us nine pizzas. Ooh, yum! Hey, if you were

in elementary school before 2006, there's a good chance you had to memorize something

similar to that sentence. This mnemonic device was used to teach children the order of the

planets in our solar system; Mercury, Venus, Earth, Mars, Jupiter, Saturn, URinus—also

pronounced UrANUS (yeah, you can’t hide from it either way), and finally Neptune,

and Pluto. Now, if you're currently in elementary school,

you might be saying, "Wait, there were nine planets?” before going back to playing Fortnite

Go or whatever kids are into these days. So, what happened to Pluto? It’s not like

it’s gone anywhere. It’s still out there on the edge of the solar system, as cold and

far away as ever, so what changed? Pluto hasn't, but our understanding of it

has. We know way more about space than we did one hundred years ago. Pluto itself was

only discovered in 1930, so it's not like the planet lineup hasn't been modified before.

Still, that doesn’t change the fact that there’s something inherently strange about

a planet being demoted. Who even knew that was a thing that could happen?

What do you think? Should Pluto be a planet or is it right where it belongs. Give me your

opinion in the comments. Things might get a little clearer once we

figure what the word planet means. The exact definition has changed a lot over time, but

from the age of Galileo to the nineteenth century, it referred to any object orbiting

the Sun. This might seem a little vague, but worked perfectly well until the year 1801.

That was the year astronomers discovered Ceres, a planet, in massive air quotes, orbiting

the Sun halfway between Mars and Jupiter. You may recognize this as where the Asteroid

belt is, and it isn’t because Ceres pulled an Alderaan and broke into a thousand pieces.

If it had, I’m pretty sure kids would pay a lot more attention in science class.

Anyway, astronomers noticed right away that Ceres was quite small, with only half the

radius of Earth's Moon. The year after Ceres’ discovery, the astronomy

community was abuzz with the discovery of another planet named Pallas. Then they found

another a few years later. And another. And another. And I think you see where this was

going. Four new planets are one thing...

Well, four things, but you get the idea. When it turns into thousands, it might be

time to reevaluate some definitions. Astronomers noticed that the rocks had more in common

with each other than any of the other planets. They were tiny; they were barren; and the

vast majority weren't even spherical. These small objects became known as the asteroids

in the asteroid belt, and the world went back to learning the seven planets, Mercury, Venus,

Earth, Mars, Jupiter, Saturn, URinus (UrANUS, whatever…). Neptune made eight in 1846,

with Pluto joining the party eighty-four years later.

With the benefit of hindsight, it might be easy to guess that Pluto would go down the

same path as it’s long lost cousin Ceres. But we all know what they say about hindsight,

and there are things we know about Pluto now that weren't obvious in the early twentieth

century. For example, while we know Pluto is nothing more than a tiny ball of ice and

rock, initial measurements gave it a similar size to Uranus or Neptune.

Pluto’s size was revised down to that of the Earth the year after its discovery. In

1948, it shrank again. It would keep shrinking until Astronomers were finally able to get

an accurate measurement in 2006. We know that Pluto is only one 459th the size of planet

Earth, making it smaller than the moon and only about twice the size of the former planet

Ceres. Pluto’s planet status was in trouble long

before that, however. In 1978 astronomers discovered Pluto's moon, Charon. At first,

this might seem to be strong evidence in Pluto's favor. If it's big enough to have a satellite,

it must be a planet, right? Not exactly. Charon may be smaller than Pluto,

but not that much smaller. One half the diameter might seem like a big difference, but not

compared to the differences in size between the other planets and their moons. In fact,

they're similar enough in mass for Charon to noticeably affect Pluto's orbit around

the Sun, causing it to wobble to and fro as it travels through space.

That’s some very un-planet like behavior, and it led more than a few astronomers to

feel uncomfortable about using that word to describe Pluto. And they got even less comfortable

every time a new, Pluto-like object was discovered beyond Neptune's orbit.

Still, Pluto had been on the list for decades by this point, so not everyone was ready and

willing to give it the boot. All of that changed with the discovery of Eris in 2005. While

Eris is slightly smaller than Pluto, initial measurements placed it as somewhat more massive.

This added one more strike against Pluto’s status as a planet, and in 2006 the International

Astronomical Union decided it was once again time to revise their definition of what is

or is not a planet. From then on, an object was only a planet

if it fits the following three qualifications. First, it must orbit the Sun. Number two is

that the object must be a sphere, or at least nearly so. Pluto checks the first two boxes

but runs into trouble with number three, which says a planet must have "cleared the neighborhood"

around it. Clearing its neighborhood means that there

are no nearby objects other than its own satellites. Pluto has failed to accomplish that feat,

so the third box remains unchecked. One strike might be okay in baseball, but

it’s a deal-breaker if you're trying to stay on the exclusive list for Club Planet.

Now, not everyone was thrilled to find out Pluto got kicked out. It's hard not to feel

bad for the little guy, and even today, there are a few scientists who disagree with the

IAU’s ruling and want to call Pluto a planet once more.

They propose that the first and third qualifier be removed. Under this definition, any object

with enough mass to maintain a spherical, or nearly spherical, shape would qualify as

a planet. While this would let Pluto back in, it would also let in our Moon, as well

as several of the moons of Jupiter, Saturn, and Neptune. That's not to mention Ceres,

Charon, and a whole bunch of asteroids and other objects. All combined, this new definition

would take us from a manageable eight planets to an unwieldy one hundred and fifteen.

Just imagine the pneumonic device you would need for that.

Now, the current definition is far from perfect. As some astronomers have pointed out, it excludes

rogue planets not orbiting any star. Some feel that it also puts too much emphasis on

what surrounds the perspective planet instead of the worlds themselves.

To quote Ethan Siegel from Medium.com, “Mercury, at the distance of Jupiter, would never clear

its orbit and wouldn’t obtain planetary status. A world much smaller than Mercury

could be a planet around a red dwarf star, while even Earth would fail to be a planet

if it were out in the Oort cloud somewhere.” On the other hand, what do you do about stars

such as Proxima Centauri, a red dwarf star that orbits the larger and brighter Alpha

Centauri A and B. Is this a planet? It fulfills all the requirements, even though it’s unquestionably

a star. Can you be a star and a planet at the same time? Conventional wisdom says no,

but this is the problem you run into when trying to define words like planet.

All of this might indicate that it may be about time to take another look at how we

define planets. That said, increasing the number by a factor of fourteen doesn't sound

like a great solution. Whatever definition science eventually settles on, sadly Pluto

probably won’t be on it. Ooh, and we didn’t even talk about how Pluto

was named after that Disney Dog! Oh wait, he wasn’t? shucks. Never mind.

Hey, if you learned something new today, then give the video a like and share it with a

friend! And here are some other cool videos I think you'll enjoy. Just click to the left

or right, and stay on the Bright Side of life!