- You know what a planet is, right?

Big, round thing that orbits a star?

Not so fast.

The surprisingly vicious debate over the planetary status of Pluto has given us a fascinating glimpse into what a scientific definition really is and that perhaps the word "planet" is too vague to be used as a scientific definition at all.

(upbeat music) We love to classify things.

Labels help us keep stuff organized in our heads.

In science, categorization provides a fast and easy way to know the properties of a member of a group just by knowing what group it belongs to.

Chemists group elements on a periodic table.

Those groups exhibit similar chemical properties that reflect outer shell electron number.

Biologists group organisms by similar physical characteristics and this taxonomy reflects genetic relationships.

Astronomers are all about space taxonomy.

We classify galaxies based on their shape, black holes based on how they feed and how their oriented, stars based on their color and brightness, and planets by, well, by a set of criteria that has caused more tension and heartbreak than any made up grouping scheme really should.

Because a change in that scheme demoted Pluto from planet to not-planet.

Today, we're gonna settle whether this was reasonable and whether we should keep the word "planet" at all.

The definition of "planet" has changed a lot.

If you were an ancient astronomer like say Ptolemy, the planets were the asteres planetai: the wandering stars.

These included Mercury, Venus, Mars, Jupiter, and Saturn, but also the sun and the moon.

Basically, anything that moved relative to the background stars.

Note that they did not include the Earth.

This definition of planet was the most sensible classification for thousands of years based on our observations and understanding of the universe.

But understanding improves.

In 1543, Nicolaus Copernicus published On the Revolutions of the Heavenly Spheres which cast the sun and not the Earth as the center of the universe and the Earth in its proper place among the planets.

This picture was cleaned up by the observations of Galileo and Brahe and the mathematical models of Kepler and Newton.

Newton's law of gravity led to consistent predictability of the motion of the heavenly spheres.

The solar system finally made observation and theoretical sense.

There were now six planets orbiting the sun in perfect mathematical harmony.

But, new discoveries were about to mess up these beautifully simple picture.

See, that's the thing about science.

No so-called fact is ever perfectly safe.

Everything is subject to revision if evidence turns against it.

That's the beauty of the scientific process.

Keep this in mind when we get to Pluto.

New wandering stars were discovered in the centuries following Newton.

Uranus had been spotted many times throughout history, but was only identified as a planet after William Herschel recorded its movement in 1781.

That same motion almost perfectly reflected the clockwork predictions of Newtonian mechanics.

Almost.

Slight deviations in Uranus' orbit portrayed the existence of Neptune which was discovered first in the mathematics and then with a telescope in 1846.

Between these two discoveries, four other bodies joined the planet.

At the beginning of the 1800s, Vesta, Juno, Ceres, and Pallas were all spotted between the orbits of Mars and Jupiter and classified as planets.

But, astronomers kept finding more and more objects in that vicinity and eventually realized that a new group was needed in our taxonomy.

So the class of asteroids came into being.

You might argue that Ceres, Juno, Pallas, and Vesta were unfairly demoted from planet.

After all, Ceres is 20% the diameter of Mercury, so it's in the planetary ballpark at least.

But, where do you draw the line?

How many asteroids should we then classify as planets?

Or do we drop Mercury which is after all only 50% larger than our moon and smaller than Saturn's moon, Titan?

Our taxonomy had to evolve as our understanding of the solar system grew.

And so, the term "planet" became reserved for the now familiar eight: Mercury through Neptune.

Although the definition of the word was still vague.

And this is where Pluto finally comes into our story.

Even with the discovery of Neptune, the orbit of Uranus still appeared to a bit off, at least in some calculations.

And so began a feverish search for yet another planet to resolve this discrepancy.

The search for the so-called Planet X became the grail quest of Percival Lowell, businessman-turned-astronomer.

He built the Lowell Observatory in Arizona with that single goal.

In 1930, 14 years after Lowell's death but still powered by his observatory and his family's fortune, Planet X was finally discovered.

Or so we thought.

Clyde Tombaugh, a 22-year old Kansas farm boy spotted a moving speck of light in a series of photographic plates taken under the guidance of Vesto Slipher, Lowell Observatory's director.

Orbital calculations put the object beyond the orbit of Neptune and it appeared within six degrees of one of Slipher's predictions for the location of the mysterious Planet X. Astronomers were expecting a planet and one was found roughly where they thought it should be, so perhaps it's not surprising that Pluto was hailed as a planet without due scientific process.

That's not to say everyone agreed.

The orbit of the new object was far more elliptical, stretched out than any other planet.

It also seemed too faint to possibly have the mass required to explain Uranus' orbital discrepancies.

By 1931, astronomers had figured out that there didn't even need to be a ninth planet to account for Uranus' strange orbit.

But because Pluto was the only such object yet discovered at that distance, it kept its classification as planet.

New astronomy textbooks included distant Pluto and generations of students memorized nine rather than eight planets.

Fast forward several decades.

With the advent of giant telescopes and digital cameras, we begin to find more and more objects that muddy the definition of planet.

In the late 80s, the first brown dwarf was discovered.

These giant orbs of gas aren't massive enough to ignite nuclear fusion in their cores like a true star, but still seemed too massive to be called planets.

And yet, some brown dwarves orbit other more massive stars just like planets do.

Our overly vague definition of planet left these brown dwarves in taxonomical limbo.

Also through the 1990s, more and more moving specks were discovered within our solar system beyond Neptune's orbit.

They were mostly much smaller than Pluto but appeared to form a belt of countless objects.

What we now call the Kuiper belt and that encompasses Pluto's orbit.

Pluto became to the Kuiper belt what Ceres was to the asteroid belt: the biggest fish in the pond.

Big enough to cling to its title of planet or so we thought.

Telescopes got bigger and our mapping of the Kuiper belt became more thorough and in the early 2000s, a number of objects in the Kuiper belt and beyond were found to be similar in size to Pluto: Quaoar, Makemake, Orcus, Sedna, Eris and more.

Eris was the last straw.

It's 28% more massive than Pluto which spurred NASA to initially hail it as the tenth planet.

But theoretical prediction suggested that we'd only seen the tip of the Kuiper belt iceberg.

There must be hundreds more objects in the mass range of Pluto and perhaps a couple of thousands.

If we class them all as planets, school children would need novella-length mnemonics to remember them all.

And so, despite the anger of school children everywhere, it was in their own interest that astronomers decided to act.

In 2006, the International Astronomical Union, the governing body of all astronomy names, designations, and definitions, met to finally define what it meant to be a planet in our solar system.

A number of definitions were debated.

They eventually everted and agreed on the following: a planet must, one, be in its own orbit around the sun, not around another planet like a moon.

Two, have sufficient mass to assume hydrostatic equilibrium, meaning it must be roughly spherical.

And three, have cleared the neighborhood around its orbit.

While Pluto satisfies the first two, it doesn't meet the third requirement.

Were it a proper planet, it would've collected or scattered all Kuiper belt objects in its orbit.

"Planet" got redefined to something that Pluto just isn't.

In the same process, the IAU created an entirely new class of object: dwarf planet, an object which has its own orbit and its spherical-ish but not massive enough to clear its orbit.

Pluto, Eris, Ceres, Haumea, and Makemake were all promoted to dwarf planet and many similar objects will no doubt follow.

While this redefinition was meant to settle the debate, some still argue that Pluto should be grandfathered in as a planet.

Even the current NASA administrator, Jim Bridenstine, is declaring Pluto a planet once more, saying it's the way I learned it.

Well, political appointees declare lots of things and not everything we learn as kids is true.

Fortunately, the actual scientific process is more rigorous than that.

Scientific definitions require careful thought, precision, and broad expert consensus and is subject to revision.

Sure, we could add a fourth requirement to the definition of planet like four, ignore all of the above if changing things makes people sad?

But because we are all curious and open-minded individuals here, perhaps it will be more fun to think about why this reclassification is so contentious.

Why all the sentimentality?

The reclassification from planet to dwarf planet does seem like a demotion.

Look, Pluto, I am afraid you haven't cleared your orbit of debris this quarter.

We're gonna have to reevaluate your role in this taxonomy.

Don't think of this as a demotion, it's more of a re-incentivizing horizontal pivot.

That conversation would make anyone cry but let me assure you, the 13 sextillion kilogram ball of rock and ice that is Pluto doesn't have strong opinions about its own taxonomic status among a few classification-crazy bipeds several planetoids away.

We perceive this reassignment as a demotion because we anthropomorphize everything.

But with such powerful imaginations, can't we just reframe this?

As we peer deeper into our universe, we've realized that it's full of weird, beautiful and important worlds.

Some we now call planets, some not.

For example, the moons of Jupiter and Saturn are active worlds that may prove to be the only other homes for life within our solar system.

And Pluto itself proved far more interesting than just another planet.

We already knew about planets, but Pluto's discovery revealed the existence of the Kuiper belt which is of fundamental importance to our understanding of how our solar system formed.

Pluto went from being the least of the planets to one of the greatest of a new class of object.

And what about the term planet?

Despite our attempts at improving the rigor and usefulness of the term, there's still some ambiguity.

The inner rocky planets are very different from the outer gaseous planets.

Should they receive different names?

There's no single, perfectly-correct way to classify groups of objects.

Taxonomy is by nature a somewhat arbitrary art.

And the new IAU definition of a planet, while being more precise, was somewhat tuned to a pre-determined desire to eject Pluto and retain the eight other planets.

A touch arbitrary perhaps, but the new definition is scientifically useful.

The eight solar system bodies currently defined as planets certainly share plenty enough in common.

Similarities in the way they formed and in the way they behave that make it useful to categorize them under a single label.

And like any aspect of the scientific process, this is subject to revision.

Maybe at some point in the future as we learn more about how different worlds form, astronomers will change the definition of planet again.

The language we use to describe the universe becomes more precise as we learn more about its nature.

And anyway, the word "world" still applies to Pluto and it's a rather more poetic label for one of the greatest dwarf planets in known spacetime.