took Pluto for granted.

Fast-forward to 2006.

Suddenly, Pluto was all Americans were talking about.

Or rather, yelling about.

Some people were so angry, they were giving astronomers

death threats.

- People were so angry that they said that

all the astronomers should be put on the wall and shot at.

- [Narrator] That's Thierry Montmerle.

He's former general secretary for the

International Astronomical Union.

The same organization of the world's astronomers

that changed Pluto's status from planet to dwarf planet

in 2006.

And today, over 12 years later,

people still have strong opinions

whenever you ask: Should Pluto be a planet again?

- No.

- Yes.

- No.

- No.

- Yeah.

- Yes.

- No.

- Yes.

- I'm not really sure that I have an opinion

of whether or not Pluto should be a planet.

- [Narrator] There's obviously some confusion

going on here.

So we did the next logical step.

We went to the experts to settle this

once and for all.

That's Alan Stern.

He leads NASA's New Horizons mission,

which flew by Pluto in 2015.

- And in planetary science, where the experts

in planets are, we call small planets "planets."

We call large moons "planets."

We call all the planets around other stars "planets."

And the astronomer's definition

wouldn't allow any of those to be planets.

- [Narrator] OK, so basically Stern says

it depends on context.

But why?

Back in the early '90s, Pluto was a planet, period.

No context needed.

So what changed?

By the late '90s, it was becoming clear

that Pluto wasn't alone.

Astronomers had discovered other worlds

in the same region, called the Kuiper belt.

And some of them looked awfully similar to Pluto.

Then in 2005, astronomers discovered Eris,

which estimates at the time suggested was even larger

than Pluto.

- On January 8 of this year,

while looking through some old data

that we had taken with the Samuel Oschin Telescope

at Palomar Observatory,

we found, much to our surprise, an object

three times further away than Pluto.

This will absolutely rewrite the history

of astronomy textbooks.

- [Narrator] And while these new worlds

looked and behaved like Pluto,

they were completely different from every other planet

in our solar system.

Something had to be done.

It was clear that astronomers were in need

of something they never had before:

a good definition for what makes a planet a planet.

So in the wake of these new discoveries,

the IAU came up with a checklist.

A planet must orbit the sun,

have a nearly round shape,

and have cleared its neighborhood,

meaning no other large objects are nearby.

And that last requirement boots Pluto off Team Planet.

Yes, it orbits the sun.

Yes, it's spherical.

But Pluto isn't always the dominant gravitational force

in its neighborhood.

For one thing, Eris shares the region

and isn't stuck in Pluto's orbit.

The end result? Pluto is bumped

from "planet" to "dwarf planet."

Now, Stern argues that a dwarf planet is a kind of planet.

Just like how

- The bonsai tree is still a tree.

And a Chihuahua is still a dog.

- [Narrator] But other experts, like Montmerle,

prefer to think of dwarf planets as their own class.

So where does that leave us?

Well, in the grand scheme of things,

it doesn't really matter what Pluto's official

designation is.

- If people don't like it, they don't use it, period.

- [Narrator] Which is exactly what planetary scientists do.

- So everyone's using the planetary scientist's

definition in the written, refereed scientific literature.

And using it at the podium

in giving scientific presentations.

That's the kind of consensus

that's very powerful in science.

- [Narrator] So maybe Pluto isn't a planet

the same way that Earth and Jupiter are planets.

But that doesn't mean we should ignore it.

Besides, there's more to this dwarf planet

than meets the eye.

The New Horizons mission has found evidence

of ice volcanoes, hidden oceans.

- There are evidence for icefalls and floes and glaciers.

Just tremendous stuff.

- [Narrator] And that's true, no matter what you call it.