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  • - [Narrator] Back in the '90s, most people

  • 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.

- [Narrator] Back in the '90s, most people

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Should Pluto Be A Planet Again?

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    Liang Chen posted on 2019/06/25
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