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Should Pluto Be A Planet Again?


– [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.

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