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Joan Cartan-Hansen, Host: From 
up here, you can see all the

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planets in our solar system, 
but do you know exactly what a

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planet is?

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Find out.

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(MUSIC)

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Mercury, Venus, Mars, Jupiter 
and Saturn were the first

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space objects called planets.

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The word planet means wanderer 
because the ancient Greeks saw

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them as moving lights in the 
sky.

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But the ancient Greeks didn't 
think of Earth as a planet.

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They thought it was the center 
of the universe and all space

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bodies revolved around it.

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Over time, humans learned the 
Sun is the center of our solar

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system.

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And all the planets, including 
Earth, orbit around it.

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Still, defining a planet was 
back then was easy.

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Everything, the Sun, the Moon, 
the Earth, all space bodies

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were called planets.

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Until a new invention came 
along.

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In 1781, Uranus became the 
first planet discovered by

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telescope.

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And by then, scientists had 
realized that the Sun was a

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star and the Moon was a 
natural satellite of the

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Earth.

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But everything else was pretty 
much described as a planet.

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Neptune was discovered in 1846 
and Pluto in 1930.

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But Pluto is different from 
terrestrial planets like

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Mercury, Venus, Earth and Mars 
or the gas giants like Jupiter

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and Saturn or the ice giants 
like Uranus or Neptune.

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Pluto is small.

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And its moon, Charon, is half 
the size of Pluto.

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It's so big that the two are 
sometimes called a double

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planet system.

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Pluto has an usual orbit that 
sometimes brings it closer to

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the Sun than Neptune.

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And its orbit isn't circular 
like the other planets.

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Pluto is different and that 
lead to confusion.

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By 1992, planetary scientists 
found a number of ice worlds

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in the Kuiper Belt.

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Brian Jackson, Assoc.

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Prof. of Physics, Boise State 
University: The Kuiper Belt is

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a donut shaped region in the 
outer solar system, beyond the

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orbit of Neptune.

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And it's filled with objects, 
made up mostly of ice, so

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water ice, but also methane 
and ammonia ice.

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And some of these objects are 
large enough that we would

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call them ice worlds.

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And some of these ice worlds 
in the Kuiper Belt, they

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look a lot like Pluto.

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And so, astronomers were faced 
with this question.

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If we were going to call Pluto 
a planet, what were we going

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to call all these hundreds of 
thousands of other objects,

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very similar to Pluto in the 
Kuiper Belt.

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Cartan-Hansen: So, astronomers, 
decided they needed to clarify

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what exactly is a planet.

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In 2006, the International 
Astronomical Union, a

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worldwide group of top 
astronomers, came up with a

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new definition...

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A planet is a celestial body 
that is in orbit around the

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Sun.

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It has enough mass so gravity 
helps make it round.

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And it has cleared the 
neighborhood around its orbit.

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Jackson: So that means that 
the object has enough mass and

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enough gravity that it 
consumes or sweeps up other

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small bodies in its orbital 
path and Pluto, since there's

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a lot of other objects out 
there, doesn't do that.

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It's just too small.

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Cartan-Hansen: So, what to do 
with Pluto?

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Astronomers came up with the 
idea of a dwarf planet.

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A dwarf planet orbits around 
the Sun.

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It has enough mass and gravity 
to form an almost round shape.

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But it isn't big enough to 
clear the neighborhood.

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It also can't be a moon.

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As of 2020, we have five dwarf 
planets in our solar system.

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In order closest to the sun 
out, they are Ceres, Pluto,

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Haumea, Makemake and Eris.

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Ceres is the closest dwarf 
planet to the Sun.

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It's located in the asteroid 
belt, between Mars and

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Jupiter, making it the only 
dwarf planet in the inner

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solar system.

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Next comes Pluto, then Haumea.

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Haumea has an elongated shape 
rather than being totally

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round, probably because of the 
speed of its rotation.

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Next is Makemake.

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Makemake has a moon and is a 
classic Kuiper Belt object.

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And finally, Eris...

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Eris is almost as large as 
Pluto and was once considered

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our solar system's tenth 
planet.

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Jackson: Planetary scientists 
are still looking for dwarf

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planets.

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This is an active area of 
research.

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There may be another hundred 
in our solar system and

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probably hundreds more just 
outside of the Kuiper Belt.

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Cartan-Hansen: Not everyone 
agrees with the new definition

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of a planet.

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Some still think Pluto 
deserves that title.

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Jackson: That's true, but the 
definition of a planet may

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still change as we learn more 
about our solar system and

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just because Pluto and Ceres 
and all those other objects

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are called dwarf planets it 
really doesn't make them any

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less interesting.

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What we call them is almost 
the least interesting aspect

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of these objects which have 
a fascinating variety of

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phenomena.

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It's just our current way of 
classifying one of the many

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wonders of space.

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Cartan-Hansen: If you want to 
learn more about planets and

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dwarf planets, check out the 
science trek website.

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You'll find it at science trek 
dot org.

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(MUSIC)

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Announcer: Presentation of 
Science Trek on Idaho Public

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Bettis family legacy of

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