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

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

planet is?

Find out.

(MUSIC)

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

space objects called planets.

The word planet means wanderer
because the ancient Greeks saw

them as moving lights in the
sky.

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

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

bodies revolved around it.

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

system.

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

Still, defining a planet was
back then was easy.

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

were called planets.

Until a new invention came
along.

In 1781, Uranus became the
first planet discovered by

telescope.

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

star and the Moon was a
natural satellite of the

Earth.

But everything else was pretty
much described as a planet.

Neptune was discovered in 1846
and Pluto in 1930.

But Pluto is different from
terrestrial planets like

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

and Saturn or the ice giants
like Uranus or Neptune.

Pluto is small.

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

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

planet system.

Pluto has an usual orbit that
sometimes brings it closer to

the Sun than Neptune.

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

Pluto is different and that
lead to confusion.

By 1992, planetary scientists
found a number of ice worlds

in the Kuiper Belt.

Brian Jackson, Assoc.

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

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

orbit of Neptune.

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

water ice, but also methane
and ammonia ice.

And some of these objects are
large enough that we would

call them ice worlds.

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

look a lot like Pluto.

And so, astronomers were faced
with this question.

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

to call all these hundreds of
thousands of other objects,

very similar to Pluto in the
Kuiper Belt.

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

what exactly is a planet.

In 2006, the International
Astronomical Union, a

worldwide group of top
astronomers, came up with a

new definition...

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

Sun.

It has enough mass so gravity
helps make it round.

And it has cleared the
neighborhood around its orbit.

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

enough gravity that it
consumes or sweeps up other

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

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

It's just too small.

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

Astronomers came up with the
idea of a dwarf planet.

A dwarf planet orbits around
the Sun.

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

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

It also can't be a moon.

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

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

Haumea, Makemake and Eris.

Ceres is the closest dwarf
planet to the Sun.

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

Jupiter, making it the only
dwarf planet in the inner

solar system.

Next comes Pluto, then Haumea.

Haumea has an elongated shape
rather than being totally

round, probably because of the
speed of its rotation.

Next is Makemake.

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

And finally, Eris...

Eris is almost as large as
Pluto and was once considered

our solar system's tenth
planet.

Jackson: Planetary scientists
are still looking for dwarf

planets.

This is an active area of
research.

There may be another hundred
in our solar system and

probably hundreds more just
outside of the Kuiper Belt.

Cartan-Hansen: Not everyone
agrees with the new definition

of a planet.

Some still think Pluto
deserves that title.

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

still change as we learn more
about our solar system and

just because Pluto and Ceres
and all those other objects

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

less interesting.

What we call them is almost
the least interesting aspect

of these objects which have
a fascinating variety of

phenomena.

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

wonders of space.

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

dwarf planets, check out the
science trek website.

You'll find it at science trek
dot org.

(MUSIC)

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Science Trek on Idaho Public

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