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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