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? [music] ?

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KIDS: Our World!

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EVAN: Hey Globey. What’s the
matter? You look sad today.

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You’re sad because your buddy
Pluto is no longer called a

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planet? Well, that’s partially
true. But that’s no reason to

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be sad. Pluto is still a
planet, just now scientists

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classify it as a "dwarf"
planet.

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Scientists classify objects
based on their traits or

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characteristics, like size and
color.

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In the case of planets,
scientists look at size and the

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orbit or path of the object in
space.

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In 1930, an American named
Clyde Tombaugh discovered

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Pluto. At that time, it was
classified as the 9th planet in

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our solar system. Today though,
many astronomers say that Pluto

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is too small to be accurately
called a planet.

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ALAN LUNSFORD: A lot of people
still consider Pluto a planet.

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They’ve just grown up knowing
it’s a planet and hearing that

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it’s a planet. What’s happened
is we’re starting to discover

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other bodies in our solar
system that are similar to

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Pluto and they’re not
considered planets yet.

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So the question was, well what
defines a planet and what

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doesn’t. We’ve never had a
really scientific definition.

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We’ve only had a list. So in
trying to come up with a

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scientific definition that
encompasses everything we might

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observe, Pluto didn’t quite
make the cut.

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So we decided on three little
stages.

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The body has to orbit a star
and not another planet.

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If it orbits another planet, it
a moon. The body has to be big

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enough basically to be
spherical.

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There are a lot of oddly shaped
asteroids. They’re not big

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enough to make themselves into
a sphere.

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And then, the third criteria is
the orbit of the planet has to

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have been such that it has
cleared its neighborhood of

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other things that are
potentially planets.

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Otherwise you just kind of have
a belt of things.

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And so Pluto didn’t quite meet
that criteria because we’ve

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learned that beyond Pluto,
there’s a whole belt called the

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Kuiper Belt, of small icy
objects

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that some are bigger, some are
smaller.

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And since they’re there, we
consider Pluto to be a member

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of the Kuiper Belt rather than
a planet on its own.

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So we call it a dwarf Planet.
And beyond the Kuiper Belt

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is the Oort Cloud. There’s a
whole bunch of other bodies

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out there. And around other
stars, we’re starting to

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discover other planets.
Although nothing as small as

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Pluto or even Earth.

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EVAN: So what happened? How did
Pluto lose its status as

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a planet? Well, in 1930,
astronomers thought Pluto was

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much larger than it actually
is. In fact, they thought it

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was the same size as earth.

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They looked at how much light
it was reflecting from the Sun

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and used that to help calculate
the size of Pluto.

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But they were way off in their
estimates.

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One of their mistakes is they
thought Pluto’s surface was

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much darker than it actually
is.

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Therefore they thought it was
much bigger.

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Let’s see how that could
happen.

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If you have two objects of
equal size, the object that is

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lighter in color is going to
reflect more

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of the sun’s light.
For example, a dark object...

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say a hill covered with grass,
doesn’t reflect as much

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sunlight as another hill the
same size covered in snow.

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Therefore a darker object would
have to be larger than a

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lighter object to reflect the
same amount of light.

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Today, astronomers have much
more powerful telescopes than

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they did in 1930.They
discovered that Pluto is much

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lighter in color than
originally thought.

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Astronomers thought Pluto was
dark gray, when in fact it’s a

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lighter gray. Therefore, it
didn’t need to be the size they

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originally thought in order to
reflect the amount

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of light they observed.
In fact, Pluto is considerably

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smaller than they thought.

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It’s about one-fifth the size
of Earth.

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With powerful new telescopes,
scientists have been

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discovering smaller and smaller
objects in our solar system.

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Many of these dwarf planets are
made of rock and ice, but they

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also have unusual orbits, much
like the orbit of Pluto.

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Their orbits cross the orbits
of other objects.

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Pluto’s orbit is so big that it
takes 249 years to go

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around the sun. In that time,
Pluto spends 20 years inside

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the orbit of Neptune.

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Since Pluto does not share the
same properties as the other

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planets, astronomers decided
they could no longer classify

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Pluto as a planet like Earth or
Mars.

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Now they call Pluto a dwarf
planet.

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Another reason for changing the
status of Pluto as a planet was

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the discovery of other objects
in our solar system

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beyond Pluto. It forced the
question: If Pluto is a planet,

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than shouldn’t these objects
also be called planets?

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Because Pluto is so small and
far away, it’s difficult for

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scientists to learn a lot about
it. Pluto is about 35 times

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farther from the Sun than the
Earth is.

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On January 19th, 2006, NASA
launched "New Horizons,"

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which is a robotic spacecraft
that will fly past Pluto and

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gather important information
that it will then send

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back to our scientists.

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New Horizons is the fastest
spacecraft to ever leave Earth.

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After new Horizons was
launched, it flew past our Moon

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in just half a day. But Pluto
is so far away that even at

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this very fast speed, it will
take 9 years for

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the spacecraft to reach Pluto.

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As New Horizons made its
journey towards Pluto,

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it actually began to slow down
due to the Sun’s gravitational

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pull. But here’s something
really cool.

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It flew by Jupiter in February
of 2007 and used the planet’s

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gravitational field to increase
its speed towards Pluto.

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"New Horizons" is part of
NASA’s "New Frontiers" Mission

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which is a series of scientific
space missions that will

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increase our understanding of
our solar system.

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So Globey, even though Pluto is
no longer classified as a major

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planet, it’s still a major
source of interest for our

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scientists. And NASA’s New
Horizons spacecraft will one

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day be sending us incredible
information on the

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dwarf planet called Pluto.

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? [music] ?

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KIDS: Our World!

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