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KIDS: Our World!
EVAN: Hey Globey. What's the matter? You look sad today.
You're sad because your buddy Pluto is no longer called a
planet? Well, that's partially true. But that's no reason to
be sad. Pluto is still a planet, just now scientists
classify it as a "dwarf" planet.
Scientists classify objects based on their traits or
characteristics, like size and color.
In the case of planets, scientists look at size and the
orbit or path of the object in space.
In 1930, an American named Clyde Tombaugh discovered
Pluto. At that time, it was classified as the 9th planet in
our solar system. Today though, many astronomers say that Pluto
is too small to be accurately called a planet.
ALAN LUNSFORD: A lot of people still consider Pluto a planet.
They've just grown up knowing it's a planet and hearing that
it's a planet. What's happened is we're starting to discover
other bodies in our solar system that are similar to
Pluto and they're not considered planets yet.
So the question was, well what defines a planet and what
doesn't. We've never had a really scientific definition.
We've only had a list. So in trying to come up with a
scientific definition that encompasses everything we might
observe, Pluto didn't quite make the cut.
So we decided on three little stages.
The body has to orbit a star and not another planet.
If it orbits another planet, it a moon. The body has to be big
enough basically to be spherical.
There are a lot of oddly shaped asteroids. They're not big
enough to make themselves into a sphere.
And then, the third criteria is the orbit of the planet has to
have been such that it has cleared its neighborhood of
other things that are potentially planets.
Otherwise you just kind of have a belt of things.
And so Pluto didn't quite meet that criteria because we've
learned that beyond Pluto, there's a whole belt called the
Kuiper Belt, of small icy objects
that some are bigger, some are smaller.
And since they're there, we consider Pluto to be a member
of the Kuiper Belt rather than a planet on its own.
So we call it a dwarf Planet. And beyond the Kuiper Belt
is the Oort Cloud. There's a whole bunch of other bodies
out there. And around other stars, we're starting to
discover other planets. Although nothing as small as
Pluto or even Earth.
EVAN: So what happened? How did Pluto lose its status as
a planet? Well, in 1930, astronomers thought Pluto was
much larger than it actually is. In fact, they thought it
was the same size as earth.
They looked at how much light it was reflecting from the Sun
and used that to help calculate the size of Pluto.
But they were way off in their estimates.
One of their mistakes is they thought Pluto's surface was
much darker than it actually is.
Therefore they thought it was much bigger.
Let's see how that could happen.
If you have two objects of equal size, the object that is
lighter in color is going to reflect more
of the sun's light. For example, a dark object...
say a hill covered with grass, doesn't reflect as much
sunlight as another hill the same size covered in snow.
Therefore a darker object would have to be larger than a
lighter object to reflect the same amount of light.
Today, astronomers have much more powerful telescopes than
they did in 1930.They discovered that Pluto is much
lighter in color than originally thought.
Astronomers thought Pluto was dark gray, when in fact it's a
lighter gray. Therefore, it didn't need to be the size they
originally thought in order to reflect the amount
of light they observed. In fact, Pluto is considerably
smaller than they thought.
It's about one-fifth the size of Earth.
With powerful new telescopes, scientists have been
discovering smaller and smaller objects in our solar system.
Many of these dwarf planets are made of rock and ice, but they
also have unusual orbits, much like the orbit of Pluto.
Their orbits cross the orbits of other objects.
Pluto's orbit is so big that it takes 249 years to go
around the sun. In that time, Pluto spends 20 years inside
the orbit of Neptune.
Since Pluto does not share the same properties as the other
planets, astronomers decided they could no longer classify
Pluto as a planet like Earth or Mars.
Now they call Pluto a dwarf planet.
Another reason for changing the status of Pluto as a planet was
the discovery of other objects in our solar system
beyond Pluto. It forced the question: If Pluto is a planet,
than shouldn't these objects also be called planets?
Because Pluto is so small and far away, it's difficult for
scientists to learn a lot about it. Pluto is about 35 times
farther from the Sun than the Earth is.
On January 19th, 2006, NASA launched "New Horizons,"
which is a robotic spacecraft that will fly past Pluto and
gather important information that it will then send
back to our scientists.
New Horizons is the fastest spacecraft to ever leave Earth.
After new Horizons was launched, it flew past our Moon
in just half a day. But Pluto is so far away that even at
this very fast speed, it will take 9 years for
the spacecraft to reach Pluto.
As New Horizons made its journey towards Pluto,
it actually began to slow down due to the Sun's gravitational
pull. But here's something really cool.
It flew by Jupiter in February of 2007 and used the planet's
gravitational field to increase its speed towards Pluto.
"New Horizons" is part of NASA's "New Frontiers" Mission
which is a series of scientific space missions that will
increase our understanding of our solar system.
So Globey, even though Pluto is no longer classified as a major
planet, it's still a major source of interest for our
scientists. And NASA's New Horizons spacecraft will one
day be sending us incredible information on the
dwarf planet called Pluto.
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KIDS: Our World!
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