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No.159 SIM音読用英文

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Pluto Demoted, No Longer a Planet
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The International Astronomical Union's general assembly,

meeting in Prague,

has voted to accept only eight planets in our solar system.
           
A U.S. astronomer who was there,

Richard Binzel of the Massachusetts Institute of Technology,

says the astronomical community has decided

Pluto is not a planet, but something else.

"The community has decided

that Pluto is best described as a 'dwarf' planet."

This is a big decline in Pluto's fortunes

since the astronomers began their meeting on August 14.

At that time, they faced a resolution

hammered out by Binzel and several others

not only keeping Pluto as a planet, but adding three more

- Pluto's largest moon, Charon; the asteroid Ceres;

and a larger, more distant body

discovered last year, provisionally named Xena.

That resolution defined a planet

as any body orbiting a star

that has enough gravity to pull itself into a sphere.

Charon was included

because it and Pluto orbit each other.

But Binzel says most astronomers at the assembly favored

a more exclusive definition

that considers more than just a body's shape.

"It's simply a matter of what criterion is the most important?

Is the shape of the object the most important criterion,

or is it how it orbits?

And ultimately, of the members present

who calculate things like orbits, 

that was deemed to be a more important consideration.

And so it was just a change of emphasis."

The newly-accepted definition says

a planet must be not only spherical,

but also must have cleared away all other objects in its vicinity.

Its gravitational force must have either pulled them in and absorbed them

or flung them away.

However, the icy, distant bodies Pluto, Charon, and Xena orbit

among thousands of other similar bodies

in a region of the solar system beyond Neptune

called the Kuiper Belt.

Furthermore, Pluto's orbit overlaps Neptune's.

Ceres circles among a large number of asteroids.

In each case, these dwarfs are too small

to exert enough gravity to give them more space.

Pressure has been building to demote Pluto

since the Kuiper Belt was discovered in 1992,

but California Institute of Technology astronomer Michael Brown says

his discovery of Xena last year forced the issue.

"I may go down in history

as the guy who killed Pluto.

No, it's not necessarily what I set out to do

when I came to work that morning,

but sometimes these things just happen."

Of course, millions of textbooks must be revised

and schoolchildren the world over taught

that our sun now has only eight planets.

David McAlary, VOA News, Washington.
(Material is provided courtesy of voanews.com.)
by danueno | 2006-08-30 16:05 | SIM音読用英文


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