Page 63 - Reading Success B7

In the nineteenth century, upon noticing discrepancies in the orbits of the planets Uranus
and Neptune, astronomers speculated that another undiscovered planet might be affecting
the two. It wasn't until 1930, though, that astronomer Clyde Tombaugh actually identified
Pluto, a body 3 million miles away from the Sun. Tombaugh and other scientists
pronounced Pluto to be the ninth planet in our solar system.

Since then, however, astronomers have been debating the definition of the term “planet”.
In 1998, the International Astronomical Union finally agreed that a planet is a body that has
two characteristics: It orbits a sun and not another planet, and it is large enough for its own
gravity to make it round. According to this definition, Pluto, which has an atmosphere and a
moon, is indeed a planet. The other eight planets in our solar system fall into two clear
categories: Four of them (Mercury, Venus, Earth, and Mars) are rocky, terrestrial bodies and
the other four (Jupiter, Saturn, Uranus, and Neptune) are giant gas planets. Pluto, which is
a ball of ice smaller than the moon that orbits Earth, is in a class by itself. It also does not
orbit the Sun on the same plane as the other planets; its orbit tilts 17 degrees, and it's
actually closer to the Sun than Neptune for 20 years out of its 248-year cycle.

These eccentricities have led some astronomers to conclude that Pluto is really a comet
or an asteroid. Comets, which are frozen balls of dirt, and asteroids, which are large rocky
bodies that can be round like planets, both orbit the Sun just as planets do. Like comets and
asteroids, and unlike the other planets, Pluto does not have sufficient gravity to clear debris
from its orbital path.

Other astronomers argue that Pluto is actually one of hundreds of icy bodies that form
the Kuiper Belt, a region of space at the outer edges of our solar system that is cluttered
with debris. These scientists argue that Pluto is merely a very large Kuiper Belt object
(KBO) in the midst of many others. Researchers at the Hayden Planetarium at New York's
American Museum of Natural History have subscribed to this view, demoting Pluto to king of
the Kuiper Belt.

The debate about Pluto is fueled by the fact that it is the only planet in the solar system
that has not been visited by a space probe. However, the mystery may finally be solved
when the New Horizons spacecraft is launched in 2006 and encounters Pluto ten years
later. The probe's telescopes, cameras, and other instruments may help us decide, once
and for all, what Pluto really is.

Main Idea

What is the main idea of this story?

a. the physical characteristics of Pluto
b. what the Kuiper Belt is

c. what a planet is
d. the debate surrounding Pluto’s status as a planet

66_Reading Success B 7