Download S-5-6-3_Pluto Graphic Organizer Why Isn`t Pluto a Planet

S-5-6-3_Pluto Graphic Organizer
Composition
Large Moon
Why Isn't
Pluto a
Planet?
Small Size
Orbit around
the Sun
Atmosphere
S-5-6-3_Pluto Graphic Organizer
Background Information for Teachers:
Why Isn’t Pluto a Planet?
Pluto was called a planet from its discovery in 1930 until it was re-classified as a "dwarf planet"
in 2006. The change in status stems from the fact that, since 1993, astronomers have discovered
thousands of objects similar to Pluto in size and composition, in the region of Pluto’s orbit. This
region is called the Kuiper Belt and the objects are called Kuiper Belt objects (KBOs). Pluto fits
with the objects in the Kuiper Belt, and does not fit with either the rocky planets (Mercury,
Venus, Earth, and Mars) or the gas giant planets (Jupiter, Saturn, Uranus, and Neptune). Factors
that distinguish Pluto from the eight planets include its composition, atmosphere, small size, a
comparatively large moon, and the shape of its orbit around the Sun.
Composition: Pluto is composed of ice, rock, and frozen gases, similar to the composition of the
comet-like objects in the Kuiper Belt (the region beyond Neptune's orbit).
Atmosphere: Pluto's atmosphere grows as it moves closer to the Sun and recedes as it moves
away. This is similar to the comet-like objects found in its vicinity. The frozen gases sublimate
(turn from a solid to a gas) as Pluto moves closer to the Sun and then condense on the surface of
Pluto as it moves away from the Sun. In contrast, the atmospheres of the planets do not appear or
disappear during their orbits.
Small size: Pluto is relatively tiny, having a mass about 1/500th that of Earth (in contrast,
Mercury's mass is 1/20th that of Earth). In diameter, Pluto is 1/5th the size of Earth. Although
puny on a planetary scale, Pluto is one of the largest objects inhabiting the Kuiper belt.
Large moon: Pluto's largest moon, Charon, is a little more than one-half the radius of Pluto and
one-eighth its mass. Pluto and Charon are in a "synchronous" orbit: they always show the same
face to each other as they orbit. Charon's orbit is only 20,000 km away from Pluto. For
comparison, Earth's Moon has about 1/80th the mass of Earth and orbits 400,000 km away.
Many astronomers consider Pluto and Charon to be a binary Kuiper Belt object since they orbit a
common center. Pluto’s two smaller moons, Nix and Hydra, are considerably smaller and more
distant from Pluto than is Charon.
Orbit around the Sun: Pluto's orbit is more elliptical than that of any of the planets (it actually
crosses Neptune's orbit). It is tilted at an inclination of 17.15 degrees relative to the plane of
Earth's orbit. The planets all have much smaller inclinations, while the Kuiper Belt objects can
have large inclinations like Pluto.
Source: http://amazing-space.stsci.edu/resources/explorations/trading/trading-inter-scientist.html#ques15