Page 314 - NS-2 Textbook

ASTRONOMY 309

of Neptune. Therefore, astronomers looked for further new planet has been proposed by its discoverers to the
explanation in the form of another planet. Finally, in 1930 International Astronomical Union, but has not been an-
an American astronomer, Clyde Tombaugh, discovered nowlced pending acceptance by thls body.
the ninth planet after examining a series of telescopic Some ash'onomers argue fhat the newly announced
photographs. Planet X, plus several other similar but smaller bodies re-
Pluto was selected as the name for the IJne\vest U cently discovered in the Kuiper belt, should not be given
planet because Pluto was the Roman god of darkness planetary status because of their extreme distance from the
and the underworld. The planet Pluto is very dark in- Sun, and their often highly elliptical orbits. Some would
deed. It orbits at an average distance of 3.67 billion miles include Pluto in this group, and they propose limiting the
from the Sun. planetary designation to only the eight traditional planets
This faraway, mysterious world is an oddity in the from Mercmy to Neptune in order to avoid future contro-
solar system. Its orbit is inclined to the plane of the rest versy as additional candidates are discovered with the
of the planets and is also highly elliptical, so that for 20 more powerful telescopes now becoming available. III
years of each of its 248-year journeys around the Sun,
Pluto is actually inside the orbit of Neptune. TIlis oc-
curred most recently between 1979 and 1999.
Mass and density calculations indicate Pluto is com- CRITICAL THINKING
posed of rock and methane ice. Pluto and its satellite,
Charon, discovered in 1978, form a so-called binary plan- 1. List some of the more important of the astronomical
etary system. Charon orbits Pluto at a distance of only and other physical factors and scientific conditions
about 10,600 miles, and it is fully half the size of the that combine to make life as we know it on Earth pos-
planet. It orbits Pluto in 6.4 days, exactly matching sible. Could these same conditions possibly exist at
Pluto's rotation rate, so that, like Earth's moon, Charon other locations in our solar system?
always shows the same face to the planet. 2. Why has there long been such fascination among as-
Because of its unique orbital characteristics, scien- tronomers, other scientists, and the general public
tists have speculated that perhaps Pluto is a comet or as- concerning the possibility of life on the planet Mars?
teroid captured by the Sun's gravity, or even a satellite of 3. What are the characteristics required for a body orbit-
Neptune thrown deeper into space by a close encotmter ing the Sun to be classified as a planet as opposed to
with Neptune's large moon Triton. In January 2006 another type of body like an asteroid or comet?
NASA launched a New Horizon spacecraft that will fly by
and photograph Pluto and Charon in 2015. Attempts to
photograph the planet using the Hubble Space Telescope
have been made, but because of the extreme distance, Study Guide Questions
not much detail is observable.
1. Name the nine traditional planets in order from the
Sun.
2. What gravitational force keeps the planets in their
PLANET X
orbits?
In 2005 astronomers using the Oschin Telescope at the 3. What type of timetable is used to keep track of the
Palomar Observatory near San Diego, California, an- movement and location of the planets?
nounced the discovery of a newly found tenth planet or- 4. What is an orbital period?
biting in the Kuiper comet belt far outside the orbit of 5. Why do some planets appear at times to be going
Pluto. About ninety-seven times farther from the Sun backward in their orbits?
than the Earth (about twice the distance of Pluto from 6. Which two planets are closest to Earth?
the Sun), the planet is the farthest known object in the 7. What information did Mariner 10 provide us about
solar system, and the third-brightest object in the Kuiper Mercury?
belt. It was originally photographed in 2003, but like 8. What have recent space probes revealed about
Pluto, was not recognized as a planet until its motion Venus?
across the backgrotmd of the stars was detected in early 9. When do we usually see Venus best, and what do we
2005. Analysis of its reflected sunlight indicates that it is call it at these times?
probably about one and a half times the size of Pluto, 10. What planet is called the "red planet"?
making it the first such object ever found in the outer re- 11. A. What are fhe canali?
gions of the solar system. Further observations of it will B. What happened to the canali theory?
be attempted using some of the larger ground and space- 12. How far from the Sun is Mars?
based telescopes now coming available. A name for the 13. Compare Mars with Earth as to diameter, atmo-