Name: ______Date: ______Per: ______
Pluto - Defining a Planet - September 1st, 2006
In 1930, Clyde W Tombaugh at the Lowell Observatory in Flagstaff Arizona discovered a moving object too big to be an asteroid. Was it a planet? To be a planet, the object would have to orbit the sun. Finally, in 1931, after a year of observations, Tombaugh concluded that this object was orbiting the sun, and announced that he had discovered a planet. At the suggestion of an 11-year-old British girl, the planet was named Pluto, after the Roman god of the underworld.
What year was Pluto announced as a planet? ______
How did Pluto get its name? ______
______
Ever since Pluto's discovery there has been much controversy about whether Pluto is really a planet. Many scientists have argued that Pluto is too small to be a planet. In 1978, they discovered Pluto's largest moon Charon, which helped scientist determine the size of Pluto to be 2,400 km in diameter. This was less than they had thought. It was only half the size of Mercury, the next smallest planet. But it was still larger than anything so far found beyond the orbit of Neptune.
According to this paragraph, why do scientists argue that Pluto is not a planet?
______
Then in 1993, astronomers began to discover many icy comets also beyond Neptune. These comets and some asteroid shared many similar characteristics with Pluto, including size, shape, and composition (what they were made of). Astronomers called these new objects the Kuiper Belt (it was named after one of the astronomers). At first, the comets found in the Kuiper Belt were smaller than Pluto, but in 2005 a group of astronomers found Eris, which was believed to be the same size or larger than Pluto. Astronomers estimate that there are at least 70,000 icy objects with similar composition, and some of them are bigger than Pluto in the Kuiper Belt. Eris was just the first example.
What is the band of comets and asteroids located beyond Neptune called?
______
After the discovery of Eris, there was new debate about how to define planets and whether Pluto should still be classified as a planet. If Pluto is a planet, then many other objects in the solar system will become new planets. Many scientists were opposed to this, because they did not want to drastically increase the number of planets. This group of astronomers argued that Pluto should not be counted as a planet.
What did this group of astronomers argue? ______
______
A second group of astronomers wanted Pluto to remain a planet because it was discovered long before other, similar Kuiper Belt Objects.
The last of the astronomers wanted Pluto and other similar Kuiper Belt Objects to be classified as something else. They suggested that Pluto and similar objects should have their own category, called “dwarf planet.”
What did these astronomers want to classify as “dwarf planets?” ______
______
Astronomers decided they would make a final decision about the definition of a planet at the 26th General Assembly of the International Astronomical Union on August 25, 2006. There were three possible decisions.
1. They could define a planet as a body that orbits the sun and that has gravity strong enough to pull its material into a spherical shape. This means that Pluto would remain a planet. But then there are at least three other bodies like Pluto that will become planets, too. We would have at least 12 planets, and maybe more later.
2. They could define a planet in a way that does not include Pluto. This would mean that there are only 8 planets.
3. They could keep 9 planets, with Pluto as the 9th planet because we have been calling it a planet for many years. Other bodies like Pluto would not be called planets, even though they are just like Pluto.
At their meeting, the astronomers decided to define a planet in this way:
A. It needs to be in orbit around the sun.
B. It needs to have enough gravity to pull itself into a spherical shape.
C. It needs to have enough gravity to “clear the neighborhood” of its orbit of other objects. That means that the planet’s gravity is big enough that all the other objects in its orbit are pulled into the planet, where they crash into the planet and are no longer flying through space.
Do YOU think Pluto is a planet? ______
1. 1931
2. Named after Roman God of
the underworld
3. Too small
4. Kuiper Belt
5. Should not be a planet
6. Pluto and other small objects
in Kuiper Belt
7. Up to you