Pluto and other Dwarf Planets

Pluto: The Planet That Wasn't

https://www.readworks.org/article/Pluto-The-Planet-That-Wasnt/fc7bcf2c-3069-462a-b594-27f6a74702b9#!articleTab:content/

Poor Pluto!

It’s bad enough to be the runt of the group, but to be told after 75 years that you’re not even a member of the club - what an insult!

Pluto was first discovered in 1930. Until 2006, students were taught that it was the ninth and smallest planet in the solar system. Smaller than Earth’s moon, it is not even as wide as the United States.

Pluto is made up almost entirely of rock and ice. It is so far away from Earth that the NASA New Horizons spacecraft took almost 10 years to get very close to it. Pluto’s full orbit around the sun lasts almost 250 Earth years!

But as small as it is, as cold as it is, as far from the sun as it is, for all those years it was considered the ninth planet of the solar system… until Eris came around.

Eris was discovered in 2005. It is about the same size as Pluto. And like Pluto, it is part of the Kuiper Belt, a ring of objects that circle the outer edge of the solar system.

After Eris was discovered, scientists had to make a decision. Either Eris was the 10th planet in the solar system or it was not a planet at all! And if Eris weren’t a planet, could Pluto be considered one?

Multiple Choice

After Eris was discovered, scientists had to decide whether to count it as a planet. Why did this make them question whether Pluto should still be counted as a planet?

• because Pluto and Eris are both space objects

• because Pluto and Eris were discovered at the same time

• because Pluto and Eris are very different in size

• because Pluto and Eris are very similar in size

Scientists made new rules for what is counted as a planet, and decided that neither Pluto nor Eris qualified.

Multiple Choice

What did the discovery of Eris make scientists do?

• rethink the rules for what is counted as a star

• rethink the rules for what counted as a dwarf planet

• rethink the rules for what counted as a planet

• add more planets to the group of planets

A new category was created: dwarf planet. The official list of planets in the solar system went from nine to eight, and Pluto and Eris became members of the dwarf planet club. So long for Planet Pluto—but at least it no longer has to be the littlest guy in the club. In fact, Pluto is one of the bigger dwarf planets! Maybe Pluto doesn’t have it so bad after all.

Scientists had to rethink the rules for what is counted as a planet. They decided on the following three criteria:

1. It must orbit the sun.

2. It must hold a sperical shape

3. It must have enough gravity to clear its orbit of debris.

NEITHER Pluto nor Eris qualified.

Multiple Choice

The discovery of Eris made scientists rethink _______.

• the rules for what counted as a dwarf planet

• the rules for what counted as a planet

After Eris was discovered, scientists had to make a decision. Either Eris was the 10th planet in the solar system or it was not a planet at all! And if Eris wasn’t a planet, could Pluto be considered one?

Multiple Choice

Pluto used to be considered a planet. Today, what is it considered to be?

• a comet

• a star

• a dwarf planet

• an asteroid

Multiple Choice

A planet must be in orbit around the sun.

• True

• False

Multiple Choice

A planet must be spherical in shape.

• True

• False

Multiple Choice

A planet must have cleared the neighborhood around its orbit.

• True

• False

Multiple Choice

What does the author mean by stating, "Maybe Pluto doesn't have it so bad after all"?

• Even though Pluto is no longer counted as a planet, it is in a new group called dwarf planets.

• Even though Pluto is no longer counted as a planet, it is one of the bigger dwarf planets.

• Even though Pluto is no longer counted as a planet, it is still part of the ring of objects that circle the outer edge of the solar system.

• Even though Pluto is now counted as a dwarf planet, it isn't alone as other space objects are counted as dwarf planets.

Multiple Choice

Pluto used to be considered a planet. Today, what is it considered to be?

• a dwarf planet

• a star

• a comet

• an asteroid

Multiple Choice

What is the main idea/gist of this text?

• Pluto was no longer considered a planet after the discovery of Eris made scientists come up with new rules for what is counted as a planet.

• Pluto is so far away from Earth that the NASA New Horizons spacecraft took almost 10 years to get very close to it.

• Eris is about the same size as Pluto, and like Pluto, it is part of a ring of objects that circle the outer edge of the solar system.

• Scientists come up with rules for what is counted as a planet and what is not.