Pluto: The Planet that Wasn't

Pluto: The Planet that Wasn't

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 wasn’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 they are both space objects

• because they were discovered at the same time

• because they are very different in size

• because they are very similar in size

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

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.

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

Pluto was discovered in what year?

• 1930

• 2005

• 1775

Multiple Choice

In what year did Pluto lose its status as a planet?

• 1995

• 2006

• 2000

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

A planet should use what to clear asteroids and dwarf planets out of its path?

• its atmosphere

• its gravity

Multiple Select

Which of the following planet criteria DOES Pluto meet? (select ALL that apply)

• Orbits the Sun

• Is spherical in shape

• Clears its orbit