The Planet Pluto

Reading and Assessment

Pluto, formerly considered the ninth planet in the Solar System, was discovered by astronomer Clyde W. Tombaugh in Arizona on February 18, 1930. Pluto is now considered a dwarf planet and a member of the Kuiper belt.

Pluto and Charon

Pluto and Charon from the Hubble Space Telescope

Symbol for Pluto

Pluto Facts:

  • Pluto is the smallest planet, smaller even than Earth's Moon.
  • Most often, it is farthest away from the Sun, but sometimes Neptune is.


How big is this planet?

Symbol for Pluto

Comparison of the sizes of Pluto, Charon, the Moon and Earth

Pluto is the smallest of all of the planets in the Solar System. It weighs 12,500,000,000,000,000,000,000 kilograms. While this may seem large, it's only about 1/500th of the Earth's mass. Pluto is between 2200 and 2400 kilometers across. Its surface area is about 17,950,000 square kilometers (or 1/30th of the Earth's). Its volume is 7,150,000,000 km2 (or 1/150th of the Earth's).

What is its surface like?

surface of pluto

Color of Pluto

We don't really know for sure. No spacecraft has ever been there, and even the best telescopes can't see any detail. It is certainly very cold, about -230 °C. The surface is covered with ice. Pluto also has a very thin atmosphere which freezes when Pluto moves further from the Sun.

The image to the left shows Pluto's color.

What is its moon like?

Pluto has five moons: Charon, Styx, Nix, Kerberos, and Hyrda. The largest is called Charon. Charon is about half as wide as Pluto. Its surface is covered in water ice. In Roman mythology, Charon took dead souls across the river Acheron to the land of the dead.

The discovery of the other two moons were announced in October 2005 by scientists using the Hubble Space Telescope. These are very small moons that are almost 40 km across. Because they are so far away almost nothing is known about them.

How long is a day on this planet?

One day on Pluto is about 6.387 Earth days long. Like Uranus, Pluto also spins on its side.

How long is a year on this planet?

One year on Pluto would be about 90,613 days or 248 years on Earth!

What is it made of?

Scientists believe Pluto is made mostly of rock and ice, but they will not be sure until more research is done. The discovery of Charon helped scientists estimate the density of Pluto. The information collected told them what Pluto was and was not made out of. If Pluto were made out of heavy solids, it would have a very high density. If it were made of gases, it would have a low density. Pluto is somewhere in between, so it is probably made of rock and ice.

How much would Pluto's gravity pull on me?

If you were on Pluto, gravity would be only 0.06 times as strong as it is on Earth. This means you could do really high jumps like people on the Moon!

Who is it named after?

Pluto was named after the Roman god of the underworld. In Roman mythology, he kidnapped Prosperina (Persephone) so he could marry her. This made her mother very sad, causing winter. To end winter, Jupiter, his brother, sent Mercury to get her back. Before he let her go, Pluto made her eat a pomegranate. Eating it meant she would always have to live with him for half the year. This is the Roman myth of Springtime. When she goes to the underworld, everything stops growing. When she comes back, her mother is happy again, and life returns.

Is Pluto really a planet?

Some scientists say that Pluto isn't really a planet. They say it is too small, and point out that Charon is half its size. Also, Pluto, like Kuiper belt objects, is affected by the pull of Neptune. This is why they think Pluto is just a very large Kuiper belt object.

Right now, there is no clear definition of a planet. One definition would make most objects in the Solar System planets. Under the other definition, most planets now would no longer be called planets!

The organization that names Solar System objects is called the International Astronomical Union (IAU). Right now, they are debating the definition of a planet. This means that, for now, Pluto is still a planet. In the world of astronomy, there are many opinions on this subject.

New Horizons: The First Mission to the Pluto 

On January 19, 2006, New Horizons was launched from Cape Canaveral Air Force Station by an Atlas V rocket. It's primary mission was to perform a flyby study of the Pluto system in 2015, and a secondary mission to fly by and study one or more Kuiper belt objects (KBOs) in the decade to follow,

surface of pluto seen from Horizon

The surface of Pluto seen from Horizon (NASA Spacecraft). See more photos from Horizon.

Is there a tenth planet beyond Pluto?

In July 2005, astronomers announced the discovery of an object they are calling the tenth planet. This object is called 2003 UB313 for now. It will be given a permanent name by the IAU once they decide whether it as a planet or a Kuiper belt object. The discoverers are calling this object a planet since it is about 1 1/2 times the size of Pluto. They argue that if we call Pluto a planet, we must also call 2003 UB313 a planet. Astronomer Mike Brown suggested the name "Persephone" for the tenth planet.


How long does it take Pluto to travel around the Sun?


2) How long is a day on the planet Pluto?


3) An object weights 100 pounds on earth. What would be its weight on Pluto?



Multiple Choice Questions

1. Why do some scientists think that Pluto is not a planet?
a) the Planet Pluto is too small
b) Pluto is affected by the pull of Neptune
c) Pluto is just a very large Kuiper belt object.
d) all of the above

2. Which statement is not true of about Pluto?
a) the surface temperature of Pluto is about -230 degrees C
b) Pluto is the second smallest planet (after Mercury) in the Solar System
c) Pluto is the tenth planet from the sun
d) Pluto has 3 moons.

3. Which statement is not true about Charon
a) Charon is a moon of Pluto
b) Charon is about half the size of Pluto
c) Charon is the tenth planet in our Solar System
d) Charon can be seen with a telescope

4. What is the surface of Pluto like?
a) scientists believe the surface is mostly rock and ice, but are not really sure
b) dry and mountainous
c) mostly liquid hydrogen and close to absolute zero temperature
d) we really don't know but the density of Pluto must be very low

5. Who is Pluto named after?
a) Greek god of the dead
b) Pluto was named after the Roman god of the underworld
c) the scientist Albert Pluto who discovered the planet
d) Pluto was named after a cartoon character