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What Is Pluto?
August 29, 2012

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Pluto was discovered in 1930 by an astronomer from the United States. An astronomer is a person who studies stars and other objects in space.

Pluto was known as the smallest planet in the solar system and the ninth planet from the sun.

Today, Pluto is called a "dwarf planet." A dwarf planet orbits the sun just like other planets, but it is smaller. A dwarf planet is so small it cannot clear other objects out of its path.

On average, Pluto is more than 3.6 billion miles (5.8 billion kilometers) away from the sun. That is about 40 times as far from the sun as Earth. Pluto orbits the sun in an oval like a racetrack. Because of its oval orbit, Pluto is sometimes closer to the sun than at other times. At its closest point to the sun Pluto is still billions of miles away.

Pluto is in a region called the Kuiper (KY-per) Belt. Thousands of small, icy objects like Pluto are in the Kuiper Belt.

Pluto is only 1,400 miles (2,300 kilometers) wide. That's about half the width of the United States. Pluto is slightly smaller than Earth's moon. It takes Pluto 248 years to go around the sun. One day on Pluto is about 6 1/2 days on Earth.

Pluto was named by an 11-year-old girl from England. The dwarf planet has three moons. Its largest moon is named Charon (KER-ən). Charon is about half the size of Pluto.

Pluto's two other moons are named Nix and Hydra. They were discovered in 2005. NASA's Hubble Space Telescope took pictures of the two new moons. Nix and Hydra are very small. The moons are less than 100 miles (160 kilometers) wide.
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Why Is Pluto Not Called a Planet Anymore?
In 2003, an astronomer saw a new object beyond Pluto. The astronomer thought he had found a new planet. The object he saw was larger than Pluto. He named the object Eris (EER-is).

Finding Eris caused other astronomers to talk about what makes a planet a "planet." There is a group of astronomers that names objects in space. This group decided that Pluto was not really a planet because of its size and location in space. So Pluto and objects like it are now called dwarf planets.

Pluto is also called a plutoid. A plutoid is a dwarf planet that is farther out in space than the planet Neptune. The three known plutoids are Pluto, Eris and Makemake (MAH-kee-MAH-kee). Astronomers use telescopes to discover new objects like plutoids.

Scientists are learning more about the universe and Earth's place in it. What they learn may cause them to think about how objects like planets are grouped. Scientists group objects that are like each other to better understand them. Learning more about faraway objects in the solar system is helping astronomers learn more about what it means to be a planet.

What Is Pluto Like?
Pluto is very, very cold. The temperature on Pluto is 375 to 400 degrees below zero. Pluto is so far away from Earth that scientists know very little about what it is like. Pluto is probably covered with ice.

Pluto has about one-fifteenth the gravity of Earth. A person who weighs 100 pounds on Earth would weigh only 7 pounds on Pluto.

Most planets orbit the sun in a near-circle. The sun is in the center of the circle. But Pluto does not orbit in a circle! The orbit of Pluto is shaped like an oval. And the sun is not in the center. Pluto's orbit is also tilted.
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How Is NASA Exploring Pluto Today?
NASA learns about Pluto from pictures taken with telescopes. Pictures from the Hubble Space Telescope helped scientists find the moons Nix and Hydra. Hubble has also taken pictures of Pluto's surface. The pictures show dark and light areas on Pluto. Pluto is so far away that even pictures taken by telescopes in space are a little fuzzy.

In 2006, NASA launched the first mission to Pluto. It is called New Horizons. New Horizons is a spacecraft that is going to the edge of the solar system. The spacecraft is about the size of a piano. It will take nine years to reach Pluto. In 2015, New Horizons will arrive at Pluto. The mission will spend more than five months studying Pluto and its moons. New Horizons will then study other objects in the Kuiper Belt.

New Horizons has cameras that will take pictures of Pluto. The spacecraft also has science tools to gather information about Pluto. These pictures and information will help scientists learn more about the dwarf planet.

Why Is NASA Exploring Pluto?
NASA sends spacecraft to other planets because exploring space is exciting. It helps people learn new things. Spacecraft have visited every major planet in the solar system. Studying places like Pluto may help scientists learn how planets form.

More About Pluto:
› Space Place in a Snap: The Solar System's Formation   →
› New Horizons for Students   →
› New Horizon Fun Facts   →
› Pluto Pals   →
› Hail King of the Ice Dwarfs!   →
› Pluto: The Ice Dwarf   →


Read What Is Pluto? Grades 5-8

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Heather R. Smith/NASA Educational Technology Services

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Pluto and its three moons
In 2005, this image from NASA's Hubble Space Telescope was used to identify two new moons orbiting Pluto. Pluto is in the center. The moon Charon is just below it. The newly discovered moons, Nix and Hydra, are to the right of Pluto and Charon.
Image Credit: 
NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST
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A drawing of the orbits of the planets and Pluto
A drawing of the solar system shows Pluto's tilted orbit. Pluto's orbital path angles 17 degrees above the line, or plane, where the eight planets orbit.
Image Credit: 
NASA
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A drawing of a spacecraft and a partial view of Pluto
An artist's drawing shows the New Horizons spacecraft as it nears Pluto. The moon Charon is in the distance.
Image Credit: 
Johns Hopkins University Applied Physics Laboratory/Southwes
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Page Last Updated: January 27th, 2014
Page Editor: NASA Administrator