Exploring Our Solar System: Thedwarf planet Pluto

The New Horizons spacecraft flew by Pluto in 2015. The spacecraft took many photos of the dwarf planet. This image of Pluto

is made up of four New Horizon photos. Photo from: JPL/NASA/Johns Hopkins University.

Pluto was considered the smallest planet in the solar system until 2006. That year a large

group of scientists decided that Pluto was not a true planet after all. They voted to call Pluto a

dwarf planet instead.

By Encyclopaedia Britannica, adapted by Newsela staff on 09.07.17

Word Count 595

Level 830L

This article is available at 5 reading levels at https://newsela.com. 1

Pluto is very far from Earth. It is difficult to observe from Earth, even with the most powerful

telescopes. It orbits the sun at an average distance of about 3.6 billion miles, and is usually

farther from the sun than Neptune. However, Pluto’s path around the sun is shaped like an

oval rather than a circle. Its distance from the sun varies greatly as it orbits. About every 228

years Pluto’s orbit takes it closer to the sun than Neptune. This last occurred from 1979 to

1999.

Pluto's unusual features make it different from the eight planets. In the 1990s scientists

discovered a ring of millions of small, icy objects, called the Kuiper Belt. These objects orbit

the sun beyond Neptune. Pluto seems to be similar to some of them.

Physical Features

The distance through Pluto's center is about 1,430 miles. That makes the dwarf planet less

than half the width of Mercury, the smallest planet.

Scientists think that Pluto consists of rocky material and frozen gases. Some areas of Pluto’s

surface are very bright, while others are dark. The bright regions are probably a frozen gas

called nitrogen. Pluto is so far from the sun that it receives only a little sunlight. Scientists

believe that the average temperature on its surface is about minus 387 degrees Fahrenheit.

That's colder than any place on Earth.

This article is available at 5 reading levels at https://newsela.com. 2

Pluto has five known moons. Their names are Charon, Nix, Hydra, Kerberos and Styx. Charon

is much larger than the other four moons.

Orbit And Spin

A year is the amount of time it takes a planet to make one full revolution around the sun. Earth

takes just over 365 days to make a full trip, so that is the length of a year on Earth. A year on

Pluto is much longer. It lasts about 248 Earth years. That is because Pluto is farther from the

sun and takes much longer to travel around it.

Pluto spins about its center slowly. A single day is equal to the time it takes a planet to make a

full rotation. That is one complete spin on the planet's axis. Earth takes 24 hours to make one

full rotation. Pluto's full rotation takes about 6.5 Earth days.

This article is available at 5 reading levels at https://newsela.com. 3

Observation And Exploration

In 1930, U.S. scientist Clyde Tombaugh discovered Pluto using a telescope. After the

discovery, scientists believed that Pluto was the ninth planet in the solar system. That

changed in 2006, when scientists decided that Pluto was actually a dwarf planet. This brought

the number of planets down to eight.

In 2006, the United States sent a spacecraft to explore Pluto. The spacecraft was called New

Horizons. Its journey to Pluto was expected to take about nine years. The spacecraft finally

made its closest approach to Pluto in July 2015. It captured amazing images of the dwarf

planet and its moons. Scientists are still studying and learning about Pluto from the images

and data that New Horizons collected.

This article is available at 5 reading levels at https://newsela.com. 4

Quiz

1 Read the section “Orbit And Spin.”

Select the sentence that shows WHY Pluto has a longer year than Earth.

(A) A year is the amount of time it takes a planet to make one full revolution around

the sun.

(B) Earth takes just over 365 days to make a full trip, so that is the length of a year

on Earth.

(C) That is because Pluto is farther from the sun and takes much longer to travel

around it.

(D) A single day is equal to the time it takes a planet to make a full rotation.

2 Read the section titled “Physical Features.”

Which sentence from the section shows WHY Pluto has such low temperatures?

(A) Some areas of Pluto’s surface are very bright, while others are dark.

(B) The bright regions are probably a frozen gas called nitrogen.

(C) Pluto is so far from the sun that it receives only a little sunlight.

(D) Scientists believe that the average temperature on its surface is about minus

387 degrees Fahrenheit.

3 One main idea of the article is that Pluto is a dwarf planet.

What is another MAIN idea of the article?

(A) Pluto has an orbit that brings it closer to the sun than Neptune.

(B) Pluto is very different from the eight planets in the solar system.

(C) Pluto was discovered by Clyde Tombaugh using a telescope.

(D) Pluto has dark spots and some bright spots with frozen nitrogen.

This article is available at 5 reading levels at https://newsela.com. 5

4 Read the sentences summarizing the main ideas of the article below.

Pluto was once a planet, but it is now considered a dwarf planet. It issmall, cold and very far from the sun.

Which answer choice would complete the summary?

(A) It orbits the sun at an average distance of about 3.6 billion miles.

(B) When Pluto was considered a planet, it was even smaller than Mercury.

(C) Pluto takes about 6.5 Earth days to do a complete spin on its axis.

(D) Since its discovery, scientists have been working to learn more about it.

This article is available at 5 reading levels at https://newsela.com. 6