Pluto, the dwarf planet

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Pluto, the dwarf planet By NASA.gov, adapted by Newsela staff on 11.10.

1 Pluto, the dwarf planet By NASA.gov, adapted by Newsela staff on Word Count 858 TOP: Pluto nearly fills the frame in this image from the Long Range Reconnaissance Imager (LORRI) aboard NASA's New Horizons spacecraft, taken on July 13, MIDDLE: Pluto and Charon are shown in enhanced color in this image. BOTTOM: In this image from NASA s New Horizons spacecraft, great blocks of Pluto s water-ice crust appear jammed together in the informally named al-idrisi mountains. Photo: Photo: NASA. Pluto was discovered in 1930 by the astronomer Clyde Tombaugh. At the time, astronomers scientists who study stars and outer space believed that Pluto was the ninth planet in our solar system. However, it was much smaller than the other planets. Pluto, Out In The Cold In 2006, scientists got together at a meeting of the International Astronomical Union, a group for astronomers. They put Pluto into a new category of space objects: dwarf planets. These small, spherical bodies orbit the sun just like planets but are much smaller. Pluto is also a member of a group of rocky objects that circle the sun in a disc-like zone called the Kuiper Belt. This region is located beyond the orbit of Neptune. Like the other icy objects in the Kuiper Belt, Pluto is extremely cold. Its average surface temperature is 380 degrees below zero. This article is available at 5 reading levels at 1

2 The diameter of Pluto is 1,475 miles, which makes it smaller than Earth's moon. It most likely has a rocky core. Scientists believe this core is surrounded by water ice. The surface of the planet is coated with more unusual kinds of ice. These are made of frozen gases, including methane and nitrogen. Pluto is much less dense than Earth, and it has a much lower mass. Earth's mass is more than 400 times greater than Pluto's. Trying To Catch Some Rays? Like Earth, Pluto orbits the sun, but Pluto's orbit is much bigger and takes much longer. One complete orbit takes Pluto 248 years. Pluto's path around the sun is elliptical, or ovalshaped. At its farthest point, Pluto's orbit takes the dwarf planet about 49 AUs, or astronomical units, away from the sun. One astronomical unit is equal to about 93 million miles (the average distance between Earth and the sun). When Pluto is close to the sun, its surface ices change from a solid to a gas. As they rise, these gases temporarily form a thin atmosphere. Pluto becomes much colder during the part of each orbit when it is traveling far away from the sun. During this time, most of the planet's atmosphere freezes, then falls to Pluto's surface as snow. Days In The Life Of Pluto In addition to orbiting the sun, Pluto also rotates or turns around, just as the Earth does. One full rotation of a planet is what we call a day. One full Pluto rotation a Pluto day takes 6.4 Earth days. Pluto has a very large moon named Charon, which is almost half its size. Charon is located 12,200 miles away from Pluto. It is grayer than the reddish Pluto, suggesting they are made of different materials. It takes Charon 6.4 Earth days to orbit Pluto. Meanwhile, Pluto is also rotating. One full Pluto rotation a day also takes 6.4 Earth days. Charon neither rises nor sets. Instead, it hovers over the same spot on Pluto's surface as the planet turns, and the same side of Charon always faces Pluto. Scientists Over The Moons Since 2005 scientists using the Hubble Space Telescope have discovered four tiny, previously unknown moons orbiting Pluto. These have been named Nix, Hydra, Kerberos and Styx. All are more distant from Pluto than Charon is. In 2006, scientists launched the unmanned New Horizons space probe. Its mission is to conduct the first detailed studies of Pluto. By the spring of 2015, it was close enough to Pluto to begin recording images of the planet's surface. This article is available at 5 reading levels at 2

3 The pictures New Horizons has sent back have left scientists greatly puzzled. For example, pictures of Pluto's surface show large mountains up to 9,800 feet high. Scientists had not believed Pluto contained materials that could hold up such mountains and maintain their shapes over millions of years. They now suspect the mountains are made of ice. Trying To Fill In The Gaps Scientists were also puzzled to see fewer craters on Pluto than expected. Still, some extremely large ones were spotted. Adding to the mystery, these large craters appear to be partially filled in. Scientists had not thought Pluto had the kind of material that would fill in a crater. Here on Earth, loose rock and dirt fill in craters over time. New Horizons has also given scientists images of Charon, Nix and Hydra. Like Pluto, Charon has large mountains, as well as plains. All three moons have a good deal of ice on them. There is no evidence that they have any kind of atmosphere. One mystery is the fact that the ice on the surface of both Nix and Hydra seems strangely clean and bright. Scientists do not understand how the ice on these small moons has stayed so bright over billions of years. Usually ice grows darker over time. How Pluto Got Its Name Pluto is the only world to be named by an 11-year-old girl. In 1930, Venetia Burney of Oxford, England, suggested to her grandfather that the new discovery be named for Pluto, the Roman god of the underworld. He forwarded the suggestion to the scientists who had discovered the new planet, and they liked Venetia's idea. This article is available at 5 reading levels at 3

4 Quiz 1 How does the following sentence from the introduction contribute to the article? It establishes the idea that space objects must meet certain criteria to be considered a planet. It explains why scientists are studying Pluto. It shows that scientists do not agree on many things about Pluto. It tells why scientists continue to study Pluto. 2 Which sentence from the section "Trying To Catch Some Rays?" develops the idea that Pluto has some things in common with other planets? Like Earth, Pluto orbits the sun, but Pluto's orbit is much bigger and takes much longer. One astronomical unit is equal to about 93 million miles (the average distance between Earth and the sun). When Pluto is close to the sun, its surface ices change from a solid to a gas. During this time, most of the planet's atmosphere freezes, then falls to Pluto's surface as snow. 3 Which paragraph in the section "Days In The Life Of Pluto" has information that could be used to write a new caption for the photo in that section? 4 Read the section "Scientists Over The Moons" and examine the photo in this section. Which of the following statements is TRUE based on the section and the image? in 2006, scientists learned that there were mountains made of gas-ice on the surface of Pluto. In 2015, scientists discovered that the mountains on Pluto were made of water-ice. The mountains on Pluto were photographed by the New Horizons in Pluto's al-idrisi mountains were formally named in This article is available at 5 reading levels at 4

5 Answer Key 1 How does the following sentence from the introduction contribute to the article? It establishes the idea that space objects must meet certain criteria to be considered a planet. It explains why scientists are studying Pluto. It shows that scientists do not agree on many things about Pluto. It tells why scientists continue to study Pluto. 2 Which sentence from the section "Trying To Catch Some Rays?" develops the idea that Pluto has some things in common with other planets? Like Earth, Pluto orbits the sun, but Pluto's orbit is much bigger and takes much longer. One astronomical unit is equal to about 93 million miles (the average distance between Earth and the sun). When Pluto is close to the sun, its surface ices change from a solid to a gas. During this time, most of the planet's atmosphere freezes, then falls to Pluto's surface as snow. 3 Which paragraph in the section "Days In The Life Of Pluto" has information that could be used to write a new caption for the photo in that section? Paragraph 9: Pluto has a very large moon named Charon, which is almost half its size. Charon is located 12,200 miles away from Pluto. It is grayer than the reddish Pluto, suggesting they are made of different materials. 4 Read the section "Scientists Over The Moons" and examine the photo in this section. Which of the following statements is TRUE based on the section and the image? in 2006, scientists learned that there were mountains made of gas-ice on the surface of Pluto. In 2015, scientists discovered that the mountains on Pluto were made of water-ice. The mountains on Pluto were photographed by the New Horizons in Pluto's al-idrisi mountains were formally named in This article is available at 5 reading levels at 5

Pluto Data: Numbers. 14b. Pluto, Kuiper Belt & Oort Cloud. Pluto Data (Table 14-5)

Pluto Data: Numbers. 14b. Pluto, Kuiper Belt & Oort Cloud. Pluto Data (Table 14-5) 14b. Pluto, Kuiper Belt & Oort Cloud Pluto Pluto s moons The Kuiper Belt Resonant Kuiper Belt objects Classical Kuiper Belt objects Pluto Data: Numbers Diameter: 2,290.km 0.18. Earth Mass: 1.0. 10 22 kg