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Could the Solar System Have Ten Planets?


Subject(s) Science --Space Science


Grades 2-up

News Content

Could a recently discovered object in space be a tenth planet?

Anticipation Guide

Before reading, ask students to agree or disagree with each of the statements below.

  • There are only nine planets in the solar system.
  • Scientists can use powerful telescopes to see objects that are more than a billion miles from Earth.
  • Scientists agree that any object in space that is bigger than Pluto should be considered a planet.

    News Words

    Which word in the News Word Box (student news sheet) matches each definition below?

  • any of the nine large bodies in space that orbits around the sun (planet)
  • a person who studies the stars, planets, and space astronomer
  • one of many objects that orbit the sun; usually small and often irregularly shaped asteroid
  • the sun together with the nine planets and all other bodies that orbit around the sun (solar system)

    Read the News

    Click for a printable version of this week's news story Could the Solar System Have Ten Planets?

    Reading the News

  • You might use a variety of approaches to reading the news:
  • Read aloud the news story to students as they follow along.
  • Project the news story onto a screen. (If you do not have a projector connected to your computer, photocopy the news story onto a transparency and use an overhead projector to display it.)
  • Students might first read the news story to themselves; then call on individual students to read the news aloud for the class.
  • Arrange students into small groups. Each student in the group will read a paragraph of the story. As that student reads, others might underline important information or write notes in the margin of the story. After each student finishes reading, others in the group might say something -- a comment, a question, a clarification -- about the text.

  • More Facts to Share

    You might share these additional facts with students after they have read this week's news story.

  • The rocky 2003 UB313 is located in the Kuiper Belt, a collection of asteroids at the edge of the solar system.
  • Scientists estimate that 2003 UB313 is about 50 percent larger than Pluto. That would make it the largest object discovered in the solar system since Pluto was first seen in 1930.
  • 2003 UB313 is more than 9 billion miles from the sun. That's three times farther from the sun than Pluto is.
  • Earth is 93 million miles from the sun. If 2003 UB313 is 9 billion miles from the sun, it is nearly 100 times farther from the sun than Earth is.
  • Dr. Michael Brown of the California Institute of Technology, one of the discoverers of 2003 UB313, said, "If Pluto is a planet, it seems reasonable that something that's bigger than Pluto, and further away than Pluto, should be called a planet, too."
  • About two years ago, Brown discovered Sedna, an object about the same size as Pluto. It has yet to officially be called a planet.
  • Some scientists say that 2003 UB313 is too much different from the other planets to be called a planet. Brian Marsden, who runs the Minor Planet Center, told that if Pluto is a planet, then other round objects nearly as large as Pluto ought to be called planets. In that case, 2003 UB313 would not be the tenth planet because "it would have to get in line behind a handful of others (including Sedna) that were discovered previously."
  • Alan Stern of the Southwest Research Institute says there could be "1,000 Plutos out there."
  • Alan Boss, a planet-formation theorist at the Carnegie Institution of Washington, says that Pluto and other small objects (such as 2003 UB313) beyond Neptune should be called, at best, Kuiper Belt planets. "To just call them planets does an injustice to the big guys in the solar system," Boss told

    Comprehension Check

    Revisit the Anticipation Guide at the top of this lesson; ask students to respond again to the statements in it.

  • There are only nine planets in the solar system. (That statement is up for debate.)
  • Scientists can use powerful telescopes to see objects that are more than a billion miles from Earth. (true, the object called 2003 UB313 is estimated to be 9 billion miles from Earth)
  • Scientists agree that any object in space that is bigger than Pluto should be considered a planet. (false, scientists do not agree on what defines a planet; that's why there is a debate over 2003 UB313)

    You might follow-up that activity with some of these questions:

    Recalling Detail

  • Why might scientists need to redraw maps of the solar system? (because some scientists say they have discovered a new planet)
  • Is 2003 UB313 a cold or warm planet? (it is far, far from the sun; it is very cold and icy)
  • When was Neptune discovered? (1846)
  • Why was 2003 UB313 not found until very recently? (Answers will vary; they might include that it is outside the orbits of the other planets or that new, powerful telescopes can see things farther out in space than ever before.)

    Think About the News
    Discuss the Think About the News question that appears on the students' news page. (Note: Michael Brown, one of 2003 UB313's discoverers, says that his team of scientists have decided on a name, but they will not reveal it until the IAU decides if the object is a planet or not. In the past, Kuiper belt objects have been named after Native American, Inuit, and minor Roman gods. Brown has stated that "our new proposed name expands to different traditions.")

    Follow-Up Activities

    Math. Pluto is about 1,420 miles in diameter. If the new planet is 1-1/2 times (or 50 percent larger) the size of Pluto, how many miles in diameter would it be? (about 1,420 x 1.5 = 2,130 miles in diameter) If Mercury is about 3,030 miles in diameter, is the new planet larger or smaller than Mercury? (smaller) Science. Use Planet Size Comparison to compare the sizes of the planets. Show students how to use this online tool. (Click on the names of two planets, then click on the word COMPARE to see a visual comparison of the planets' size. Note: You can also compare the size of planets to the size of the sun or the moon.) Which planet in each of the following planet pairs is the larger one?

  • Earth and Neptune (Neptune)
  • Jupiter and Mars (Jupiter)
  • Earth and Uranus (Uranus)
  • Mars and Saturn (Saturn)
  • Neptune and Saturn (Saturn)


    Use the Comprehension Check (above) as an assessment. Or have students work on their own (in their journals) or in their small groups to respond to the Think About the News question on the news story page or in the Comprehension Check section.

    Lesson Plan Source

    Education World

    National Standards

    National Standards

    GRADES K - 4
    NS.K-4.4 Earth and Space Science
    NS.K-4.5 Science and Technology
    GRADES 5 - 8
    NS.5-8.4 Earth and Space Science
    NS.5-8.5 Science and Technology
    GRADES 9 - 12
    NS.9-12.4 Earth and Space Science
    NS.9-12.5 Science and Technology

    See recent news stories in Education World's News Story of the Week Archive.

    Article by Gary Hopkins
    Education World®
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