If you are old enough, you may remember a time when we thought Pluto was a planet. In 2006 the International Astronomical Union (IAU) adopted a standardized definition of what is and isn't a planet. Unfortunately Pluto no longer made the cut. However, the IAU also adopted a new term: "dwarf planet." A dwarf planet is a planet that orbits the sun, has enough mass for its gravity to form it into a nearly round shape, has not cleared other large objects from the region it crosses during its orbit, and is not a satellite of another object in space. Pluto fits this definition perfectly. 

Two people place a red circle with a slash through it over the word "pluto" outside an exhibit entrance.

If you are old enough, you may remember a time when we thought Pluto was a planet. In 2006 the International Astronomical Union (IAU) adopted a standardized definition of what is and isn't a planet. Unfortunately Pluto no longer made the cut. However, the IAU also adopted a new term: "dwarf planet." A dwarf planet is a planet that orbits the sun, has enough mass for its gravity to form it into a nearly round shape, has not cleared other large objects from the region it crosses during its orbit, and is not a satellite of another object in space. Pluto fits this definition perfectly. 

Two people place a red circle with a slash through it over the word "pluto" outside an exhibit entrance.

Stories

How Did We Discover the Planets? What an Accidental Discovery Told Us About Pluto
Inside Look: Celebrating New Horizons With the Mission Team New Horizons: The Gift That Keeps Giving First Mission to Pluto: The Difficult Birth of New Horizons

Exhibitions

Learn more Kenneth C. Griffin
Exploring the Planets Gallery
National Air and Space Museum in Washington, DC
Learn more U.S. National Science Foundation Discovering Our Universe National Air and Space Museum in Washington, DC

Learning Resources

Flights of Fancy Story Time: "Pluto’s Secret" In this special author session, Margaret Weitekamp reads the book, Pluto’s Secret: An Icy World’s Tale of Discovery https://youtu.be/0bKXCaD4toc Grade PK, Grade K, Grade 1, Grade 2, Grade 3 Dwarf planets, People, Science, Small solar system bodies, Solar System

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My Path: "Don't Be Afraid of Who You Are." Hannah Wakeford, Astronomer Dr. Hannah Wakeford is on the cutting edge researching and hunting for exoplanets. Many of these newly discovered planets are in the Goldilocks Zone where conditions may be right to support life. https://www.youtube.com/watch?v=SM4gSVAhMIQ&list=PL6RlkQnoCx_VvN6SkGaFShkAyOcNX8Rry&index=9 Grade 5, Grade 6, Grade 7, Grade 8, Grade 9 Astronomy, Dwarf planets, Exoplanets, People, Solar System, Spaceflight

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What’s New in Aerospace: Inside the First Epic Mission to Pluto (Alan Stern) or Chasing New Horizons A conversation with New Horizons principal investigator Alan Stern and astrobiologist David Grinspoon about their new book, Chasing New Horizons: Inside the Epic First Mission to Pluto. https://www.youtube.com/watch?v=NsNk9ylvP7Y Grade 3, Grades 15-16, Grades 17-20, Grades 13-14, Grade 12, Grade 11, Grade 10, Grade 9, Grade 8, Grade 7, Grade 6, Grade 5, Grade 4 Dwarf planets, Uncrewed spaceflight

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What’s New in Aerospace: New Horizons at Pluto Alan Stern, principal investigator of NASA’s New Horizons mission, shows the exciting images and science results yielded from the New Horizons mission as the spacecraft flew through the Pluto system https://www.youtube.com/watch?v=9LOfSYJeINE&feature=youtu.be Grade 6, Grade 7, Grade 8, Grade 9, Grade 10, Grade 11, Grade 12, Grades 13-14, Grades 15-16, Grades 17-20 Astronomy, Dwarf planets

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Flights of Fancy Story Time: Pluto’s Secret In this special author session; Margaret Weitekamp reads the book; Pluto’s Secret: An Icy World’s Tale of Discovery https://youtu.be/0bKXCaD4toc Grade PK, Grade K, Grade 1, Grade 2, Grade 3 Dwarf planets, People, Science, Small solar system bodies, Solar System

Student

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E.Z. Science: Exploring the X-ray Universe By studying X–rays in deep space, we can learn about some of the most violent and extreme objects in the universe, such as black holes and the remains of stars that have exploded https://youtu.be/tBbdtV53OSc Grade 3, Grade 5, Grade 6, Grade 4, Grade 7, Grade 8, Grade 9, Grade 10, Grade 11, Grade 12 Dwarf planets, Exoplanets, Solar System, Stars, Sun, Telescopes, Spaceflight

Student

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Publications

Learn more Pluto's Secret: An Icy World's Tale of Discovery