In the first few years of the 21st century, several new objects that resembled planets were discovered in our solar system. Finding these new objects prompted the International Astronomical Union (IAU) to clearly define what a planet is. In 2006, the IAU announced a planet must:
1. Orbit its host star. In our solar system, the host star is the sun.
2. Be mostly round.
3. Be big enough that its gravity cleared away other objects of similar size from its orbit around the host star.
After defining what a planet is, the IAU then created a new category of objects known as "dwarf planets." Dwarf planets are similar to planets in that they orbit their host star and are mostly round, but there is one big difference between them and regular planets. Dwarf planets are not big enough to create enough gravity to clear away debris and other objects in their orbit.
This new category of dwarf planet reclassified five objects in the night sky: Pluto (formerly called a planet), Ceres (formerly called an asteroid), and the newly discovered objects named Haumea, Makemake, and Eris. Scientists believe there are many more dwarf planets in our solar system that have yet to be discovered.
Source: NASA
| Name | Pluto | Ceres | Haumea | Makemake | Eris |
|---|---|---|---|---|---|
| Named for: | Roman god of the underworld | Roman goddess of corn and harvest | Hawaiian goddess of fertility | Rapanui god of fertility | Greek goddess of discord and strife |
| Year Discovered: | 1930 | 1801 | 2003 | 2005 | 2005 |
| Location: | Kuiper Belt | Asteroid Belt | Kuiper Belt | Kuiper Belt | Kuiper Belt |
| Size: | 1,477 miles in diameter | 592 miles in diameter | 1,080 miles in diameter | 888 miles in diameter | 1,500 miles in diameter |
| Moons: | Charon, Nix, Hydra, Kerberos, Styx | None | Namaka and Hi'iaka | MK 2 | Dysnomia |
| Rings: | None | None | Yes | None | None |
| Temperature: | -387 degrees Fahrenheit average | -105 degrees Fahrenheit average | unknown | unknown | -382 degrees Fahrenheit average |
| Distance from Sun: | 3.7 billion miles | 257 million miles | 4 billion miles | 4.3 billion miles | 6.3 billion miles |
| # Earth Days to Orbit Sun: | about 90,000 | 1,682 | about 104,000 | about 111,000 | about 203,000 |
Source: NASA
Dwarf Planets by 
The Dwarf Planets by
Many people are familiar with Pluto, but what about the other dwarf planets in our solar system? This nonfiction title takes beginning readers to the solar system's different dwarf planets. Narrative text and playful illustrations identify what makes each one unique, and fun facts add bonus information. A special feature includes moons and size rankings, while questions encourage readers to think beyond the text. This fun title takes young readers on a learning adventure!
Far-Out Guide to the Icy Dwarf Planets by
Our solar system used to have nine planets, but recently, scientists ruled that Pluto was actually a "dwarf planet." So what exactly is a dwarf planet, how many of them are there, and how do scientists study and classify them? All the facts readers need, and lots more, are included in this book, featuring a center spread with fast facts.
Pluto and the Dwarf Planets by
In 2006, Pluto was reclassified as a dwarf planet. Small, rocky dwarf planets make their home on the outer edges of the solar system. Scientists use telescopes to study Pluto and the dwarf planets as they orbit the Sun. Their fascinating findings are on display in this informative title about some of the solar system's most far-away objects!
Beyond by
Journey far beyond our solar system and explore the marvels of interstellar space. A wonder-filled poem and spectacular illustrations bring readers across the observable universe to encounter dwarf planets, black holes, brand-new stars, and other incredible phenomena. Award-winning author Miranda Paul and illustrator Sija Hong present a fresh and fascinating journey to the outer reaches of outer space.
The Dwarf Planet Pluto by
A lively and informative look at the dwarf planet Pluto, from its discovery to the recent controversy over its status as a planet. the book describes Pluto's place in the solar system, its moons, its orbit, what we know of its physical features, and the difficulties scientists encounter in studying this mysterious body.
Pluto by
Did you know that Pluto has mountains, glaciers, and even blue skies? In July 2015 a spacecraft called New Horizons finally reached Pluto--almost ten years after it had launched! The spacecraft flew by Pluto to take the first close-up photos of the dwarf planet. When it began sending those photos back to Earth, scientists were amazed by what they saw. Pluto was more fascinating than scientists ever imagined! Read this book to learn more about New Horizons, its long journey to Pluto, and the things scientists learned from the photos of this distant dwarf planet.
Pluto by
Pluto is a dwarf planet with three known moons--Hydra, Nix, and Charon. Children will explore Pluto's physical features, its position in the solar system, and why it is no longer considered a planet.
Exploring Planets and Dwarf Planets with Velma by
A giant storm is raging on the planet Jupiter. Scientists have tracked it for about 150 years, but much about the storm remains a mystery. Science expert Velma and the rest of the Mystery Inc. gang are ready to investigate! Learn how the inner planets are different from the outer planets, how fast the wind blows on Neptune, what the newest rovers are studying on Mars and more!
Pluto and Other Dwarf Planets by
A basic introduction to dwarf planets, such as Pluto, their sizes, compositions, and places within our solar system.
Pluto and the Dwarf Planets by
Jump into orbit around some amazing objects in our solar system: dwarf planets! Take a fact-filled flight around Pluto, Ceres, Eris, Haumea, and learn what makes them so special. Explore their icy surfaces, their moons, and more. Includes a support page of teaching tips for caregivers and teachers. Downloadable Teacher's Notes available.
Mission to Pluto by
In July of 2015 a robotic spacecraft reached Pluto after a nine-and-half-year journey. New Horizons is the first spacecraft mission to Pluto and revealed its five moons as never before seen. Images from the mission show a reddish surface covered in ice-water mountains, moving glaciers, and hints of possible ice volcanoes and an underground ocean. Pluto is geologically alive and changing! This addition to the Scientists in the Field series goes where no person or spacecraft has ever gone before. Follow along with the team of scientists as they build New Horizons, fly it across the solar system, and make new discoveries about a world three billion miles away.