Earth's Journal

Space Journal Entry

Pity Poor Pluto: It's Been Demoted (August 28, 2006)


View of Pluto (lower left) and its moon Charon from the Hubble Space Telescope. The planet was 2.6 billion miles (4.6 billion kilometers) away when the picture was take, nearly 30 times Earth's distance from the Sun. NASA/European Space Agency.

Suddenly, every solar system poster hanging in every classroom around the world is out of date. After years of lively debate, astronomers announced some stunning news: Pluto is not a planet any more.

The International Astronomical Union's (IAU) decision changes the list of planets for the first time since Pluto was discovered in 1930. From now on, poor Pluto will be considered a dwarf planet.

The change in Pluto's status follows from the IAU's new definition of a planet. To be a planet, a solar system body must be massive enough for its gravity to pull it into the shape of a ball. Secondly, the body must orbit the Sun. It also cannot share its "neighborhood" in space with any other bodies.

Pluto meets the first two conditions but not the third. Thousands of other bodies are in Pluto's celestial neighborhood in the Kuiper Belt, a region of icy debris beyond the orbit of Neptune. Pluto's elliptical orbit also crosses that of Neptune during its 248-year journey around the Sun. Two other bodies, Xena (officially called 2003 UB313) and the asteroid Ceres, were also named dwarf planets.

For those keeping score, our solar system now includes eight planets instead of nine and three dwarf planets. It's also filled with tens of thousands of other objects called smaller solar system bodies, including comets and asteroids.

Xena is bigger than Pluto. Like Pluto, it orbits the Sun beyond Neptune in the Kuiper Belt and so doesn't qualify as a planet. Astronomers expect hundreds of other dwarf planets will soon be discovered in the Kuiper Belt.

Ceres doesn't make it as a planet because it orbits the Sun in the crowded asteroid belt between Mars and Jupiter. Ceres was discovered in 1801, before either Neptune or Pluto. At the time, some astronomers wanted to name it the eighth planet. They soon changed their minds after spotting more and more icy rocks just like it in what was later called the asteroid belt.