All posts by David L. Kaufman

What Happened to Planet Pluto?

planet_pluto_facesFor 75 years, kids around the world learned about the nine planets that go around (orbit) the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. And then in 2006, Pluto was gone; there were only eight planets! What happened to Pluto?

Pluto was discovered in 1930 by Clyde Tombaugh at the Lowell Observatory in Arizona. And from the beginning we knew that Pluto was weird. It was not like the other eight.

It was usually the farthest planet from the sun, though sometimes it was closer than Neptune. It was tiny, smaller than Earth’s Moon. It wasn’t round either, more like a potato than a ball.

And while the other eight planets orbits are almost circular, Pluto’s orbit was a deep oval (like the shape of an egg). And it has 4 moons, one almost as big as Pluto itself.

None of that bothered space scientists (astronomers) until 2005 when they found a 10th planet, Ceres! Ceres is bigger than Pluto, but just as weird. But just when they were about to update all the textbooks to include #10, they found an 11th, Eris.

Soon astronomers realized that Ceres and Eris were just some of hundreds of objects orbiting way out past Neptune. If Ceres is a planet, then they all are! Imagine having to memorize all of them for a science quiz!

So the scientists changed the definition of “planet.” Definitions are really important in science. They keep everyone in the world talking the same language. So making the change was a big deal.

After much debate, they agreed that to be called a planet, you needed to:

  1. Be nearly round like a ball
  2. Have an orbit close to a circle around the Sun
  3. Have cleared out the neighborhood (orbit) of other floating junk

Pluto, Ceres and all the others did not meet the definition and were officially renamed “Dwarf Planets.”

So, Pluto is still out there, but it is no longer considered a planet. However, it’s still a weird, cool rock. And we are about to learn even more about it In 2015, a NASA rocket arrives at Pluto! The “New Horizons” mission will take the first ever close up look at the most famous dwarf planet.

Fun Phineas Fact: Planet Pluto was named by an 11-year-old girl from England.


WEB: Dwarf Planets


WEB: UniverseToday – Interesting Facts About Pluto

PODCAST: Astronomycast Episode #1

PHOTO CREDIT: NASA, ESA, and M. Buie (Southwest Research Institute): This is the most detailed view to date of the entire surface of the dwarf planet Pluto, as constructed from multiple NASA Hubble Space Telescope photographs taken from 2002 to 2003.