Jan
15
2009

Why is Pluto no longer considered a planet?

This drawing, modified by Image Editor from a NASA graphic, shows the orbits of the eight planets. Also shown are Ceres (between Mars and Jupiter) and three dwarf planets in the outer reaches of the solar system: Pluto, Eris and Makamake. CC-BY

This drawing, modified by Image Editor from a NASA graphic, shows the orbits of the eight planets. Also shown are Ceres (between the orbits of Mars and Jupiter) and three dwarf planets in the outer reaches of the solar system: from top to bottom, Pluto, Makemake and Eris. CC-BY

All of us who write here at Quezi learned in school that our solar system has nine planets. But if you have kids in school, you might have heard that they’re learning now that there are only eight planets in our solar system. The reason: Pluto is no longer counted as a planet.

Pluto has long been an oddity:

  • Although all planets have oval orbits, their orbits are fairly close to circular. But Pluto’s orbit is so eccentric that frequently it is closer to the sun than Neptune is.
  • Although the first eight planets are the largest objects in our solar system after the sun, there’s at least one object (it’s called Eris) that’s larger than Pluto.
  • Pluto has a mass that is far less than our moon’s and some of the other moons in the solar system.
  • If you could stretch a huge piece of paper across our solar system, you would see that the orbits of the eight plants would be close (in astronomical terms) to that piece of paper; in other words, all the planet’s orbits are at the same “level” (the fancy term is “ecliptic plane”) relative to the sun. But the orbit of Pluto is at an angle to those of the planets.
  • The largest moon of Pluto, Charon, is so massive relative to Pluto that they can be thought of as orbiting around each other rather than Charon orbiting around Pluto.

For these reasons and others, the International Astronomical Union in 2006 developed a definition of “planet” that no longer includes Pluto. It classified Pluto as a dwarf planet along with Ceres (discovered in 1801, more than a century before Pluto) and three astronomical bodies that were discovered early in this century: Eris, Haumea and Makemake.

Although there are some astronomers who still prefer to recognize Pluto (and perhaps one or two of the dwarfs) as a planet, the IAU’s planetary definition has been widely recognized, changing the way we learn about our solar system.

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1 Comment

  • eiffel says:

    When I’m talking about the “olden days” to my kids, I sometimes throw in the phrase “Back when Pluto was a planet…” just for fun!

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