Why Pluto isn’t considered a planet anymore

Photo of Pluto

Pluto was first designated a planet in 1930 when it was discovered by Clyde W. Tombaugh at the Lowell Observatory in Flagstaff, AZ.

The tiny planet is about two-thirds the diameter of Earth’s moon and most likely is made up of a rocky core surrounded by ice and coated with methane and nitrogen frost. It has a low density — about one-sixth that of our moon — with a thin atmosphere that expands when it comes closer to the sun and collapses as it moves farther away, similar to comets.

It is located beyond the orbit of Neptune, in a region known as the Kuiper Belt, about 3.6 billion miles from the sun.

However, from the beginning, Pluto has been something of an oddity as a planet. Its orbit is inclined 17 percent compared to other planets, although its uneven shape does make it intersect with Neptune’s orbit, and actually was closer to the sun than Neptune 1979-99. It takes 248 Earth years for it to complete one orbit, and it also spins slowly, with one Pluto day equal to about six Earth days.

Pluto's orbit

In 1978, Pluto’s main moon, Charon (pronounced Karen), was discovered and is now known to be one of five moons around Pluto. However, instead of Charon orbiting Pluto, they both orbit a gravitational center not located on Pluto. Pair that fact with the discovery of other large objects in the Kuiper Belt — a body called Haumea that is just a little smaller than Pluto and has two moons of its own, Makemake, which is only slightly smaller than Pluto, and Eris, which is the same size or larger than Pluto — its designation as a planet came into question.

A vote was taken at the XXVIth General Assembly of the International Astronomical Union in 2006 to define a planet.

(See planet fun facts)

Astronomers decided a planet must have three qualifications to retain its designation.

  1. It must orbit the sun.
  2. It must be nearly spherical in shape.
  3. It must have cleared the neighborhood around its orbit. This means it is the dominant gravitational body in its orbit. Any other small objects in the orbit would be consumed or slung away from the planet.

Pluto fits the first two qualifications, but it falls short on the third as it is only 0.07 times the mass of other objects in its orbit. It is actually one of 70,000 large objects in the Kuiper Belt, and it was demoted to the status of dwarf planet with Eris, Ceres, Makemake and Haumea. The definition of dwarf planet is that a body meets the first two qualifications of a planet but not the third. There may be as many as 200 dwarf planets in our solar system.

It all seemed cut and dry as of 2006, but the New Horizons spacecraft flew by Pluto in 2015 and sent back photos of the dwarf planet and its complex geological landscape of mountains, valleys, plains and craters that have convinced some scientists that Pluto should actually be considered a planet.

As we find out more and more about the world beyond our planet, perhaps Pluto will be upgraded once again to a planet. Or perhaps it will become something entirely different.

We’ll just have to wait and see!

Graphic of a Pluto fun fact

1 Comment

  1. Jan Grant on January 11, 2019 at 3:30 pm

    I really appreciate these facts about Pluto. Thanks!

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