Planethood and Pluto weirdness

Maybe you’ve heard about the controvesy over Pluto, how its status as a planet is disputed by some. If it had been discovered today, it might not have been considered a planet by the IAU, the body in charge of naming and defining astronomical objects.

Recently we’ve learned that there are larger bodies than Pluto out there orbiting the sun. We simply lacked the technology to find them until just a few years ago.

The IAU has gone back to the drawing board in light of growing questioning of traditional views of what defines planethood, and is expected to rule on Pluto’s status this September.


What calls Pluto’s status as a planet into question? It’s hardly only its size (tiny Pluto is smaller than our moon). The above image (made with John Walker’s Solar System Live) shows the orbits of all the planets of the solar system. All of them are in roughly the same plane — except one. The orbit of Pluto the oddball is inclined 17 degrees from the ecliptic plane, the plane that Earth (and all the other planets) moves within. It seems that Pluto was snatched into its current orbit by the sun’s gravity some time after the other eight planets were already in place.

Here’s a Wikipedia diagram showing the same tilt:

Pluto's orbit

Pluto also isn’t even always the farthest planet from the sun. Not only is its orbit at odds with the plane of the ecliptic, its eccentric egg-shaped orbit occasionally takes it inside the orbit of the eighth planet from the sun, Neptune — sometimes making Neptune, not Pluto, the farthest planet from the sun.

For twenty years in the current era, from 1979 to 1999, Neptune, not Pluto, was the farthest planet from the sun. In the diagram below, look how Pluto’s orbit slips within Neptune’s. This is what was going on in space when we were all wearing Members Only jackets and enduring Nelson and Martika.

Pluto won’t slip behind Neptune again for over 200 years.

For twenty years of my life, a “fact” I learned in grade school was absolutely wrong. And for all my childhood and a chunk of my early adulthood, my conception of Pluto as a planet just like the others was completely wrong.

For more on Pluto and the planet controversy, have a look at the links below.

Wikipedia entry on Pluto

Farewell Pluto?

What is a Planet?: Debate Forces New Definition

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