So, whatever happened to Pluto, anyway?

by Jason Wolvington, Associate Director, Discovery Science Center

I’m still bitter.

OK, so maybe “bitter” isn’t the right word. Heartbroken? Disappointed? It’s probably more that I’ve always known Pluto as a planet, and so that’s how it always will be in my mind. That’s legit, right?

Actually, no … it’s not.

Back in August 2006, at a meeting of the International Astronomical Union, astronomers made a final decision about the definition of a planet. For an object to be considered a “planet,” it needs to meet these three requirements:

  • It must orbit around the Sun.
  • It must have enough mass and gravity to pull itself into a nearly-round shape, called hydrostatic equilibrium.
  • It needs to have “cleared out” its orbit, meaning it has to have joined with or consumed all smaller objects in its orbit.

From this, Pluto meets only 2 of the 3 requirements – it has not “cleared out” the neighborhood of its orbit. Simply put, Pluto is just one object within the Kuiper Belt, a large collection of celestial bodies similar to an asteroid belt. And wiithin the Kuiper Belt, there are several objects, some of which are even larger than Pluto. So Pluto is not a classified as a planet anymore, just another object in the Kuiper Belt. Make sense?

So what does this mean? In a nutshell, it means that science is doing its job.

Science is a way of seeking to understand our natural world, through observation, experiments, and analysis. Science relies on examining evidence and interpreting its findings through logic; it is tested, repeated, and verified. A scientific theory is a well-substantiated explanation for a set of phenomena that has been tested and verified.

And that’s the key: while scientific theories may be well-supported by evidence, they can be modified or replaced as new evidence appears. Science changes – and it should! – as new discoveries are made.

A perfect example: for thousands of years, people believed the Earth was stationary – and flat! – located at the center of the universe. It wasn’t until scientists such as Copernicus, Galileo, Kepler, and Newton – using a combination of advances in mathematics and developments in telescope optics – developed a new understanding of astronomy and physics that led to new discoveries. You know, things like correctly recognizing that the Earth revolves around the Sun, and that all other planets are ruled by the same physical laws as the Earth.

And that’s what happened to our now non-planet Pluto. As scientific discoveries were made, science adapted … and the Solar System now has 8 planets. It’s not so much that Pluto was demoted, but rather scientific thought refined the classification of what constitutes a planet. And unfortunately, Pluto no longer meets the requirements.

Pluto is now considered a dwarf planet, along with Ceres, Eris, Haumea, and Makemake,  all found in the Kuiper Belt. Will it someday regain its planetary status? Perhaps. But with dwarf planet Eris being larger than Pluto, we may have to adjust from the current 8 planets to 10.

And that would just be crazy, right?!

Postscript: It’s only appropriate that we pay homage to Venetia Phair, who, as an astronomy-loving 11-year old in 1930, proposed the name “Pluto” for the newly-discovered “Planet X.” She passed away on April 30th, at the age of 90.

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2 Responses to “So, whatever happened to Pluto, anyway?”


  1. 1 Laurel Kornfeld May 12, 2009 at 11:25 am

    Your initial response regarding Pluto IS legitimate, as Pluto still IS a planet. It should be noted that the IAU’s controversial demotion of Pluto is very likely not the last word on the subject and in fact represents only one interpretation in an ongoing debate. Only four percent of the IAU voted on this, and most are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. Stern and like-minded scientists favor a broader planet definition that includes any non-self-luminous spheroidal body in orbit around a star. The spherical part is important because objects become spherical when they attain a state known as hydrostatic equilibrium, meaning they are large enough for their own gravity to pull them into a round shape. This is a characteristic of planets and not of shapeless asteroids and Kuiper Belt Objects. Pluto meets this criterion and is therefore a planet.

  2. 2 Jamie Miles August 13, 2009 at 2:25 am

    I agree
    For so long has Pluto been called a planet is does not matter if the most powerful of men tell one that its not a planet.
    Pluto has…..
    1. A Sphericle(Round)shape.

    2. It…like said…orbits a star our sun.

    3. it also has its very own moon.

    So if Pluto is not a planet then Charon is not a moon

    It was once said that putting a man on the moon was impossible.

    As it is with Pluto being a simple object….

    in my book its a planet that has a moon, is round and orbits the sun as well as the other planets do.

    to be honest Pluto does NOT follow an annual path around our star

    Niether Neptune. The two planets switch spots in orbit….changing the pattern of planets.

    In Technicallity…Pluto is not a planet, but a mere comet astray on a set orbit, Charon is not a moon, and Neptune is not a planet as well as Uranus.

    Uranus-Theory says that after being hit by a massive object that it sits on its side, compared to other planet in the Solar System Uranus is not a planet.

    So MY Summary is….

    1. Pluto is not a planet wether it be dwarfed it is NOT.

    2. Neptune does not follow on its regular path and also is not a planet.

    3. Uranus sits on its side..though it may seems to not be a planet…it actuallity it IS A PLANET.


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