WRAL WeatherCenter Blog

What happened to Pluto

Posted February 7, 2012

New Horizons mission is 2/3 of the way to Pluto
(credit: JHUAPL/SwRI)

"People love Pluto!" says Morehead Planetarium Educator, Amy Sayle.

Referring to the 8 planets in our solar system during planetarium shows still brings up lots of questions about Pluto. Some visitors have gotten quite worked up about demotion. It's not just adults that grew up with Pluto the planet, kids have lots of opinions about the subject as well. Even mentioning the the IAU brings some "good natured boos" from audiences young and old according to Sayle.

So why did astronomers feel the need change something as seemingly basic as membership in our solar system? The answer lies in the definition of, or lack there of, the word "planet".

It all started in 2005 when CalTech professor of planetary astronomy discovered Eris, icy dot in his telescope beyond Pluto with an even more eccentric orbit. Initially Brown was championed as the discoverer of the tenth planet, something that had not been done seen since Tombaugh discovered Pluto in 1930, but this got astronomers thinking. Is Eris really a planet? Beyond Pluto (and now Eris) there's a lot out there, over 1000 objects are known today. Are they all planets? What is a planet?

The following year, The International Astronomical Union (IAU) the professional organization of astronomers responsible for naming celestial bodies, provided a more formal definition during its annual meeting. To be considered a planet a body has to meet 3 criteria, 2 of which gauge its size: it must be in orbit around the sun, must have assumed a spherical shape, and "cleared the neighborhood". It's that last one that cost Pluto it's planetary status. It lacks enough mass to clean up it's path around the sun.

The IAU offered an olive branch of sorts to Pluto fans with the creation of the designation "Plutoid" for anything that met those first 2 criteria. Pluto fans weren't appeased, even prompting student protests.

Astronomers didn't anticipate the backlash this created, or the confusion among some kids who thought this meant that Pluto had somehow gone away or even ceased to exist. Not only did countless textbooks instantly become outdated, the mnemonic that we learned in elementary school to remember the order of the planets had to change as well. Our very eager/earnest/educated mother started serving nachos instead of nine pizzas. 3rd grade boys studying the topic seem to prefer: "My Vicious Earthworm Might Just Swallow Us Now."

The controversy remains a boon for planetariums who have called it "the greatest thing to happen to astronomy education". 6 years later it still sparks questions and conversations about what lies beyond the orbit of Neptune. Morehead's fulldome digital video system installed
2 years ago helps keep that going that as audiences travel out to Pluto and it's "friends" in the Kuiper belt, as Sayle calls them.

For all this attention being paid this little icy ball just 0.21% the mass of Earth, we know relatively little about Pluto. That will change when NASA's New Horizons reaches there in July 2015. Launched 8 months before that faithful IAU meeting, the spacecraft is traveling at nearly 36,000 miles per hour towards its target. To put that into perspective, if you could get the family minivan up to that speed it would cut that summer vacation trip from Raleigh to Orlando to just under 59 seconds. That is assuming the family minivan can handle mach 50.

In the end, Pluto hasn't changed, only a label has. It is still making its way around the sun, though it takes nearly 250 Earth years to do so. The same sunlight that shines on us, shines on Pluto, though it takes another 6.6 hours to get there. It's still in the same eccentric orbit tilted away from the rest of the solar system.
Its still kept company by Charon (the two are called a double planet by many) along with moons Nix and Hydra as well as S/2011P1. That last moon with the boring name was discovered with the Hubble Space Telescope last year theorized be the result of a collision between Pluto and one of it's "friends" in the Kuiper belt.

Even under optimal conditions with a massive telescope, Pluto will appear as an easily overlooked dull gray dot. If you are up just before dawn look to the east above the handle of the the teapot in Sagittarius to Pluto's location.

Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on twitter @rtphokie. 


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  • ascalaphus799 Feb 16, 2012

    The mistaken notion that Pluto was named after the cartoon dog seems to have originated with Neil de Grasse Tyson, who apparently had difficulty seeing beyond his own viewpoint regarding what Pluto should be considered to be. It was named for the Roman god of the underworld. The idea that Pluto is not a major planet does remain controversial. It is not accepted by many astronomers (particularly those specializing in planetary science) and even some of its original supporters now reject it or have reservations about it. You don't need an advanced degree in astronomy to know a planet when you see one and the illogic of the current definition of major planet is clear even to a great many lay people, to say nothing of a considerable number of astronomers. Let's put Pluto back where it belongs in the planetary lineup and let's put Eris in with it.

  • ascalaphus799 Feb 16, 2012

    Actually, it does matter that the IAU has mislabeled Pluto. Even non-astronomers can tell something's not right here. The criterion used to "demote" Pluto makes no sense. Earth would not dominate its orbit gravitationally at Pluto's or Eris's distance from the Sun and the fact that their orbits are elliptical and tilted to the ecliptic don't seem to matter to the IAU when the subject is exoplanets. Both objects possess enough mass so that their gravity forms them into round shapes, and since they orbit the Sun independently, even a young child would see them as planets, and there is no point in classifying Pluto (or Eris) otherwise. Here we have objects that would be considered planets if they orbited a star other than the Sun, which are not planets to the IAU basically because they orbit this star. It is ridiculous and the IAU needs to own up to their mistake and correct it.