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Something New Under the Sun

Aspiring astronomers find a big point in little Pluto


Illustration by Carl Wiens

Pluto, once considered the solar system’s ninth planet, was officially demoted in 2006. But in Michael Mendillo’s Cosmic Controversies class, the matter is still up for discussion.

In early April, Mendillo stands before his students compiling a list of reasons for Pluto’s reprieve. He is acting as a devil’s advocate; Mendillo (GRS’68,’71), a College of Arts & Sciences professor of astronomy, voted in favor of exclusion at the 2006 International Astronomical Union (IAU) conference in Prague and still has the yellow voting card. But in Cosmic Controversies, the version of introductory astronomy he’s been teaching with graduate assistant Josh Wing (GRS’11) for two years, questioning established wisdom isn’t simply encouraged, it’s the point. And in Mendillo’s mind, there’s no better opportunity for evidence-based analysis than a study of the solar system, which changes as new technology enables new discoveries.

He tells his students that “20 years ago, when most of you were born, the question of Pluto, the hypothesis of dark matter — these weren’t issues at all. Throughout your life, you’ll have things to evaluate. Evidence should be what you use to make a decision.”

In Cosmic Controversies, students explore the building blocks of scientific discovery via three still-developing concepts of contemporary astronomy: dark matter, planets beyond our solar system, and extraterrestrial life. The idea is to demonstrate how astronomy has evolved during their lifetimes, offering scientific literacy to understand future changes.

“The conclusion that the majority of our universe is filled with dark energy is new —  my students in the 1970s didn’t hear about that,” Mendillo says. “And evidence of extra-solar-system planets didn’t occur in the 1980s. But these students are going to hear about it, and I think they ought to be prepared.”

The Pluto controversy is of particular interest to this demographic; more than 1.6 million people have joined the Facebook group called “When I was your age, Pluto was a planet.” By the time Mendillo’s students reach the topic, they’re as well versed in the drama as they are in the facts. That’s important, he says, because it demonstrates how science and popular belief still clash, 400 years after Galileo published his heretical theories of a sun-centered solar system.

“I get yelled at over Pluto every time I get in a taxicab,” Mendillo tells his class. “So what evidence made the astronomers take the step that would turn out to be so unpopular? I’d like to present the evidence both ways, and then we’ll have the vote that was held in Prague.”

The evidence Mendillo lays out for Pluto’s planetary status meets two of three criteria set by the IAU in 2006: it orbits the sun and has adequate mass to form a sphere. But is it the dominant gravitational body at its distance from the sun? In 1930, when Pluto was discovered, it appeared to be alone. But modern high-powered telescopes have revealed thousands of large (and sometimes larger) objects nearby.

At the now-infamous IAU meeting, astronomers first considered increasing the number of planets in our solar system to 12. Then they set the current criteria and voted Pluto out. Now, Mendillo puts it to his students: based on the evidence, what is Pluto?

Students say that pondering such questions is an opportunity they hadn’t expected from a 100-level science course.

“It’s an eye-opener,” says Becky Rosevalt (CAS’10). “This is something you don’t get to talk about every day.”

“With the whole Pluto thing, I didn’t really have the arguments for and against,” says
Nick Bove (COM’12). “Now I can make my own judgment.”

In Prague, Mendillo held up his yellow card to vote. In class, he asks for a show of hands. The final verdict? Poor Pluto gets the boot again, 70 to 1.

Jessica Ullian can be reached at jullian@bu.edu.

This article, in slightly different form, originally appeared in the summer 2009 Bostonia.


6 Comments on Something New Under the Sun

  • Laurel Kornfeld on 07.30.2009 at 12:49 am

    Pluto IS a planet

    Please do not blindly accept the controversial IAU decision as the "official" version, as it is just one side of an ongoing debate. I am a writer, amateur astronomer, and astronomy student in Swinburne Astronomy Online’s Graduate Certificate of Astronomy and have been advocating either overturning and/or ignoring the IAU decision since day one.
    Pluto did not stop being a planet because 424 astronomers made a controversial decision and adopted a vague, unusable planet definition. The requirement that an object "clear its orbit" was concocted specifically to exclude Pluto and keep the number of planets in our solar system low. The IAU definition makes no sense in stating that dwarf planets are not planets at all, a departure from the use of the term "dwarf" in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies. Also, the IAU definition classifies objects solely by where they are while ignoring what they are. If Earth were in Pluto’s orbit, according to this definition, it would not be a planet either. A definition that takes the same object and makes it a planet in one location and not a planet in another location is essentially useless.
    The IAU should take responsibility for the highly flawed definition adopted by only four percent of its members, most of whom are not planetary scientists, in 2006. However, the IAU should not be viewed as the sole authority on the definition of planet. Many planetary scientists do not belong to the IAU. Should they not have a say in this matter? Something does not become fact simply because a tiny group that calls itself an authority says so. It is significant that hundreds of planetary scientists led by New Horizons Principal Investgator Alan Stern immediately signed a formal petition opposing the IAU definition.
    There are other venues through which a planet definition can be determined, such as last year’s Great Planet Debate at the Johns Hopkins University Applied Physics Lab. Audio and video proceedings from this far more balanced conference, which I was fortunate to attend, can be found at http://gpd.jhuapl.edu/ . You can also read more about this issue on my blog at http://laurele.livejournal.com .
    Please, Professor Mendillo, rethink your decision and support initiatives within the IAU to amend the resolution and make dwarf planets a subclass of planets (as would have happened if resolution 5b, which failed by a small margin, had passed).

  • Jim Lombardo on 07.30.2009 at 9:47 am

    Grandfather clause for Pluto!

    I think the scientific community should invoke a “grandfather clause” for Pluto. Since it was originally deemed a planet, and any literature on the subject prior to 2006 would identify it as a planet, they should make an exception.

  • Anonymous on 07.31.2009 at 8:40 am

    Well yes, those enrolled in online schools are often the brightest and most informed people. I know because I go to the University of Phoenix’s online law school and am pretty sure I am the next president.

  • Anonymous on 07.31.2009 at 10:56 am


    I agree with the first comment. Astronomers shouldn’t strip planetary status once it has been granted. When we restore Pluto’s planetary status, we should also restore the planetary status of Ceres, Pallas, Juno, Vesta, and the dozen or so other asteroids that had their planetary status unfairly stripped from them by astronomers in the 1850’s when it became clear asteroids were a new class of object. We should also restore the Sun and Moon’s status as planets, which was unfairly removed thanks to that troublemaker Copernicus who first showed that objects once thought to be planets actually had substantially different behavior and properties. It is more important for terms to keep their meaning than for them to reflect our constantly-increasing understanding of the universe.

  • andrew on 07.31.2009 at 3:07 pm

    Pluto is at Riverside station

    If you take the Green D line out to Riverside, pluto is outside of the train platform. Evidently there are planets spaced out throughout Boston in relation to where they would be in the solar system. Some public school project if I remember right.

  • Giana Bennett on 02.20.2014 at 8:24 pm

    thnxs pluto is not only apart of our solar system it should be a planet but we should be more consurned about how .life is growing on mars

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