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CosmosUp | August 14, 2022

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Pluto is a Planet Again?

By | On + -
Pluto is a Planet Again?

When the International Astronomical Union came up with an official definition of a planet in 2006, they booted Pluto out of the club and reclassified it as a dwarf planet. But some say the discovery of exoplanets requires that we revisit this definition and give Pluto a second chance.

The current, official definition says that a planet is a celestial body that:

1. is in orbit around the Sun
2. is round or nearly round, and
3. has “cleared the neighborhood” around its orbit.

But this definition only applied to planets in our solar system. What about all those exoplanets orbiting other stars? Are they planets?

But in America, democracy reigns, and last week the Harvard-Smithsonian Center for Astrophysics re-addressed the debate, putting Pluto’s planetary status to a vote once again. A debate preceded the vote, with three planetary experts offering differing points of view.

 Dr. Gareth Williams, associate director of the Minor Planet Center in Cambridge, said the icy body does not qualify for planetary status. But Dr. Dimitar Sasselov, director of Harvard’s Origins of Life Initiative, argued otherwise. And Dr. Owen Gingerich, professor emeritus of astronomy at Harvard and a senior astronomer emeritus at the Smithsonian Astrophysical Observatory, argued that defining what a planet is shouldn’t be up to scientists.

“What is a planet is a culturally defined word that has changed over the ages. The IAU was foolhardy to try and define the word planet,” he said.

Bill Nye didn’t participate in the debate, but the self-described “longtime fan of Pluto” told The Huffington Post in an email:

If astronomers want to call Pluto a “planet,” that’s fine with me. If that is the route they choose, I believe they will add the several other objects way out there that have enough gravity to be spherical. … I love Pluto as much as the next guy, but it has a different origin from the traditional planets and orbits in a different plane. It might be exciting to have names for hundreds of new (very old) planets, but I would be fine with 8 “traditionals” and hundreds of “Plutoids.” These objects are out there and have the characteristics they have regardless of what we call them. But I know, people get pretty passionate about it.

 After the experts argued their cases, the audience was allowed to vote on what a planet is or isn’t and whether Pluto should be considered a planet. The results were conclusive: Pluto should be a planet once again. The video of the debate is now available online:



  1. jim altfeld

    About three decades ago I had the honor and privilege to spend a couple of hours with Dr. Clyde Tombaugh and his wife in their home in Las Cruces, NM. What a fascinating man. The two hours felt like ten minutes. For his sake, in his honor, and as a tribute to him it would be a whole lot more wonderful than somewhat if they reinstated Pluto back to being a planet.

  2. BCL1

    “When the International Astronomical Union came up with an official definition of a planet in 2006, they booted Pluto out of the club and reclassified it as a dwarf planet.” Um. . . Pluto was never out of the club. A dwarf planet is still a planet.

    • Laurel Kornfeld

      That’s what one would think, but the iAU specifically voted that dwarf planets are not planets at all. If they amended this, most of the controversy would evaporate.

      • Graeme Cree

        That’s why they got so much heat. They came up with confusing definitions (a dwarf planet, which is not a planet even though it has planet in the name).

        And they came up with nonsensical definitions. Specifically, a definition of “planet” that excluded a lot of other planets besides Pluto, including Earth, from being planets. Earth does NOT have a clear orbital path. Neither do a lot of planets. They knew they wanted to defrock Pluto, but clearly hadn’t thought out their reasons for doing it very clearly. If they’d come up with a clear and reasonable definition, they might have gotten a lot less heat.

  3. Laurel Kornfeld

    Pluto never stopped being a planet. There never was a consensus among astronomers to demote Pluto, and the media really messed up in acting as though such a consensus exists. Only four percent of the IAU voted on the controversial 2006 resolution, and most are not planetary scientists. Their decision was immediately opposed by hundreds of professional astronomers in a formal petition led by New Horizons Principal Investigator Dr. Alan Stern. In 2009, many of these dissenting astronomers asked the IAU to reopen the discussion at its 2009 General Assembly. The IAU leadership adamantly refused, leading the petitioning astronomers to boycott that General Assembly and a subsequent one in 2012. According to the equally legitimate geophysical planet definition, a planet is any non-self-luminous spheroidal body orbiting a star or free floating in space (or possibly orbiting another planet). In other words, if the object is large enough and massive enough to be squeezed into a round shape by its own gravity but not massive enough to generate hydrogen fusion in its core, it is a planet.

    • Graeme Cree

      You’re right that the media took the IAU vote as gospel (I’m not sure if how many of them actually voted is relevant one way or the other). They probably figured that an “organization” carried more clout than individual astronomers.

      Your definition sounds better than theirs, although of course, you would have to add the caveat that it cannot be in orbit around a larger such body (otherwise it would be a moon). We’d also have to some kind of leeway about what “round” means, since no planets are exactly spherical.

      Where does this geophysical planet definition come from?

  4. stephen s

    Clearing the orbit? That definition excludes the Earth, as an estimated several thousand NEOs can attest. I think the IAU vote was totally political. Pluto is the planet discovered by an American. The vote was primarily of French “scientists” putting in their two centimes worth of opinion of the Iraq War.
    p.s. Is it true that Michael Brown is the second-most despised astronomer in the world?

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