Pluto: A planet once again?

The recent discovery of a fourth moon orbiting Pluto has reignited one of science's most vociferous debates: Is Pluto a planet? The celestial body was downgraded to "dwarf planet" status in 2006 by the International Astronomical Union (IAU) after the group recast its definition of "planet" and invented the new "dwarf planet" term for Pluto. The reclassification set off a storm of protest from Plutophiles worldwide. Should Pluto now rejoin the solar system as our ninth planet?

No. Pluto can't be compared to real planets: To meet the definition of a planet, an astronomical body has to orbit the sun, be spherical in shape, and be the dominant body in its path of orbit, says Mark Thompson at Discovery News. The last criterion is "the nail in the coffin for Pluto," which shares its orbit with several other celestial objects, including "ice giant Neptune." Pluto is a dwarf planet that's even smaller than our moon. "The recent discovery of another moon in orbit around the tiny distant world doesn't change anything."

Yes. Pluto is just a special kind of planet: Though the IAU downgraded Pluto, "it's counter-intuitive to think of it as just a giant asteroid," says Jordan Yerman at NowPublic. "As we see more and more of the universe around us, we're forced to rethink the concept of the planet," and it's quite possible that Pluto is a planet after all — just a different sort of planet. "'Dwarf planet,' indeed! Surely a planet of small stature is still a planet."

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Besides, the decision to reclassify Pluto was flawed: When the IAU voted on Pluto's fate, "only a handful of IAU members were present to vote," says Victoria Jaggard at National Geographic. Furthermore, the IAU's specialty isn't planetary science, it's astrophysics. Even astronomers who disagree over Pluto's classification agree that "the IAU's current definition of a planet only works for bodies orbiting our sun." The more we see outside our solar system, the less relevant current definitions become.