Pluto might actually have a shot at becoming a planet again

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Sep 10, 2018, 11:55 AM EDT (Updated)

When Pluto was demoted, a gasp echoed across the internet. There were tweets. There were memes. There were empty pleas to the International Astronomical Union, which inevitably ended up getting lost in space. Now a team of scientists have found a way to possibly reinstate Pluto as Planet 9.

The outrage blew up in 2006, when the IAU redefined a planet as having the largest gravitational force in its orbit, which disqualifies Pluto because it is under the influence of Neptune’s gravity and, to add insult to injury, has to share its orbit with frozen gases and random Kuiper belt objects. UCF planetary scientist Philip Metzger and his team say that is beyond unfair — and have the research to back it up.

"It's a sloppy definition," Metzger, who is the lead author on a study recently published the journal Icarus, told, openly criticizing the IAU’s method of classifying what is and isn’t a planet. "They didn't say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit."

That includes Earth, Mars, Jupiter, and (ironically) Neptune, which are always being stalked by asteroids in their orbits. Metzger was insistent enough on changing this definition to delve into astronomical literature from the past two centuries. What he found was that the only publication that supported the IAU’s requirements was from 1802, and even the thinking behind that that has been disproved.

What about the research that was supposed to be the last word on what was and wasn’t a planet, published by Gerard Kuiper, whose name has been immortalized by the Kuiper belt? It separated planets and other things floating around in the cosmos by their formation processes, but still didn’t sway the IAU.


Pluto and its moon Charon. Credit: NASA

Metzger also unearthed something of a shocker. Planetary scientists have been referring to moons like Titan and Europa as planets as far back as the 15th century. Think Galileo’s era. The standard that Metzger and co-author Kirby Runyon have proposed for planethood is that a celestial body is large enough for its gravity to shape it into a spherical form. Stars, asteroids, meteorites, black holes would still be ruled out, but that would up the number of so-called planets in our solar system from eight to—wait for it—110.

"It turns out this is an important milestone in the evolution of a planetary body,” Metzger said, “because apparently, when it happens, it initiates active geology in the body."

Pluto has so much going for it in terms of being viewed as a planet again by the scientific community. Geologically, it is almost as fascinatingly complex, with several moons, a multilayered atmosphere, evidence of organic compounds and ancient lakes, and a subsurface ocean. Water doesn’t always mean life, but if there is a hot water vent like those found on Earth’s ocean floor, you never know.

Runyon agrees that Pluto “has everything going on on its surface that you associate with a planet” and that there is “nothing non-planet about it,” as the Johns Hopkins University planetary scientist said in a JHU press release.

He even believes it has more potential to reveal incredible things than Mars. Considering the hype behind landing on and possibly colonizing the Red Planet, the revival of Pluto as a planet could be huge.

Maybe we should start changing our space travel plans.