New Horizons Principal Investigator Thinks Flyby Will Prove Pluto Is Still a Planet

Monday, 15 June 2015 - 2:51PM
Solar System
Monday, 15 June 2015 - 2:51PM
If you're anything like me, you were incredibly sad when underdog little guy Pluto was declared "not a planet" in 2006. Since then, we've all taken for granted that Pluto is, in fact, a dwarf planet, and that only eight celestial bodies in our solar system qualify as "real" planets. But according to New Horizons principal investigator Alan Stern, Pluto has been a planet all along, and the upcoming New Horizons flyby will prove it.

"If it looks like a duck, swims like a duck, and quacks like a duck, then it's probably a duck," Stern told Popular Science. "If you go to planetary science meetings and hear technical talks on Pluto, you will hear experts calling it a planet every day."

Pluto was considered to be our solar system's ninth planet until a 2006 meeting of the General Assembly of the International Astronomical Union (IAU) in Prague. Strangely enough, there had never been an official definition of a planet until this meeting, and the official definition the IAU came up with excluded Pluto as a planet. Stern, and many other astronomers, have been fighting that decision ever since, contending that the astronomers who wrote the definition were not planetary experts, and therefore were not qualified to make the decision. 

"Astronomers who are non-experts in planetary science basically passed a bunch of B.S. off on the public back in 2006," said Stern. "A week later, hundreds of planetary scientists, more people than at this meeting in Prague, signed a petition that rejects the new definition. 

According to the current official definition delineated by the IAU, a planet is "a celestial body" that:

1) is in orbit around the Sun,

2) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

3) has cleared the neighborhood around its orbit.

Pluto fulfills the first two requirements, but doesn't meet the third qualification, as it has many objects around it, from asteroids to other Pluto-like dwarf planets. But according to Stern, the only reason Pluto fails this test is its location in the Kuiper Belt, an area in our solar system filled with small celestial bodies, similar to an asteroid belt.

"If you put Earth out in the Kuiper Belt, it couldn't clear it either," said Stern. "Does that mean Earth's not a planet?"

Stern opines that the third criterion for a planet is meant to ensure that it has a sufficiently strong gravitational force to repel objects away from its orbit. But this definition of a planet is flawed, if only because it rules out other celestial bodies other than Pluto: "In reality, every planet in the solar system has other small bodies next to it that it has not cleared. So, this lousy definition from non-specialists rules out everything. It rules out Jupiter, because Jupiter has Trojan asteroids. It rules out Neptune, because Pluto crosses Neptune's orbit. It's just so poorly thought-out that it's laughable."

But all this might change on July 14, when New Horizons makes its closest approach to Pluto. The NASA probe will take pictures that will reveal Pluto in more detail than ever before, and may show the scientific community that Pluto is too similar to other celestial bodies that are considered "planets" to be excluded. Stern believes that the images from the New Horizons mission could lead to an entirely new planetary definition, which would not only mean that Pluto would be considered a planet, but that many other comparable celestial bodies will be invited to the cool kids' table as well.
"I think that one of the things that will come out of the New Horizons mission is that the public will take a look, and they won't know what else to call Pluto but a planet, and a pretty exciting one."
Solar System

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