FromQuarkstoQuasars

Why Pluto is Not Being Reclassified as a Planet

Daniel BarkerJune 23rd 2014

Many of you may have noticed the below Salon article (originally published in Scientific American) making a few rounds on the internet lately. If you are among the few—few being relative, given that the original source acquired more than three thousand shares on Facebook —that had hope for Pluto’s return to planetary status, I’m here to tell you this is not going to transpire (despite what our epic April fool’s joke suggested).

Screen shot of Salon article showing article title and caption.
Screen shot of Salon article showing article title and caption.

The Salon article about Pluto’s comeback is actually the transcript of a sixty second podcast written by PhD astronomer Ken Croswell. I will tell you all, the article (and its title) are a bit misleading. The article boasts of Pluto’s five moons, it’s dominant size, and its tenuous methane atmosphere—All reasons why Pluto should (and probably will be once more) a planet. I want to tackle all of these claims individually.

Though the claims made by Croswell aren’t false, they have little to no impact on Pluto’s planetary status. Pluto does indeed have five known moons, but it is just barely the dominant body in the system. In fact, Pluto’s moon Charon is so large that the system’s barycenter (center of orbital mass) lies outside of Pluto’s interior. All orbital systems have a point around which all the bodies mutually orbit, including the Earth-moon system. The difference is, the center of mass in most systems (systems with a more massive dominant object) is beneath the surface of the dominating object. Even the sun “wiggles” in response to the orbit of its planets, and even asteroids can have moons (or a ring system!).  But Pluto actually makes a visible revolution…

Pluto and Charon's Orbit (Credit: Stephanie Hoover)
Pluto and Charon’s Orbit (Credit: Stephanie Hoover)

Croswell also notes Pluto’s dominant size in the list of known dwarf planets. However, in reality, Pluto boasts a diameter that is a mere 26 miles (practically within a margin of error) larger than Eris. It is also worth noting that, were we to change a dwarf planet’s status because it was the biggest of them, then we would start down a slipper slope where (possibly_ all dwarf planets would be planets, as changing the category of the largest would create a new largest, and so on, and so on.

One more factor Croswell puts up for consideration is the presence of an atmosphere on Pluto. This also has nothing to do with an objects planetary status. The inner solar-system dwarf planet Ceres could have an atmosphere (and an ocean), though the outcry for Ceres’ planethood seems absent.

For clarification, this debate is against the definition of “planet”, which the International Astronomical Union is credited for creating. In 2006, the IAU voted to accept this definition of planet:

(1) A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

[Reference: “Pluto and the Developing Landscape of Our Solar System“]

Pluto simply does not meet this criteria. Croswell’s case for Pluto includes traits of the dwarf planet that have nothing to do with whether or not it is a planet, as the definition stands. In order for Pluto to become a planet, we would need to entirely do away with the definition of a planet. Reinstating it would require us to restructure the definition in such a way that we would end up with (potentially) a whole slew of planets, and we’d have no real hierarchy to distinguish them. If the IAU changes the definition of planet to include Pluto then we would have to include perhaps hundreds of other objects, for consistency.

Consequently, the definition is not likely to change. This means that the only way Pluto will ever become a major planetary body again is if it clears its orbit of other debris (which wont happen for a very long time, if ever).

The fight for Pluto seems to be never ceasing, Croswell’s article even sites a petition to reestablish Pluto’s planethood. Recently, Neil deGrasse Tyson turned down a request to debate Alan Stern (head of the New Horizons mission) on the status of Pluto. Many hold Tyson partially responsible for Pluto’s declassification, as he was very supportive of the change. In response, Stern posted on his Facebook page, saying that Tyson’s refusal to debate was, “evidence he knows the position isn’t supportable.” It would seem our nostalgia for Pluto has gotten the better of us, but the harsh truth is that science cares very little for our sentiment.

Pluto & Its Moons From the Surface
Pluto & Its Moons From the Surface

Just because it was reclassified, doesn’t mean that Pluto has changed in any way. It seems strange that we let categorization of space stuff to get to us. Pluto is exactly as it used to be, and the definition change is simply for clarity, and to make our study of the solar system easier. So Pluto took a hit, but it was in the name of discovery. Pluto is a dwarf planet.

All images courtesy of NASA
All images courtesy of NASA

 


Scientists recently learned that Charon – Pluto’s largest moon – might have hosted an ocean in the distant past. Learn more here, or see the first look at the Plutonian system, as seen by New Horizons here.

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