Sep 11 2018

The Pluto Debate Rages On

Published by under Astronomy
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If you have not watched Rick and Morty, I highly recommend it. It is an adult-themed cartoon, very entertaining, and extremely well written. On one episode Jerry, while trying to help his son Morty with a science project, mistakenly says that Pluto is a planet. When Morty corrects him, Jerry doubles-down and will not relent, going as far as to call NASA to insist that Pluto be reclassified a planet. This gets the attention of the Plutonians, who are themselves engaged in a raging controversy over whether or not Pluto is a full planet, and who enlist Jerry as an “Earth scientist” to support the planet position. This plot line is really a commentary on science denial, specifically global warming denial.

The Pluto controversy back on Earth is less intense, and the stakes are lower, but it remains and interesting debate about how to optimally categorize things in science. What categories should we have, and what criteria should we use? Should scientific utility be the only measure, or should public understanding also play a role?

Pluto was discovered in 1930 by Clyde Tombaugh and was designated as the ninth planet. Pluto has always been, for some reason, a popular favorite, maybe because of the name, maybe because it was the smallest planet. Pluto has the most eccentric orbit of any of the classic 9 planets, and it’s orbit actually crosses over the orbit of Neptune. It is also the first Kuiper belt object discovered – a region of our solar system beyond Neptune that is full of icy objects.

Problems started for our nice planetary system when other Kuiper belt objects started to get discovered. In 2005 Eris was discovered – it is slightly smaller than Pluto but is 27% more massive (because it is more dense). Eris also has a moon, Dysnomia. Two other large Kuiper belt objects have since been confirmed and named – Makemake and Haumea.

All of this motivated the International Astronomical Union (IAU) to formally define “planet” for the first time. The concern was, if Pluto and Eris are to be considered planets, then we may soon have dozens of planets in the Kuiper belt and our neat classification system might get messy. They ultimately settled on the following definition of a planet:

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.

By this definition, they concluded, Pluto is not a planet, because it has not cleared out its orbit. So, they also created another category, the dwarf planet, which would then include Pluto, Eris, Makemake, and Haume in the Kuiper belt, and Ceres in the asteroid belt. That is where things stand now. The world, however, was not happy. It was perceived that beloved Pluto was “demoted” and that just didn’t sit well.

There was also serious scientific objection to the new classification system, and that controversy has never fully settled down. Now a new paper seeks to reignite the discussion. UCF planetary scientist Philip Metzger from the university’s Florida Space Institute argues that the criterion that a planet must clear its orbit is both arbitrary and not scientifically useful.

His basic argument is that no astronomer uses the concept of clearing one’s orbit in their research. He found only one paper in over 200 years, from 1802, that referred to this concept. From the press release:

“We showed that this is a false historical claim,” Runyon said. “It is therefore fallacious to apply the same reasoning to Pluto,” he said. Metzger said that the definition of a planet should be based on its intrinsic properties, rather than ones that can change, such as the dynamics of a planet’s orbit. “Dynamics are not constant, they are constantly changing,” Metzger said. “So, they are not the fundamental description of a body, they are just the occupation of a body at a current era.”

I only sort-of agree. I do think they make a solid argument for dropping the criterion that a planet must clear its orbit. This is also arbitrary – to what extent must the orbit be clear? There are asteroids is every planet’s orbit.

But according to Metzger, then, many current moons would be reclassified as planets. There are seven moons larger than Pluto, including our own moon. Basically he would have every object in the sun’s gravitational orbit that is large enough to be pulled into roughly a sphere be considered a planet. I think this is going too far in the other direction.

Here is my humble suggestion. We keep criteria “a” and “b” and drop “c”. However, we add that the object must not be in a subservient orbit around a larger object. What does that mean? If two objects, like the Earth and Moon, are in orbit around each other, and the center or gravity (barycenter) lies beneath the surface of one of the bodies, then the smaller object will be said to orbit the larger object, and is a moon. Therefore Europa, which is large enough by itself to be a planet, would instead be considered a moon because it orbits Jupiter.

Here is an interesting implication of this – the barycenter between Pluto and Charon (its largest “moon”) lies outside of either body. Therefore, by this definition neither would be a moon, and Pluto-Charon would be a double planetary system – they would both be planets.

Ceres, Eris, and Makemake would be promoted to planets. Haumea is arguably not spherical enough to be a planet, and would remain a dwarf planet. All other confirmed spherical objects would be moons. This means we would go from 8 to 13 planets (the current 8 plus Pluto, Charon, Ceres, Eris, and Makemake). This number is almost sure to grow and more Kuiper belt objects are confirmed.

There is no objectively right answer to the question of how to classify planets. The point is that valid and scientifically useful concepts should be used. Ideally criteria would be operationally defined, so there are no fuzzy boundaries.

But there is another criterion that always seems to be lurking in the background, even if it is not stated explicitly – the aesthetics of the resulting system. Does the classification seem neat and pretty, does it seem intuitive? Having our Moon become a planet just feels wrong, as much if not more than having Pluto not be a planet. I also understand that having lots of planets may feel wrong to some, but that just doesn’t bother me. If there are lots of planets in the Kuiper belt, so be it. We can divide planets into inner planets (Mercury through Mars or Ceres), outer planets (Jupiter through Neptune) and Kuiper belt planets (Pluto-Charon and beyond).

I do hope the IAU reconsiders its 2006 decision. Perhaps this new paper will be the spark. But I am also concerned that whatever new decision they make will be even less appealing. At the very least I think there needs to be a longer and more transparent conversation about how to best classify planets. It’s the least we can do for the poor Plutonians.

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