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Posts Tagged ‘planetary science

how to define a planet: the problematic case of Pluto

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Pluto, with its ‘heart-shaped’ area known as Sputnik Planitia, imaged by New Horizons, July 14 2015

A while back I listened to a podcast from Point of Inquiry, in which two planetary scientists, Alan Stern and David Grinspoon, involved in NASA’s New Horizons mission to Pluto, were separately interviewed, and were inevitably asked about Pluto’s demotion from planet status. Having not followed this issue, I was surprised at the response. So it’s time to take a closer look.

Of course I should be writing ecstatically about the New Horizons mission, not to mention those of Juno, Cassini, Mars’ Curiosity and so forth, and hopefully that will come, but the controversy about Pluto immediately struck me, as I thought, in my naïveté, that its demotion was a consensual thing amongst astronomers, with only the ignoroscenti (my neologism) left to mourn the fact (not that I mourned it particularly – Pluto still existed after all, and it didn’t care a jot what we thought of it).

Pluto, discovered by Clyde Tombaugh in 1930, was accepted as the ninth and final planet in our solar system for decades until the nineties, when another Kuiper belt object was discovered (besides Charon, Pluto’s large moon), and the Kuiper belt itself became a thing, in fact a massive thing, far bigger than the ‘familiar’ asteroid belt between Mars and Jupiter. We now know of more than 1000 kuiper belt objects, with at least 100,000 believed to exist. The Kuiper belt is widely spread out from the orbit of Neptune, and though Pluto is its largest and brightest object, it’s not the most massive. Presumably it’s for this reason that Pluto was demoted – what with the scattered disc and the Oort cloud there seemed to suddenly be a host of objects that could be included as planets, so it was thought better to exclude Pluto, or to demote it to dwarf planet status, presumably along with other assorted Kuiper belt objects (KBOs), rocks and iceballs that were worthy of the designation. That seemed okay to my thoughtless mind, but here’s what Alan Stern had to say on the subject:

Well, you know, we don’t really honour that classification in planetary science, that was really done by a group of different astronomers who don’t know much about planets. Let me give you a technical term, we call it BS. You know what BS stands for don’t you? Bad Science. Now you wouldn’t ask a podiatrist, a foot doctor, to help you if you had a cardiovascular problem with your heart, that’d be the wrong expertise, though they’re both doctors you’d be going for a cardiologist. And if you had a real estate problem you probably wouldn’t go to a divorce attorney, even though they’re both attorneys. In the space field we have many professions, we have engineering professions, we have many different scientific specialties, etc. Astronomers really don’t know much about planets any more than I’m an expert in black holes in faraway galaxies. They had a little meeting in 2006, they were worried that school children would have to memorise the names of too many planets, so they wrote a definition that limited the number of planets to eight. Now, right after that, Ira Flatow called me up on Science Friday and said, would you debate Mike Brown, who was one of the proponents of ‘let’s limit the planets to eight’, and I said, sure, and we got on the phone and it’s Science Friday live, and Mike Brown makes his case and says, ‘look we just can’t have 50 planets, it’s too many to remember.’ Now, I found that anti-scientific, it seems like engineering the definition, versus letting it inform you, but Ira said, Alan what’d you think, ‘can’t have 50 planets’, what d’you say back to MIke? I said, ‘well if you can’t have 50 planets then we’re probably going to have to go back to eight states, I guess’. And he was speechless…

I love that story – though no doubt Mike Brown would’ve told a different one. So let’s turn Stern’s objection into an inquiry. Was it scientifically correct/accurate/fair to reclassify Pluto as a dwarf/minor planet?

Happily I just happened to listen to a podcast of the Skeptics’ Guide a few days later, which has led me to a more detailed piece on Steven Novella’s Neurologica blog on the Pluto controversy. Apparently, in the above-mentioned 2006 meeting they decided that to be classified as a planet, a body in our solar system should meet 3 criteria:

  • it has to orbit the sun
  • it has to be spheroid (i.e. have the mass to be so, due to its gravity),
  • it must have cleared its orbit of other objects.

Now this third criteria immediately seems the dodgiest, as it sounds like it’s designed to eliminate any KBOs. And how do we know an orbit is cleared? After all, one day, a comet or asteroid may strike us, because our orbits have coincided this time around. And why is that third criterion even important?

Novella cites a recent paper by planetary scientist Phillip Metzger who argues that the third criterion is invalid and that nothing about a body’s orbit should be in the definition since orbits can alter due to external influences. Only characteristics intrinsic to the body should be included in the definition. This would essentially leave one criterion standing – that of sphericity. And even then, how sphere-like does a planet have to be? Another ‘problem’ with Metzger’s definition is that it would include moons, such as our own, and many others. Novella has his own classifying suggestion, which sounds promising to me:

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 of 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.

I need to further explain the term ‘barycentre’, for my own sake. Think of two bodies in gravitational relationship to each other. Inevitably, one of them will be more massive, and will exert a greater gravitational force. An obvious case is the Earth and the Moon. Between the two there is a point, the ‘centre of gravity’, or barycentre,  around which the two bodies revolve, but because the Earth is a lot more massive that the Moon and they’re relatively close to each other, that barycentre is actually close enough to the Earth’s centre to be within the mass of the Earth, with the result that only the moon revolves. The Earth, though, is very much affected by the Moon’s gravitational field, which causes a slight wobble as well as tidal effects on the Earth’s surface. 

Interestingly, Novella’s reclassification would include Charon, Pluto’s ‘moon’, as a planet (as well as Pluto of course) because its size relative to Pluto puts the barycentre at a point between the two bodies, rather than within Pluto. So Pluto-Charon would be reclassified as a binary-planet system. It would also promote Ceres, in the asteroid belt, and Eris and Makemake, two recently discovered Kuiper belt objects, to planetary status. That takes the current eight up to thirteen, with others yet to be discovered. 

It’s unlikely of course that the astronomical overlords who reclassified Pluto would be swayed by any mere outsider’s view, however well-reasoned, but this examination of the issue is a reminder of just how dubious the reasoning of ‘experts’ can be, and how important it is to question that reasoning. Size apparently does matter to these guys, but this new category of ‘dwarf’ or ‘minor’ planet seems inherently unstable, and will probably become even more so as the number of discovered exoplanets increases. Will it be mass or volume that’s the decider, and what will be the mass or volume that decides? And does it really matter? It’s only nomenclature after all. And yet… The difference between an asteroid and a comet is important, is it not? And so is the difference between a planet and an asteroid. And so is the difference between a moon and a planet. And so… is it not? 

Written by stewart henderson

October 14, 2018 at 1:09 pm