Thousands of Planets

Why are 8 planets bad science August 12, 2009

Posted by Jorge Candeias in Definition of planet, Plutophiles.
Tags: asteroids, Definition of planet, history of astronomy, IAU, Mark Sykes, Pluto, Plutophiles, Titus-Bode Law, twitter
21 comments

Yesterday, there was a small rebellion of “plutophiles” on twitter. A hashtag, #bringbackpluto, made it to number one in the trending topics list, and the messages that came along with it were, in general, as silly as you might expect. People just don’t get it.

The people who took part in that particular hashparty vastly misunderstand the reasons why the whole business of Pluto’s “demotion” came about. And their revolt does nothing to further their case and actually “bring back Pluto” (as if Pluto went anywhere; as if it isn’t right where it has always been, going round the Sun beyond Neptune). Quite the contrary. By showing so eloquently that they don’t get it, they simply won’t sway any of the people who actually have some knowledge about this stuff. The only way to sway them is to play their game, which means learning the science and discuss it scientifically. And learning some history of astronomy as well. And remember my mantra: “this ain’t about Pluto!”

Hop aboard. I’m taking you in a small historical trip. A trip you may get from plenty of other sources, but in this there’s no such thing as too many sources of information. And besides, nobody tells it quite like I do. In the end of this necessarily long text, I’ll tell you the main reason why I think that to speak about 8 planets is bad science. You can jump immediately to that point, if you think you already know all the historical stuff, but you’ll be missing my emphasis, on which I base my conclusions. It’s up to you.

Ready? Allright then. Fasten your seatbelts and let’s go visit the ancient Greeks.

Not that those were the guys who discovered the first planets. Ever since the first records of celestial movements were made, probably by the very first astrologers, people knew that there were some lights in the sky that stayed put, wereas other lights walked about. The Greeks were simply the guys who came up with the word “planet”. It means, aptly enough for the level of their understanding, wanderer.

Back then there were two different kinds of wandering celestial objects: those with an obvious disc, and those that looked like point sources of light, like moving stars. The first kind encompassed the Sun and the Moon, and there were all kinds of legends about them; the second kind was composed by 5 objects: Mercury, Venus, Mars, Jupiter and Saturn. These five were always thought of as planets, the Moon and the Sun kept coming and going from that category. The Earth, of course, at first was not thought of as a planet like the others, being as it was the center of it all (and flat). But whatever the actual numbers and groupings were, one thing remained constant: planets were special. Worthy of being used as characters for all sorts of myths and stories. How could they not be special? They were a handful of wandering lights in an otherwise static sky! They had to be pretty important and unique indeed! Right?

Then happened the first revolution in our understanding of these things, when the geocentric models of the Universe gave way to Copernicus’ vision of a universe centered in the Sun, a heliocentric vision. Planets, once rotating around the Earth, were now circling the Sun.

And the Earth with them.

This meant that planets were not point sources of light after all, but (probably) solid, round worlds like our own, maybe even with their own inhabitants. It also meant that the Moon was not a planet, but a satellite, for it circles not the Sun, but the Earth. The Sun? Ah, not a planet either. The Sun now became the center of everything. Not a star, as yet, but so unique it had no category to belong to. It was just the Sun.

This was a complete turnaround in our understanding of what a planet is. But, despite that, the now 6 planets remained very special places indeed. Think about it: thousands and thousands of stars, and only six worlds like our own? They’re special, no question about it!

And more: there was an order to them, an order that was often used as an evidence of divinity, for only an allmighty God could create such perfectly harmonious structures. When Galileo peeked through his telescope and saw for the first time that the other planets were, indeed, discs, that seemed to confirm this notion, although shortly after two discoveries shook things a bit: the discovery of the four galilean moons of Jupiter (which were also called “planets” for a while, as were, later, the first moons of Saturn to be discovered), and a pair of strange “ears” protruding from the sides of Saturn, which even changed shape over time. It was only in mid XVII century that these ears were recognized as rings, and that the first moons of Saturn (starting with Titan, of course) became known. There was something else that also tainted these notions of divine astronomical perfection: the discovery, by Kepler, that the planets did not follow perfectly circular paths, as previously thought, but moved along ellipses.

In the next century two relevant things happened. First, some astronomers noticed that the planetary distances to the sun followed closely a mathematical relation which came to be known as Titus-Bode Law. There was a gap between Mars and Jupiter, though. And the law said nothing about ending the fun at Saturn. So everyone began looking for new planets in the gap and beyond Saturn, and Uranus was found right where the law said something should be. You can imagine by yourselves how that bolstered up its credibility and the notion that, despite some annoying facts, God really did have a finger in making an orderly and predictable universe, in which the planets had their very special parts to play.

When Ceres was found in 1801, again right where Titus-Bode predicted it, it all seemed to be proved beyond a doubt. And Ceres quietly became the 8th planet of the Solar System. But then, shortly after, 3 more planets were discovered in the same general area, and heads began to be scratched.

And then stranger things began to happen. Uranus wasn’t behaving: instead of peacefully following its path, it wobbled back and forth, as if something unseen was pulling it. So the astronomers crunched the numbers, determined the position where the perturbing object should be, pointed their telescopes to that position, and there was Neptune, yet another planet, just waiting to be discovered. This happened in 1846. Great news, right? Wrong. Neptune’s position deviated significantly from what was predicted by the old Titus-Bode Law.

Oops! Could it be that such a venerable law of nature was wrong?

To make things worse, the year before a 5th body had been found between Mars and Jupiter, and from 1847 on new discoveries around the same zone happened at a steady pace. By 1900 they were already 450. Things were a lot more chaotic than they had seemed to be. The neatly ordered plan of God was taking a beating from reality.

These were the signs of a revolution to come.

That’s when astronomers noticed two things: firstly all the chaos was restricted to the zone between Mars and Jupiter, where Titus-Bode predicted there should be a planet. Maybe it exploded, and what was being discovered were mere fragments? All the other planets seemed to behave, kinda. The divergence between Neptune’s position and Titus-Bode could perhaps be a fluke? A statistical outlyer? Astronomers also noticed that all of the well-behaved planets showed typical planetary discs. But the annoying rebels beyond Mars didn’t. Like the planets in the old days, they looked just like moving stars.

And so they were christened “asteroids”, a word that means “similar to stars”, and the number of planets was reduced to 8. And the order was preserved. And the planets continued to be special objects in the sky.

Ah! What a relief! Sometimes you need a revolution to keep things as they were.

Pluto came about in 1930 (although it had been detected much earlier), and deviated so much from Titus-Bode that effectively killed it for good. At first its size was greatly overestimated, but there was little question that it had to be called a planet, even though no disc could be seen and even though its orbit was weird. It was alone out there, very far from the area where asteroids dwell, and much bigger than asteroids were. But that weird orbit… many people found it really hard to swallow. It seemed too odd, too distant from the orderly display the other 8 showed. But, hey, 9 planets in such a large Universe are still pretty special, aren’t they? So they went with it anyway.

But then came the 1990′s. Astronomers began an amazing series of discoveries in the outer Solar System. Small and not so small icy bodies in orbits similar to Pluto’s became commonplace, a chaos of intersecting, eccentric, inclined orbits that seemed to mirror closely what happens in the Main Asteroid Belt. Those that were uncomfortable with Pluto’s oddity became increasingly more uncomfortable. And when finally an object larger than Pluto, Eris, was found, something just had to change again. It was inevitable. We just had to fundamentally rethink what makes a planet for the third time in our history.

It could be simple. Just make with Pluto the same that was made with Ceres, Pallas, Juno and Vesta in the XIX century, reduce once more the number of planets to 8, and get on with it. Keep the order. Keep the specialness of planetary status. That’s what the IAU astronomers did, and that’s the source of the current definition of planet.

But it really is everything but simple. At the same time trans-neptunian objects were being found everywhere, exoplanets were also being found by the hundreds. Around “normal”, sun-like stars, around stars smaller and larger, around red dwarfs, around pulsars, even free-floating, roaming alone the empty spaces between the stars. Other planetary systems were found that didn’t look anything like our own. Systems with planets larger than Jupiter in orbits much more eccentric than those of any Solar System dwarf planet. Systems with 2, 3 or more giant planets packed inside what in the Solar System would be the orbit of Mercury. Systems with resonant giant planets. A wide variety of outcomes of a process that is apparently universal: planetary formation.

And all of a sudden there’s no order, only different outcomes of a process that is inherently chaotic. And all of a sudden planets are no longer special: we already know where are hundreds of them, and it’s now clear that we’ll end up finding many billions in our galaxy alone. Planets are literally everywhere.

And this is why 8 planets are bad science.

By insisting on a small number of planets, the astronomers are trying to perpetuate a notion that science itself has already defeated: that planets are rare and special bodies, that they are well-behaved and orderly, that it’s still possible to find in them the music of the spheres. When none of this is true.

This time, no revolution can leave things as they were. This time, we simply cannot avoid a true, paradigm-shifting revolution.

As Mark Sykes puts it, “we are in the midst of a conceptual revolution [...], shaking off the last vestiges of the mythological view of planets as special objects in the sky – and the idea that there has to be a small number of them because they’re special.” That’s exactly it. And that’s why the most amazing part of all this is, to me, that the IAU definition was already obsolete when it was created and approved.

Which is to say, bad science.

This is also why I’m absolutely certain that it will end up being defeated. This definition will not stand. Not because thousands of “plutophiles” go do some agitprop to twitter, but because it just doesn’t fit reality. Not because people are annoyed by the “demotion” of Pluto, but due to the wide diversity of planets that exist out there. In the end, the only possible outcome of all this is a broad definition of what planets are, as broad and inclusive as planets are varied in this vast universe we live in, and a classification scheme that sets up categories within that definition. They are already emerging, even. The literature is crawling with “jupiters”, “neptunes”, “super-earths”, “hot neptunes”, “gas giants”, “ice giants”, “terrestrial planets”.

And, yes, “dwarf planets”, why not?