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Grouping the planets December 4, 2009

Posted by Jorge Candeias in Definition of planet, Dwarf planets, Giant planets, Sedna, Terminology.
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9 comments

Thoughts are like pistachios: you put one in your mouth (or in your head… doesn’t matter) and you’re on for a long ride. So, when I ranted about the terminology astronomers come up with, that sent my head spinning in new directions. However, as often happens, I’ll have to take a step back in order to explain it all properly.

As most people who deeply dislike the definition of planet the IAU came up with, particularly those who aren’t obsessed with Pluto (yeah, I know, there should be more of us), I think that a planet, like a human, a tree or a cloud, should be defined by what it is, i.e. by its own characteristics, and not by where it is. You don’t say that a human in space or under water is no longer a member of the human race, trees are trees no matter if they belong to a forest, are planted in urban streets or grow isolated in some field somewhere, and if something is composed by countless liquid or solid particles suspended in a gaseous medium, it’s a cloud, be it on Earth, on Venus or on 47 Ursae Majoris b. By the same kind of reasoning, to define what a planet is, where it is should matter not at all.

And the single most obvious thing that sets planets apart from other substellar objects is shape. Despite all their differences, they all show the same overall shape, a shape we know is due to a fundamental physical process that rounds them up if their mass is high enough to crunch them into a relatively low-energy state. Hence my definition for planet.

This means that all planets have differentiated and at least partially layered interiors, which implies the presence of geological processes going on at some point in their history (although you may have a tough time if you try to study the geology of gas giants. Still, they are differentiated like the others).

And this is where we come back to my little rant below.

So. Let’s suppose astronomers have the sense to start calling belt planets to what they currently call dwarf planets, using a location qualificative to set a subcategory that is based on location, and saving a size qualificative for another subcategory based on size. If they do, all of the planets that are currently known as dwarf planets would be both belt planets and dwarf planets, but you can use the term “dwarf planet” with other small planets that, as far as is known, do not reside in belts. Sedna, for instance, which is almost certainly a dwarf planet although it hasn’t yet been declared as such, was in that situation for a while. Its discovery was somewhat surprising, because it was too far to be a Kuiper Belt object but too close to be a denizen of the Oort Cloud… and for a while it was alone in its area. Actually, it still is very much alone out there. Sedna is dinamically classified as a detached object, together with only a dozen or so other known objects. If you consider that the outer edge of the Kuiper Belt lies at about 55 AU from the Sun and the theoretical inner limit of the Oort Cloud (also pretty much theoretical at this point) lies at about 2,000 AU, you can get a pretty good idea of how isolated Sedna really is out there. Even if you throw in the scattered disc objects to the mix, a relatively small population of objects with very elliptical orbits that make them travel from within the Kuiper Belt to large distances, sometimes well beyond 100 AU. Eris among them.

So, as far as we know for a fact, Sedna is not a belt planet because there’s no belt out there. And none is thought to exist. Astronomers think that is a very scarcely populated area, although they also say that discoveries out there are mostly a thing of the future. And yet, Sedna is undoubtedly a dwarf planet: with a diameter estimated at more than 1000 km, it’s definitely massive enough to have been rounded by its own gravity… and with a diameter of no more than 1600 km (yeah, the uncertainties are large), it’s definitely a small planet. Therefore a dwarf planet.

And then, of course, there’s PSR B1257+12 D. Also a dwarf planet which is not a belt planet, as far as we know.

But hang on: how can we draw a border between what’s an average sized planet as our own and a dwarf planet?

Well, ideally, we’d look at other planetary characteristics and find a suitable one. For instance, the presence of an atmosphere capable of creating all sorts of processes that transform the planet’s surface and of protecting it against at least some of the impactors. In other words, yet another layer of geology that sets living planets such as the Earth, Mars or Titan apart from pretty dead worlds like the Moon, Mercury or Mimas.

This, however, won’t work, because there’s a whole range of gases that remain gaseous at the various temperatures the distance from the Sun creates and that don’t get blown away to space, especially at large distances. As a consequence, Pluto has an atmosphere, at least during part of its orbit (temporary atmospheres are another reason why this is not a good criterion for the same reason the barycenter criterion is bad to define double planets. See below), Triton, also smallish, too, and Mercury, much larger, does not. And all the other criteria that were thrown back and forth during the early times of the planet redefinition debate (presence of satellites, presence of volcanism, etc.) are so flawed that I’m afraid we’d only have one alternative: go arbitrary on this. As I wrote several times, I really hate arbitrary groupings, but I have to admit that sometimes we just don’t have any good choice. This is one of them.

So the problem becomes finding a number that suits us well. Let’s see… I’m sure most people would want to keep Mercury as a medium planet, for all sorts of reasons, which gives us a maximum diameter for the limit of 4879 km. Most people would also want all the belt planets to fall in the dwarf planet category, which means that the limit has to be superior to the diameter of Eris: 2600 km. It would be nice to be a neat, round number, which leaves us with 4000 or 3000 km. Just pick one.

Personally, I prefer 4000. If you do it my way and add satellites to the mix as secondary planets (in italics), you end up with these three size-based subcategories of planet in the Solar system:

  1. Giant planets: Jupiter, Saturn, Uranus, Neptune. 4 in total.
  2. Medium planets: Earth, Venus, Mars, Ganymede, Titan, Mercury, Callisto. 7 in total, 3 of which secondary.
  3. Dwarf planets: Io, Moon, Europa, Triton, Eris, Pluto, Titania, Rhea, Oberon, Makemake, Iapetus, Charon, Umbriel, Ariel, Haumea, Dione, Tethys, Ceres, Enceladus, Miranda, Mimas plus a large number of other objects that are still in the lists of dwarf planet candidates. 21 for the time being, 16 of which secondary, a few dozens more already discovered (Sedna, Quaoar, etc.) and maybe many hundreds to be discovered.

(If you prefer setting the limit at 3000 km, Io, Moon and Europa go up to the medium planet zone, increading their numbers to 10; Dwarfs remain in the hundreds.)

And, according to location (in italics the belt planets except Charon, the only secondary, in bold the main planets):

  1. Inner planets: Mercury, Venus, Earth, Moon, Mars. 5 in total, one secondary.
  2. Asteroid belt planet: Ceres. 1 in total.
  3. Outer planets: Jupiter, Io, Europa, Ganymede, Callisto, Saturn, Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus, Uranus, Miranda, Ariel, Umbriel, Titania, Oberon, Neptune, Triton. 21 in total, 17 of which secondary.
  4. Kuiper belt planets: Pluto, Charon, Haumea, Makemake. 4 for the time being, 1 secondary, more already discovered and waiting for classification, probably more yet to discover.
  5. Scattered disc planet: Eris. 1 for the time being, a couple more already discovered, pretty certainly more to discover.
  6. Detached planets: none as yet, but at least Sedna will most certainly make the list, sooner or later. And more discoveries are likely.

Workable? I think so. And much better than what we have today because not only this planet subdivision keeps the actual structure of the Solar System visible (small number of large objects, increasingly larger numbers of increasingly smaller objects; each zone has its own planets in the list), instead of simplifying it to the extreme as the 8-planet approach does, but it can also be neatly used with extrasolar planets, demanding very little information to start with. Which is good.

So you want to talk about double planets? No sweat. November 30, 2009

Posted by Jorge Candeias in Definition of planet, double and multiple planets.
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3 comments

The post where I explain why 8 planets are bad science has been generating both good traffic and a rather interesting discussion in the comment boxes. Part if it is about double planets.

If you check the page, on this blog, where I present the current (and highly flawed) definition of planet and my alternative, you’ll find two things. One is that my alternative is quite simple and quite radical. Those long posts I keep mentioning but never get the time to write are mostly meant to explain all the reasoning behind that simplicity and radicality, along with why I think so poorly of the IAU’s definition. But I have been lacking the time to dive in those waters, and the best you may find for now are some hints spread here and there. One of the places where hints are to be found is the thread of comments in that post.

But maybe it’s time to actually write something a bit more solid than mere comments. And, since any place is good to start, why not taking the lead from the visitors to this blog and write about double planets?

The concept of double planet is very similar in its essence to that of a double star: two objects that share, more or less, the same characteristics, and that are gravitationally bound to eachother. However, whereas a double (or its extension: a multiple) planet has no definition anywhere, there is no question about what a double (or multiple) star is. A star is multiple if there is more than one star revolving about the same center of mass, the system’s baricenter. Note that nowhere is there any reference to where that barycenter lies. A small-mass star may be so close to a heavy star that the system’s barycenter lies inside the heavy one, and the system is still a double star. Undoubtedly.

The problem with planets arises because the only objects that are considered planets are those that revolve around stars (according to the IAU, it’s even worse: only the Sun can have planets, which is the most ridiculous aspect in that definition, but let’s forget about that particular nonsense for now). The fact that every planet that is part of a multiple-body system (i.e., the planet and its satellites) also revolves around that system’s center of mass murks the waters. True, in most situations the planet is much larger than its satellites, and the system’s center of mass lies deeply within it. But what if some day we’ll find two bodies of very similar sizes revolving around a center of mass that lies outside the planet? Which one is the planet then? Both? None?

And what to you mean “what if”? We already know one such system: Pluto-Charon. Even the Earth-Moon system may one day be in that scenario, for the Moon is constantly drifting away from our planet, which means that the system’s center of mass gets closer and closer to the Earth’s surface. But so far, it’s only Pluto-Charon. Pluto has traditionally been considered the planet and Charon the moon, but Pluto’s traditional standings have been getting a serious beating recently, and that one is no exception. In the first draft of the IAU definition of planet, swiftly defeated, Charon was to be “promoted” to the condition of planet and, together with Pluto, would form a double planet. The criterion was the position of the system’s barycenter.

That criterion is, however, just plain awful. Since the position of a system’s barycenter depends on the mass of the system’s components and on the distance between them, such a criterion could result in absolutely ridiculous situations. Imagine we find some fine day a system where the satellite’s mass is close to the planet’s and it’s on a highly eccentric orbit, meaning that the distance between the two objects varies a lot during an orbit. With the right masses and distances, when the two bodies get closer, the barycenter dips within the heaviest of the two bodies, and when they drift apart, the barycenter jumps from within the heaviest, hovers for a while above its surface only to dip again in the next orbit. Or, in other words, using that criterion, for part of each orbit the system would be composed of one planet and one satellite, and for the rest of each orbit it would be a double planet, obviously composed of two planets.

Sheer nonsense, don’t you agree? You do. I’m sure you do.

There are ways to solve this problem, of course. One is to say that there is no such thing as double planets: the heaviest of the set is a planet; the others are satellites and that’s it. Another one came up in the discussion of that post of mine: just establish an arbitrary limit of mass ratio between the two, above which the system would be considered a double planet, and below which it would just be a planet-satellite system. Since I’m very strongly opposed to establishing arbitrary limits (which is one of the reasons why I really hate the current IAU definition, but that’s for subsequent posts), I dislike the second option almost as much as I dislike the barycenter criterion. The first one is not arbitrary, so it’s fine with me.

Except that I have a better idea.

Let’s cover every part of the sizes’ scale. We’ve talked about stars and saw no problem there, we’ve talked about planets and saw a complete mess, let’s now see what happens in the lowest area, the asteroid, or small body, zone. Asteroids have also been found in associations of two or more gravitationally bound sets. The first asteroid found to be a binary was Ida, when Galileo (the probe, not the astronomer) photographed its moonlet Dactyl, in 1993, but in the last 16 years we’ve found almost 200 more such systems. Including systems with more than two components, the first of which was Sylvia, which has two (much) smaller companions: Remus and Romulus. What’s the terminology there?

Unsurprisingly for such a new set of concepts, it’s also a mess. People talk about asteroids and their moons, or moonlets, like they talk about planets and their satellites. However they also talk about binary asteroids and triple asteroids, without taking mass into account. The Ida-Dactyl system is a binary asteroid, despite the large difference in sizes between the two bodies. Hermes, number 69230 in the asteroid list, and composed of two components of almost the same size, is also a binary. That’s because, if taken independently, they both would surely be considered asteroids, so there’s no ambiguity. An asteroid moon is also an asteroid.

And that’s my great idea. If you look at my definition of planet, you’ll see that it only mentions roundness caused by self-gravity, not the position each body occupies in the great merry-go-round in the sky. This means that, yes, the Pluto sistem is a double planet, with two planets and two smaller bodies. An ice dwarf / ice dwarf kind of double planet. The Earth system is also a double planet, this time a terrestrial / terrestrial dwarf kind of double planet. Mars, on the contrary, is a single planet, despite being accompanied by two small bodies. Jupiter isn’t single and isn’t double: it’s a multiple planet, with 5 planets belonging to different categories (gas giant, terrestrial dwarf, maybe also ice dwarf) and a lot of smaller bodies. Saturn and Uranus are the “multiplest” of the planets, the first composed of 8 planets and a lot (really, a lot) of smaller bodies, the second comprising 6 planets plus debris. And Neptune is, again, a double planet. A gas giant / ice dwarf kind of double planet. Or perhaps an ice giant / ice dwarf. Plus small worlds, of course.

This way you get coherence along the whole scale of celestial objects. And solve easily and without ambiguity the whole double planet controversy. That’s on the plus side. On the minus side, it would make us change radically the way we look at these things. But maybe that’s not really a minus; you see, there are other reasons to do it.

But that would be for other posts.

On placemats and other mostly cultural stuff August 24, 2009

Posted by Jorge Candeias in Definition of planet.
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4 comments

Today, Mike Brown (I keep talking about this guy, for some reason) decided to post about placemats. I agree, they’re evil, and promote a very erroneous picture of the planetary fauna that exists out there and of the distances between the large and small chunks of rock, ice, gas and a few liquids that circle the sun. They are far from being unique in that, though. More often than not, even scientific illustrations fall prey to the same kind of reality-bending depictions most solar system skematics show. That’s actually part of the reason why I posted those two size comparisons below, and why I may follow with some more. Thanks to all those non-accurate renditions of planets’ sizes and distances, people are very often left with distorted notions about space. They think they know stuff, but they really don’t.

So he’s right. Mostly. Where he really gets it wrong is in thinking his version of placemat could really make a difference in the public perception of the solar system. People want simplicity: that’s why so many of the people complaining about the notion that evertything in hydrostatic equilibrium should be called a planet did it while brandishing the probable number of planets that would ensue. 200 planets? What an absurd, they said. Eight is much better: kids can learn their names by heart, they said. And, of course, have placemats with eight very incorrectly rendered planets instead of nine, much less 200.

That’s one of the cultural consequences of reducing the number of planets to 8. Instead of learning about the real solar system out there, about the various classes of planets that circle the sun, about how they interact with eachother, people will satisfy themselves with parroting some kind of mnemonic and end up knowing less about the solar system than they did when they thought the planets were 9, and much, much less than they could know with the term planet defined as a vast umbrella where every gravitational ball has its place.

And there’s another, recurrent, error 8-planet advocates fall into: to speak of these things as if the Solar System was the only planetary system of the universe. It most definitely is not.

Here’s a great reason to make dwarfs planets too August 20, 2009

Posted by Jorge Candeias in Definition of planet, Dwarf planets.
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You know, Mike “Plutokiller” Brown estimates that there may be about 200 objects larger than 400 km in diameter in the Kuiper Belt, and guesstimates the number of similar objects beyond the Kuiper Belt to be around two thousand. He thinks all of these should be in hydrostatic equilibrium, and therefore should be considered dwarf planets. I’m not convinced (the smallest body actually known to be in hydrostatic equilibrium is Saturn’s moon Mimas, which is indeed about 400 km in diameter, but I think satellites will probably be found to have lower limits because tidal stresses should help gravity in the process of rounding them up; in the absense of these stresses, they won’t round up that easily), but I ain’t complaining. And I actually think that this should be a great reason to make all of them planets too. Or at least all of those that actually are in hydrostatic equilibrium.

You see, I’m sick and tired of astrological BS. And you just try to imagine the chaos astrologers would find themselves into if they had to deal with more than two thousand planets in order to make their so-called “predictions”. Ha! Wouldn’t that be a blast?

It would be worth it, just to make these guys’ lives considerably harder, methinks.

Disclaimer for the humour-impaired: this is a tongue-in-cheek post, not a scientific one.

Disclaimer PS: The part about Mimas is serious, though.

Why are 8 planets bad science August 12, 2009

Posted by Jorge Candeias in Definition of planet, Plutophiles.
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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?

Pluto? Who cares? August 29, 2006

Posted by Jorge Candeias in Definition of planet, Pluto.
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9 comments

One of the things that has surprised me the most in all this debate on what is a planet is the obsession that so many people seem to have with Pluto. I expected it from people who didn’t know about the Solar System much more than the names of the “nine planets”, but the passion so many of the scientists involved, even those that qualified the whole debate as silly, seemed to have about the status of Pluto frankly amazed me. People seemed to decide first if they thought that Pluto was a planet or not and only then chose a definition for planet that placed Pluto where they thought it should be.

In reality, Pluto shouldn’t matter at all. The debate should be centered on what should be the criteria for an object to be qualified as planet regardless of what would happen to Pluto or any other planet in the Solar System or elsewhere. The questions that must be answered are not “is Pluto a planet?”, but “what is a planet?” and “is there any good difference between what’s a planet and what isn’t?” and “of all the things that could be used to set apart planets from non-planets which are the best ones?” It should be only after finding a good answer to these questions that the one about the status of Pluto (or any other planetary object, really) must be answered.

In science, prejudice should not have a place. Whenever it does find its way into scientific theories the result goes from simply wrong to disastrous. We’ve seen it happen over and over again, particularly in human studies, in theories about racial superiority, or about the intrinsic intellectual inferiority of women, or about sexual minorities. But we’ve also seen its nasty work in astronomy, and I’m not talking about those astronomers that were imprisoned or killed by other people, for defending “blasphemous” cosmological theories, for instance, such as Galileo or Copernicus: I’m talking about the astronomers that spent their entire life, or a good portion of it, trying to fit data to their particular pre-conceived ideas on how the universe should work. The great ones, such as Kepler, who spent long years trying to fit planetary movements in circular orbits due to a religious notion that the work of god should result in the perfection of the circle, managed to rise above their prejudice and abandon it at some point. The lesser ones persisted… and were forgotten.

I would like to see Pluto being put aside for a while. I would like to see people discussing the characteristics of the planets regardless of the characteristics of Pluto or its orbit. That would be good science. To decide first if Pluto is a planet or not and only then trying to find a formulation that fits is not.

A definition of planet must be universal August 28, 2006

Posted by Jorge Candeias in Definition of planet.
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After writing a couple of pages where I explain the purpose of this blog and the definitions of planet that will be used in it (the one the IAU proposes and the one I’m using), linked from the header (go check them out) here’s the first post proper. It’s more an ideological post than a scientific one in the sense that I think that a true definition for what a planet is has to be universal. There’s not really too much science in the reasons for my thinking so and that’s why I’m saying it’s ideological. You could think otherwise and your thoughts would be just as meritable. Still, I hope to persuade you I’m right.

So, why do I think that a definition for planet must be universal? And what does it mean “universal”?

Universal means that it must be adaptable to and usable in any place in the Universe. That’s a common thing in science: when things are defined, they are usually defined for the whole wide wilderness out there. A prime number isn’t one thing here and something else an Alpha Centauri; An orbit as an ellipse here and in M31, the Andromeda Galaxy; A fatty acid is composed of the same atoms in your body and in the Orion Cloud Complex; A star is a star if it shines above your head in a sunny day or in some globular cluster in the galactic halo or in one of the Magellanic Clouds; The quarks in the nail of your left thumb are just like the ones produced or released in the Big Bang.

The only situation where we admit the possibility that something we know here may not be defined in such a way that it can be applicable elsewhere is when we don’t know any other example of it out there. Such is the case of life. We’ve only met life in our planet, and therefore our hands and feet are tied: we must define it within the limits, that may be quite narrow, of what we know on Earth. But once our understanding expands, assuming it ever will, we’ll probably gain a new insight of what life is and will have to adapt our definitions accordingly.

Now, we had a similar problem with the planets until some 15 years ago: we assumed that there must be tons of them out there, it was even a given in science fiction, but we really didn’t know because we hadn’t detected a single one. If this issue had arisen back then, we would have to look only to what we have here in our cosmic backyard in search for information and some orientation.

That, however, has now changed. The same technologies that allowed us to detect the planets in the outer system that led to the need for a redefinition of what a planet is gave us also the information that other planetary systems exist around other stars as well (well, some of the same technologies, at least). And that’s why I believe that any definition of planet has now to take into account not only the populations of planetary objects that orbit our sun, but also all the planets found around other stars… and other objects.

Here lies the first, but huge, flaw in the “IAU planets”: the definition adopted in Prague is limited to the Solar System and to the Solar System alone and cannot be applied to any other system, not only because the word “Sun” is explicitly stated in the definition, but also due to its very nature. That, alone, is more than enough to reject that definition as far as I’m concerned.

Convinced? Not yet? Then wait until I add more stuff to the blog. I strongly recommend the RSS feed, in case you’re interested. See ya.