jump to navigation

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.
Tags: , , , , , , , , ,
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.

Advertisements

Wrapping our head around proportions August 24, 2009

Posted by Jorge Candeias in Planets.
Tags: , , , , , , , , , , , , , ,
4 comments

After writing the previous post, I was left with this uneasy feeling of not having been entirely fair towards not only placemats, but Solar System skematics in general. The truth is, it’s impossible to draw the Solar System to scale. The distances between the various bodies are so mind-boggingly vast, that something just has to be distorted, usually planet sizes. The only way to actually have everything to scale and to convey a real sense of sizes and distances is to scatter planet models over vast areas, and travel around the Solar System model thus created. Never in a skematic to be found online, in publications or in placemats.

We can also, of course, use numbers that are closer to our day-to-day experience. Inches, feet and miles for the americans; centimeters, meters and kilometers for the rest of the world. Shrink everything to fit into something a bit more palpable than thousands of kilometers and astronomical units. We all know what a meter is, more or less; we can stand up, put a hand somewhere along our torso and say “it’s about this high”, and we shouldn’t be wrong by much. So, if we divide all the true Solar System numbers by the same constant, we can provide a much more palpable notion of the real proportions out there. For instance…

Say the Sun’s diameter is not more than a million km (1 392 000 km, to be exact), but 100 meters. That’s still a pretty big ball: higher than the first level of the Eiffel Tower, in Paris, and wider than the tower, too. Still, if the Sun is that big, the first of the planets is another ball… with a diameter of 35 centimeters. That’s not even twice the size of a football ball (americans: I’m referring to soccer here). And to find that 35-centimeter ball called Mercury, you’d have to walk more than 8 kilometers!

Next is Venus. To find it, you’d have to travel another 7 km, and when you finally do, you’d see a largeish 87 cm wide ball. You are now 15.5 km from your starting point already and your trek is just beginning. Next, the Earth, another largeish 92 cm-wide ball, is found 6 km further along the road, 21.5 km from your starting point. See a pattern here? Centimeter-wide balls separated by kilometers? Yeah, that’s how things will be till the end. Only more so.

Next: Mars. Mars is, of course, smaller, only 49 centimeters in diameter, and to reach it from the Earth you have to travel 11 km more, away from your 100 m Sun ball. You are now 32 km from it, and unless you have been climbing a mountain of some sort, you probably won’t be able to see it anymore. And you’re still in the inner Solar System.

The next planet, Ceres, is also the smallest. At only 7 centimeters in diameter, you can pick it up with ease, but you’ll probably have a real hard time finding it, after travelling almost 27 km from your last stop. The Sun, almost 60 km away, is nowhere to be spotted already.

Now you have a long travel to make: 52 km. That’s about half an hour if you have a car and a highway handy, but a neverending hike if you try to go on foot. At the end, you’ll find the second largest ball of all, a 10 meter wide cliff of a thing, which dwarfs you for the first time since you left the sun behind. That was, remember, almost 112 km ago.

Hop on the car, go back to the highway: you’ll be driving for almost an hour to cover the 93 km that separates you from your next destination: a more than 8 meter wide ball called Saturn. 8 meters would seem a lot, if you weren’t 205 km from your starting point already. That far from the Sun, it strikes you as a positively lonely chunk of planetary real estate. But hey, it’s a beautiful one, with all those rings and stuff, and with many other centimeter-wide balls hundreds of meters distant, in all directions. So it’s fine, kinda. But you have to keep going, so you return to the car, stop at the next gas station and fill your tank, because your next travel is long.

208 km long to be exact. There are capitals in Europe separated by less than that. And yet, it’s simply the distance between Saturn and Uranus in our model. The Sun is 413 km away. And when this long voyage finally ends, what you find is a blue ball with a diameter of three meters and 64 centimeters. You’re tired. But you’re stubborn and you want to reach the end of this, so you go find Neptune. To do it, you’ll have to travel 233 km more, and when you finally reach your destination, you find another blue, 3-meter wide ball. For a moment you may think you went in a circle and returned to Uranus, but when you measure the ball you discover that it’s 10 cm smaller than the previous, so you’re really where you should be. Phew! But where is that? That’s 646 km away from your starting point. In Europe, you’d probably be in another country already. In the Americas, in another state or province.

Now, you know that whoever made the model you’re travelling through didn’t bother with orbits and actual positions in space, only with the average distance to the Sun. Had he taken orbits into the model, you’d be now in big trouble, because the next planet, Pluto, actually gets closer to Uranus than to Neptune due to its orbital resonance with the latter planet. You’d have to make a really long travel to find it. But since the model creator didn’t bother with that, you can go on in a somewhat straight line, and after travelling another 202 km, you’ll find a 17-cm wide ball waiting for you with a slightly smaller one right next to it. You try to get your bearings from the Sun, but it’s no use. It’s now almost 850 km away.

Next stop: Haumea. To reach it, you have to travel another 78 kilometers, and once you do you find a weird ellipsoid some 8-10 cm in diameter. You’ve travelled for so long and so far, that your vision has become blurry, and you begin to have a real hard time seing the planets you’re trying to find. But you push on, travel for another 57 kilometers, and find another ball around 11 centimeters in diameter: Makemake. You think this has to stop somewhere, but you know you’re still to find Eris, so you get back to the car, and start driving.

This time it’s the largest travel of all: 470 km, no less, and when you finally stop, after almost falling asleep during the long hours of driving, you’re a whooping 1455 km from your startng point. You pick up the Eris ball. 19 centimeters in diameter. A foot ball is 22. And it’s cold, oh, so, so cold. You know there’s more. Orcus, Ixion, Varuna, Sedna, Quaoar. But you’re so tired you thank the IAU for its slowness in making officially new dwarf planets. Only one more stop and that’s a wrap. You’ve heard so much about the Oort cloud that you’d like to pay a visit. But when you ckeck your map, you have a surprise: it ain’t there. In fact, you find out you’d have to leave the Earth and almost Earth’s orbit to reach it, for its outer edge is supposedly more than a million km away, almost three times the distance to the Moon. You swear profusely, and all we can hear is a succession of beeps, but you finally give up and go find a hotel. You’ll have a very long way to go back tomorrow. A very long way indeed.

And remember: the Earth is not even one meter wide at this scale.

That’s how huge the Solar System is.

Why are 8 planets bad science August 12, 2009

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

Stop it already August 7, 2009

Posted by Jorge Candeias in Plutophiles.
Tags: , ,
add a comment

Enough is more than enough.

Just saw, on twitter, someone calling Mike Brown a “twit” because he is, says the guy, “against Pluto” and “names a planet after a teevy (sic) show”. Really? A twit?

Yes, Mike Brown is a provocateur, otherwise he wouldn’t have chosen the alias “plutokiller” for his twitter account. But just how dumb and hysterical do you think calling him names makes you look like?

Can this nonsense stop? Can we please talk about planets and what distinguishes them from other objects in the vast Universe without this kind of childishness? And can we please put Pluto aside while we do that?

You too, Mike.

Pluto? Who cares? August 29, 2006

Posted by Jorge Candeias in Definition of planet, Pluto.
Tags: ,
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.