Well, with my apologies to this Christopher Sirola, who wrote the article and should really know better, but it’s dead wrong.

Dead.

Wrong.

The thing is: every ball is an odd ball.

Mercury is an oddball because it’s way denser than anything else of similar size in the Solar System and has a day (a solar day, that is) which is twice as long as its year. Yes, you need two mercurian years to complete only one mercurian day, which means that Mercury has the simplest calendar in the Solar System.

Venus is an oddball because, as mr. Sirola states in his article, rotates backwards. The Sun rises in the west and sets in the east, a long 58 or so (Earth) days later. If you could see it, that is, because Venus has a dense atmosphere with hellish temperatures and is permanently overcast by clouds of sulfuric acid, among other migraine-inducing compounds.

The Earth is an oddball because its surface is largely covered by a several-km deep layer of liquid water. And because of that green stuff that gets everywhere, that chlorophyl or whatever its name is. And because it’s dotted with strange lights in its night side. And… hell, there are so many unique characteristics about it that the Earth is the oddball of oddballs. ‘Nuff said.

Mars is an oddball because of those gigantic pimples it shows, those enormous volcanos in Tharsis and, of course, that behemoth 27 km high known as Olympus Mons. It’s also an oddball because of another behemoth in the canyon department, known as Valles Marineris. Because of its global dust storms. And, of course, because of all that rust.

Jupiter is an oddball because it has a red hurricane that has been going round in its atmosphere for centuries. Because it’s by far the most massive object in the system after the Sun. Because it emits more radiation than it absorbs. Because of all those multicoloured cloud bands, whirling at different speeds around and blurring its oh-so-short day.

Saturn is an oddball because of its rings. One could speak of many other features (polar hexagon, anyone?) but, really, the rings are more than enough.

Uranus is an oddball because it’s laying down on its orbit, of course. And Neptune is an oddball because it’s the only one left, apart from all those planets we still know too little about to really understand how oddballish they are: Ceres, Pluto, Eris, Makemake, etc., etc., etc. And don’t get me started on the secondary ones. One is yellow with sulfuric volcanos everywhere, another is orange with a thick atmosphere and lakes of hydrocarbons, another has jets of ice in its south pole, another is white and cracked and has a subsurface ocean, another is half pitch-black, half snow-white, another… pfuah! Let me breathe here!

The truth is, in the Solar System each world is unique. One of a kind and full of surprises. They are all oddballs, each in its own way, shaped by its own unique history to become what we see today. Maybe one day, when the number of known and well-studied extrasolar planets becomes as mindboggling as the number of stars is, we’ll find close twins to all of them, but I’d bet that we’ll be finding surprises just about everywhere, subtle differences that make all the difference.

I’d bet that we’ll end up discovering that, indeed, every ball is an odd ball. Everywhere, not only in the Solar System.

As I told him, with less than 500 known planets (primary and secondary, belt and main, solar and extrasolar) I don’t really see the need for such a detailed scheme for the moment. But I certainly admit that it may be useful in the future, when the number of known planets starts getting astronomical, pun definitely indended. And I may even be wrong right now about this lack of necessity. You see, a more detailed classification scheme is already emerging in exoplanetology. Organically, kind of .

If you browse the literature you’ll find terms such as hot and cold jupiters, cold and hot neptunes, super-earths, etc. These classes of planets are usually not very precisely defined, but that doesn’t stop them from being profusely used, which is a clear indicator that it is felt that they are needed. A “jupiter”, for instance, is defined as a planet whose mass “is close to or exceeds that of Jupiter”, and Jupiter and Saturn are usually indicated as Solar System examples of such planets. Since Saturn’s mass is less than 30% of that of Jupiter, this means that this category might range from some 0.25 M_J to the limit of brown dwarfs (or, as I prefer calling them, planetars as in “intermediate object between planets and stars”), which is about 13 M_J.

However, the “neptune” class of planets gets more definitive limits, ranging from 10 to 30 Earth masses (or M_E). In our system, Neptune and Uranus are included in this class and, unless someone comes up with an intermediate class between jupiters and neptunes, this means that jupiters in fact range from 0.0945 M_J to 13 M_J. It’s quite a large interval, including the vast majority of extrasolar planets discovered so far, so it’s possible that intermediate class will indeed appear.

Both the neptunes and the jupiters would fit under my giant planets category, but the next class that has emerged organically in extrasolar studies, the “super-earth” class, would belong to the medium-sized planets. This one, however, is very poorly defined indeed. Although the upper limit is pretty solidly set at 10 M_E, some astronomers set the lower limit at 5 M_E, wereas for others any planet that is more massive than the Earth is a super-earth. Personally, I think these two perspectives may be a bit too extreme. An interval of 5-10 M_E seems too restrictive, while starting super-earths with planets that are basically Earth twins, only slightly more massive, seems to stretch the term a bit too much. I’d call super-earth to planets of no less than 2 or 3 Earth masses, with a slight preference to a range of 3-10 M_E.

And that’s it, really. No other size-based classes of planet have been widely used outside theoretical studies of planetary formation, i.e., with real exoplanets, which is, of course, explained by the fact that the first planets to be spotted are always the larger ones and also the closest to their stars. With the exception of pulsar planets, only one planet has been found below 2 M_E: Gliese 581 e, a terrestrial planet of 1.94 M_E, so close to its star that a year out there lasts little more than 3 days. So there’s no subgroupings below that.

But these three groups are definitely a start in the kind of thing Bob Shepard suggests, only in an ad-hoc, unplanned way. They have the advantage of being born out of necessity and therefore being immediately adapted to the real world, and the disadvantage of not being very orderly.

Hey, nothing is perfect.

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.

Last chance to go read something else. No?

OK, you were warned. Here goes.

Often, I get the feeling that astronomers should be kept under a tight leash when it comes to naming things. Even when they do it kinda right, given what they know at the time of the naming, they usually show an appaling lack of vision, and then we’re stuck for all eternity with all these oh-so-misleading terms.

Take “asteroid”, for instance. OK, fine, at the time of naming, they looked through their telescopes and saw only an unresolved point of light, like a star, hence “asteroid” (which means “star-like” for those who don’t know). But they did already know that those objects were circling the sun, they did know that telescopes were constantly getting better, couldn’t they have, you know, forseen that one day we would probably be able to actually see asteroidal shapes? And that once we did, they would not look anything like stars anymore?

Another instance is planetary nebulae. Someone peeked at a telescope, saw a diffuse and faint disc and decided to make an association with the planets, despite the fact that nebulae were fixed in the sky, not at all wandering around as planets are inclined to do. And then, inevitably, it was found that planetary nebulae have absolutely nothing to do with planets. Obviously.

Stars don’t come in intermediate sizes, you know?, although they actually do. In astronomerland, they are either giants or dwarfs, no middle term. And who was the genius that came up with a name such as “brown dwarf”? Brown isn’t even a spectroscopic colour, for Pete’s sake!

And now, we have the dwarf planets. Oh, where to start with the dwarf planets? Well, here, for instance: they want to persuade us that dwarf planets are not planets. Beauty! But it actually gets much better. One would think that they are dwarfs because they are small, right? Oops: wrong. They are dwarfs because they belong to donut-shaped swarms of objects called “belts”. So, since the term dwarf doesn’t have anything to do with size, despite having, some day we’ll inevitably discover a non-dwarf planet which is smaller than part of the dwarfs.

Actually, we may have already found one: PSR B1257+12 D is the fourth planet discovered around pulsar PSR B1257+12 and, despite what wikipedia says, is not a dwarf planet because it’s the only body in its orbital zone, speculations of a Kuiper belt analogue notwithstanding. Yet, at some 0.0004 Earth masses it’s much smaller than Eris, which is about 0,0028 Earth masses.

Yeah, that’s right. PSR B1257+12 D, a non-dwarf planet, isn’t even 15% as massive as Eris, a dwarf planet. It’s your cue to facepalm.

Dwarfs, however, aren’t set in stone, unlike asteroids. Yet. We’re stuck with asteroids, but to avoid the dwarf disaster there’s still time. So how about this: want to have a term designating planets (or planet-like objects, if you prefer) that reside in belts? Fine, I think it’s a good idea. But if you are naming them after where they are for the sake of the holy FSM don’t choose a name that has to do with their size. A dwarf planet should be a small planet, regardless of where it is. Oh, but there’s that terrible question about what to call a planet that resides in a belt! Gosh! Hell, I hadn’t thought of tha… oh, wait! I know! How about belt planet?

Sometimes I scare myself. Eerie.

End rant. You can come back now.

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.

You can find the first part here, showing us the planets Mercury, Venus, Earth, Mars, Ceres, Jupiter and Saturn, the planetary moons Luna, Io, Europa, Ganymede, Callisto, Mimas, Enceladus, Tethys, Dione, Rhea and Titan, the minor moons Phobos, Deimos, Amalthea, Thebe, Pan, Daphnis, Atlas, Prometheus, Pandora, Epimetheus, Janus, Telesto, Calypso and Helene, and the minor bodies Itokawa, Eros, Ida (and Dactyl), Gaspra, Mathilde, Šteins, Annefrank, Borrelly, Wild 2, Tempel 1 and Halley.

The second part, found here, features the planets Uranus, Neptune, Pluto, Haumea, Makemake and Eris, the planetary moons Iapetus, Miranda, Ariel, Umbriel, Titania, Oberon, Triton and Charon, the minor moons Hyperion, Phoebe, Puck, Larissa and Proteus and the minor body (albeit dwarf planet candidate) Quaoar, plus a bunch of schematics and plots.

Wonderful stuff. Enjoy.

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.

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.

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.

BANG! Strike one.

But what I find really juicy is another fact: WASP-17, the planet’s star, rotates in one direction, and the planet goes round it in the opposite direction.

WHAM! Whoa!

Why is this vastly interesting? Because planets form out of the same rotating clouds of matter that creates stars, which means that the direction of planetary movement around the star, at the time of its formation, has to be the same as the direction the star itself rotates. That’s what happens in the Solar System: not only all the planets go rond the sun in the same direction the sun itself rotates, but the same is true for the vast majority of planet’s moons and smaller objects… all the way down to comets. The only significant exception is Triton, the largest moon of Neptune, whose retrograde orbital motion (it’s the name this phenomenon has) led scientists to believe that at some point it was captured by Neptune from an orbit around the sun. That is: Triton is thought to have been a dwarf planet at some point. Fun fact: it’s about the same size as Eris.

OK, this may be true for almost all Solar System objects but isn’t for WASP-17b.

What this means is that something major happened to it after it was formed. Astronomers don’t seem to be giving much credit to a capture scenario, probably because the planet is so close to its star: it’s a “hot jupiter”. Instead they are speculating that at some point, probably after it was fully formed, it must have had a close encounter (a very close encounter!) with another giant planet that sent it spinning into the traffic, so to speak. The other, unknown, planet, was either sent into a very highly elliptical orbit, or ejected alltogether from the system. This last sentence is me, speculating, so don’t go thinking it’s the truth, OK?

Fascinating stuff!

And it gets better: the very next day, two teams announced the discovery of another retrograde planet, HAT-P-7b. This one wasn’t a new find; the planet was already known. The novelty here lies in the disalignment between the plane of the planet’s orbit and the rotation of its star. In this case, numbers are somewhat conflicting and unclear, so the only thing that is an absolute certainty is that HAT-P-7b does not orbit along the equator of HAT-P-7, as happens with all the major Solar System planets (i. e., all planets excluding most dwarfs): it may be highly inclined, orbiting along the poles or close to them, or, which seems more likely, it’s another retrograde, orbiting along the equator but in the opposite direction, like WASP-17b.

Double Whoa!

Planets, special, orderly, well-behaved objects? Yeah, right…

Addendum: Take a look at Exoplanetology blog, where the method used to make the discovery is very well explained. The only thing I don’t think is correct is where he talks about a violent collision: no collision with an object moving in the same direction could make a planet move in the opposite direction… but a very strong transfer of orbital momentum could.

Size comparison between Neptune and Mercury

Size comparison between Mercury and Ceres

Isn’t this cute? The proportions look very much like those of the largest planets in each group, and if you prefer some numbers here they are: Neptune is 10 times larger than Mercury, wereas Mercury is nearly 5 times larger than Ceres. If you go check the masses, you’ll find that Neptune is almost 290 times heavier that Mercury, and Mercury 375 times heavier than Ceres. Everything very similar to the proportions between the biggest planets in each class. It should be noted, though, that Neptune may be the smallest of the giants but is not the lightest; that is Uranus’ claim to fame. Or one of them, anyway.

And, again, there isn’t much of a point in this. It’s just a visual reminder that if you look at the objects without taking into consideration their positions relative to eachother, the differences between giant and terrestrial planets tend to be larger than the difference between the terrestrials and the dwarfs.

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?

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.

Size comparison of Jupiter and the Earth

Size comparison of the Earth and Eris

The Earth in the bottom image is slightly larger than Jupiter in the top image (it isn’t easy to get this just right in Celestia without doing some math, which I didn’t), but I think the comparisons are effective even so. Eris (which doesn’t look like that, by the way; since we’ve never seen its surface, Celestia uses by default a generic texture, the same for all bodies in the same situation) is closer to the size of the Earth than the Earth is to the size of Jupiter. If you need numbers, then they are approximately as follows: the diameter of Jupiter is 11 times that of Earth. The diamater of the Earth is 5 times that of Eris (and no, the rather large uncertainties in Eris data don’t change this by much; at most they may drop that number to 4). More interestingly, if you compare not sizes but masses, which are actually more relevant, you get a couple of very similar numbers: Jupiter is about 320 times more massive than the Earth; the Earth is approximately 360 times more massive than Eris.

And the point is?

There isn’t much of a point, really. This just goes to show you that when it comes to compare sizes we’re not all that gifted. The big boys in the block are really big. And if you look at them from this perspective, the dwarfs don’t seem all that insignificant anymore.

And remember: if you look beyond the Solar System you’ll find other big boys that are even bigger than the big boy from our own neighbourhood, making our planet seem even more puny and helpless. HD 139357 b, for instance, is a behemoth 9.76 times more massive than Jupiter, which is to say 3100 times more massive than the Earth. Yes, that’s three thousand Earths needed to make only one gas giant.

Good thing that it strolls around almost 400 light years away, huh?

So OK, fellas, I’m in.

Now, don’t expect this blog to be anywhere near as active as those blogs linked above. I don’t have that much to say, in truth, nor do I have the first hand access to sources major league space bloggers have. Plus I spend a lot of time writing each post in an English that wouldn’t embarrass me too much. Not my mother tongue and all that, you know? I’m likely to go through weeks without posting anything (particularly whenever the deadlines in my day job start to tower over me, which is something that does happen relatively often), although in some occasions I may put out two or three posts in a row. The word is erratic. Expect this blog to be just that.

To stay in the loop without having to come back regularly just to see that nothing has changed, you have, of course, my RSS feed. I toyed with the idea of creating a new twitter account just for this baby, but the two email addresses I check regularly were already taken, and it may not be worth the effort of creating a new one or paying closer attention to the ones I basically ignore. You can, if you will, follow my main twitter account, where I’ll post update notices under the hashtag #1000plans, but be advised that I tweet mostly in Portuguese and on other, non-space, issues, so you may find my twitter feed quite uninteresting.

But do follow @astroengine and @exoplanetology. Those two are most certainly worth it.

One last thing on commenting policy: to avoid spam and the hassle of captchas and logins, everyone’s first comment is moderated. That means that it may take a while to show up, especially if you’re commenting while I’m sleeping or otherwise away. Rest assured, though, that it will show up (unless it’s some sort of spam or particularly trollish), and once you have one comment cleared, you’re free of moderation and your comments will show up as fast as wordpress allows. Just so you know.

See you soon.

As the very few who have visited may have noticed, this blog has been pretty much abandoned for the last three years. The reason is simple, even if somewhat embarrassing: I had misplaced the password for my wordpress account and had a few email issues as well which didn’t make it easy to recover it. Silly, huh? That’s life, the universe and everything for you.

But now, hurray, I found it. Just popped up suddenly, like an old friend who decided to renew an old friendship all of a sudden. Go figure.

So. What to do? I still have to say all those things I promised three years ago, but quite a few were said already by other, more knowledgeable people, people with much better reps than I when it comes to these issues. And even so, I myself kinda spread bits and pieces of them here and there, in the comment boxes of places such as Astroengine or Universe Today. I could go fish out those comments and use them as source for a whole series of posts, but is it worth it? Weren’t those things already said? Could a blog that has been abandoned for so long find any new readers for my thoughts, readers that don’t know all about them already?

Decisions, decisions… Life is made of decisions.

Stay tuned; I’ll let you know what I decide to do. In the meanwhile, if you wish you may tell me your opinion: the comment boxes are open, as they have been all along.

Cheers.

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.

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.