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Grouping the planets December 4, 2009

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


How about hiring a linguist? December 1, 2009

Posted by Jorge Candeias in Terminology.
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1 comment so far

Warning: this post will be a wee bit ranty. Well, perhaps more than just a wee bit.

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.