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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.

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

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

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

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

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

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

Size comparisons, take two August 14, 2009

Posted by Jorge Candeias in Ceres, Mercury, Neptune.
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I’ve already shown you a comparison between the largest Solar System planets in each category, and then I thought, heck, for the sake of completeness let’s do the same with the smallest ones, also with the help of Celestia. So here you go:

Size comparison between Neptune and Mercury

Size comparison between Neptune and Mercury

Size comparison between Mercury and Ceres

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.

Some size comparisons August 7, 2009

Posted by Jorge Candeias in Earth, Eris, Jupiter.
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Well, I think it’s about time this blog includes a few pictures. And, since posts with pictures tend to require less words, it’s also a great way to give it content without spending in it too much time. So here are two quick renditions I made with Celestia, showing side by side the largest of the Solar System’s giant, terrestrial and dwarf planets:

Size comparison of Jupiter and the Earth

Size comparison of Jupiter and the Earth

Size comparison of the Earth and Eris

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?

Pluto? Who cares? August 29, 2006

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