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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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10 comments

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

How about hiring a linguist? December 1, 2009

Posted by Jorge Candeias in Terminology.
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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.

Wrapping our head around proportions August 24, 2009

Posted by Jorge Candeias in Planets.
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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.

Odd balls August 15, 2009

Posted by Jorge Candeias in Extrasolar planets.
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2 comments

Here’s something you don’t see every day: last week brought news of not one, but two extrasolar planets that are far more oddballish than anything we have in our Solar System. It started with the announcement that the newly discovered WASP-17b had two very bizarre characteristics. For starters, a planet with its mass, about half that of Jupiter, should be smaller than Jupiter… but that one is much larger. In fact, at twice Jupiter’s diameter (i. e., about 140 000 km), it is the largest planet known so far.

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.

Some size comparisons August 7, 2009

Posted by Jorge Candeias in Earth, Eris, Jupiter.
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2 comments

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?