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Every ball is an odd ball December 16, 2009

Posted by Jorge Candeias in Earth, Jupiter, Mars, Mercury, Neptune, Saturn, Uranus, Venus.
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No, this isn’t going to be a more or less pythonesque version of that famous song that goes “every sperm is sacred”. It’s a reaction to this article, that claims that Venus and Uranus are “the Solar System’s oddballs”. Had it been written a handful of years ago, it would have been Pluto to get the honor. Now, it’s Venus and Uranus.

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.

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.

Wrapping our head around proportions August 24, 2009

Posted by Jorge Candeias in Planets.
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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.

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.