Every ball is an odd ball December 16, 2009Posted by Jorge Candeias in Earth, Jupiter, Mars, Mercury, Neptune, Saturn, Uranus, Venus.
Tags: Earth, Jupiter, Mars, Mercury, Neptune, Saturn, Uranus, Venus
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.
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.
So you want to talk about double planets? No sweat. November 30, 2009Posted by Jorge Candeias in Definition of planet, double and multiple planets.
Tags: asteroids, Definition of planet, double planets, Earth, Jupiter, multiple planets, Neptune, Pluto, Saturn, Uranus
The post where I explain why 8 planets are bad science has been generating both good traffic and a rather interesting discussion in the comment boxes. Part if it is about double planets.
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.
Wrapping our head around proportions August 24, 2009Posted by Jorge Candeias in Planets.
Tags: Ceres, Earth, Eris, Haumea, IAU, Jupiter, Makemake, Mars, Mercury, Neptune, Pluto, Saturn, size comparisons, Uranus, Venus
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.
Some size comparisons August 7, 2009Posted by Jorge Candeias in Earth, Eris, Jupiter.
Tags: Celestia, Earth, Eris, HD 139357 b, Jupiter, size comparisons
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:
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?