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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.

How about hot jupiters and super-earths? December 5, 2009

Posted by Jorge Candeias in Extrasolar planets, Giant planets, Terminology, Terrestrial planets.
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A few more pistachios in the belly, and some more ideas coming out of both the posts themselves and the comment boxes. Particularly this comment by Bob Shepard, where he proposes a very detailed classification scheme for the planets, inspired by the spectral classification of stars.

As I told him, with less than 500 known planets (primary and secondary, belt and main, solar and extrasolar) I don’t really see the need for such a detailed scheme for the moment. But I certainly admit that it may be useful in the future, when the number of known planets starts getting astronomical, pun definitely indended. And I may even be wrong right now about this lack of necessity. You see, a more detailed classification scheme is already emerging in exoplanetology. Organically, kind of .

If you browse the literature you’ll find terms such as hot and cold jupiters, cold and hot neptunes, super-earths, etc. These classes of planets are usually not very precisely defined, but that doesn’t stop them from being profusely used, which is a clear indicator that it is felt that they are needed. A “jupiter”, for instance, is defined as a planet whose mass “is close to or exceeds that of Jupiter”, and Jupiter and Saturn are usually indicated as Solar System examples of such planets. Since Saturn’s mass is less than 30% of that of Jupiter, this means that this category might range from some 0.25 MJ to the limit of brown dwarfs (or, as I prefer calling them, planetars as in “intermediate object between planets and stars”), which is about 13 MJ.

However, the “neptune” class of planets gets more definitive limits, ranging from 10 to 30 Earth masses (or ME). In our system, Neptune and Uranus are included in this class and, unless someone comes up with an intermediate class between jupiters and neptunes, this means that jupiters in fact range from 0.0945 MJ to 13 MJ. It’s quite a large interval, including the vast majority of extrasolar planets discovered so far, so it’s possible that intermediate class will indeed appear.

Both the neptunes and the jupiters would fit under my giant planets category, but the next class that has emerged organically in extrasolar studies, the “super-earth” class, would belong to the medium-sized planets. This one, however, is very poorly defined indeed. Although the upper limit is pretty solidly set at 10 ME, some astronomers set the lower limit at 5 ME, wereas for others any planet that is more massive than the Earth is a super-earth. Personally, I think these two perspectives may be a bit too extreme. An interval of 5-10 ME seems too restrictive, while starting super-earths with planets that are basically Earth twins, only slightly more massive, seems to stretch the term a bit too much. I’d call super-earth to planets of no less than 2 or 3 Earth masses, with a slight preference to a range of 3-10 ME.

And that’s it, really. No other size-based classes of planet have been widely used outside theoretical studies of planetary formation, i.e., with real exoplanets, which is, of course, explained by the fact that the first planets to be spotted are always the larger ones and also the closest to their stars. With the exception of pulsar planets, only one planet has been found below 2 ME: Gliese 581 e, a terrestrial planet of 1.94 ME, so close to its star that a year out there lasts little more than 3 days. So there’s no subgroupings below that.

But these three groups are definitely a start in the kind of thing Bob Shepard suggests, only in an ad-hoc, unplanned way. They have the advantage of being born out of necessity and therefore being immediately adapted to the real world, and the disadvantage of not being very orderly.

Hey, nothing is perfect.

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.

A magnificent photo tour November 25, 2009

Posted by Jorge Candeias in Blogroll, Planets.
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No, my friends, unfortunately I still lack the time to feed this blog properly, with the long posts I plan to write. But in the meanwhile I may as well call your attention to a couple of posts in a blog called Daily Kos, which gather together a very, very nice set of photos (mostly) of pretty much all of the Solar System planets, satellites and minor bodies that have been imaged by spacecraft thus far.

You can find the first part here, showing us the planets Mercury, Venus, Earth, Mars, Ceres, Jupiter and Saturn, the planetary moons Luna, Io, Europa, Ganymede, Callisto, Mimas, Enceladus, Tethys, Dione, Rhea and Titan, the minor moons Phobos, Deimos, Amalthea, Thebe, Pan, Daphnis, Atlas, Prometheus, Pandora, Epimetheus, Janus, Telesto, Calypso and Helene, and the minor bodies Itokawa, Eros, Ida (and Dactyl), Gaspra, Mathilde, Šteins, Annefrank, Borrelly, Wild 2, Tempel 1 and Halley.

The second part, found here, features the planets Uranus, Neptune, Pluto, Haumea, Makemake and Eris, the planetary moons Iapetus, Miranda, Ariel, Umbriel, Titania, Oberon, Triton and Charon, the minor moons Hyperion, Phoebe, Puck, Larissa and Proteus and the minor body (albeit dwarf planet candidate) Quaoar, plus a bunch of schematics and plots.

Wonderful stuff. Enjoy.

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.

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.

Odd balls August 15, 2009

Posted by Jorge Candeias in Extrasolar planets.
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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.

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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9 comments

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.