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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.

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?