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Planetary taxonomy

February 10th, 2009 (08:40 am)

I was looking at this image:

and I was struck by the difference between Jupiter and Saturn and the planets of the inner system. It wouldn't be a stretch to say that they're so dissimilar they shouldn't be covered by the same word; that the division should be "planets" vs. "giant planets", not "planets" vs. "dwarf planets". The argument would be that Jupiter is 11 times Earth's radius; Earth is only 5 times Pluto's. (The "dominating its orbit" test makes more sense, but it doesn't have a monopoly.)

And then I thought of stories which postulate intelligent life on Jupiter, and have the Jovians refer to Earth as "the third planet from the Sun". If there were Jovians, and they did astronomy, there is no reason to presume they would categorize Earth on an equal footing with Jupiter. They might have a taxonomy which lumps together the asteroids and planets inside Jupiter's orbit as "Inner System dwarves", and those outside Neptune's orbit as "Outer System dwarves"; for some uses, they might even categorize the inner planets together with the Jupiter's own moons. Or the taxonomy might be "hot dwarves" vs. "cold dwarves".

Comments

Posted by: C. Virtue (cvirtue)
Posted at: February 10th, 2009 02:32 pm (UTC)

Wellllllll, remember that although there is some scale here, it's not precise. Ceres isn't marginally smaller than Earth, it's a lot smaller. Then there's the whole atmosphere issue.

I haven't seen (and haven't looked for) a comparison between the inner planet sizes and the asteroids, but we did find this when looking for volcano images:


Except for the moon, these are all Kuiper Belt Objects, I think.

Posted by: Who, me? (metageek)
Posted at: February 10th, 2009 02:50 pm (UTC)
Oh, yeah, it's not definite

Wellllllll, remember that although there is some scale here, it's not precise.

OK, but I didn't rely on the picture; the 11:1 and 5:1 numbers came from looking up the planets in Wikipedia.

Ceres isn't marginally smaller than Earth, it's a lot smaller.

<dig, dig> ...true. Roughly the size of Western Europe.

Then there's the whole atmosphere issue.

Compared to Jupiter, Earth's atmosphere is practically a vacuum; they might not consider it a reason to categorize Earth separately from Ceres.

It's not a perfect taxonomy; it's just one that would be defensible among Jovians, so that it'd be valid to write a story where Jovians are using it.

Posted by: C. Virtue (cvirtue)
Posted at: February 10th, 2009 06:35 pm (UTC)
Re: Oh, yeah, it's not definite

I figured you'd looked the numbers up, never fear.

"It's not a perfect taxonomy; it's just one that would be defensible among Jovians, so that it'd be valid to write a story where Jovians are using it."

Right. A jovial thought.

Posted by: Who, me? (metageek)
Posted at: February 10th, 2009 06:41 pm (UTC)
Re: Oh, yeah, it's not definite

Right. A jovial thought.

Thank you. :-)



Edited at 2009-02-10 06:42 pm (UTC)

(Deleted comment)
Posted by: Who, me? (metageek)
Posted at: February 10th, 2009 03:15 pm (UTC)
Really?

Remember, you can't get much larger than Jupiter.

Are you sure? I thought there were extrasolar planets as much as 10 times the size of Jupiter.

As far as your picture, it gets rid of a very interesting fact (to me). Saturn's orbit has a radius about twice as large as Jupiter's orbit's radius.

True. The picture seems to use a log scale (approximately?); the distance between Venus and Earth is shown as about the same as the distance between Jupiter and Saturn (center-to-center, that is).

Posted by: Who, me? (metageek)
Posted at: February 10th, 2009 08:44 pm (UTC)
Re: Really?

I thought there were extrasolar planets as much as 10 times the size of Jupiter.

Hmm. A skim of Wikipedia shows there are such planets, but at least one of them, XO-3b, is thought to be generating its own heat, and there's ongoing debate over whether it should be classified as a brown dwarf instead.

On the other hand, XO-3b isn't 10% bigger than Jupiter; it's 11 times as big. Another, HD 17156 b, is 3 times as big, and I can't find anybody saying it's fusing.

Hmm...<google, google>...OK, here's a paper mentioning that the minimum mass for deuterium fusion is 13 Jupiters; for hydrogen fusion, it's 80 Jupiters.

So, no, Jupiter is nowhere near ignition size.



Edited at 2009-02-10 08:45 pm (UTC)

Posted by: Hey You (martianfencer)
Posted at: February 10th, 2009 10:10 pm (UTC)
Re: Really?

Jupiter emits radiation too. Heck, the earth even emits some (geothermal heat, anyone?)

Determinations of planet radius are really inconsistent. Titan is no longer considered the largest moon, because without its atmosphere it's smaller than Ganymede. But what would you have left if you stripped Jupiter of its atmosphere?

I think they should either all include the atmosphere as part of the radius, or they should all ignore the atmosphere as part of the radius.

As I recall, the image you used in your post is to scale for the planet sizes.

When Amy studied planets in about 7th grade, she made models of the Earth and Moon to scale, and then we hung them at the correct scaled distance from each other. Next time you're at our house, remind me to show you, because they're still on display in the sunporch. It's pretty impressive how tiny and distant they are.

Another time, with a group of homeschoolers, we walked a scale model of the solar system and placed objects of appropriate size at each of our stations. It was pretty cool.

Here is a field trip for you: The Maine Solar System Model http://www.umpi.maine.edu/info/nmms/solar/, which a 40 mile long scale model of the solar system, with the planets at the correct scaled sizes and distances.

Posted by: Who, me? (metageek)
Posted at: February 11th, 2009 01:06 am (UTC)
Re: Really?

There's a Solar System model around Boston, too—centered on the Museum of Science. I'm not sure how wide it is; looks like its furthest points are in Newton and Saugus.

But what would you have left if you stripped Jupiter of its atmosphere?

A lot of angry astronomers.

I think they should either all include the atmosphere as part of the radius,

That's the option that makes sense to me—the mass has to include the atmosphere, after all, since we've got no good way to measure them separately.

On the other hand, the radius without the atmosphere is important for landing spaceships. Perhaps the two measurements should be reported individually. For Jupiter, the inner measurement would just be marked "unknown".



Edited at 2009-02-11 01:06 am (UTC)

Posted by: Hey You (martianfencer)
Posted at: February 11th, 2009 04:23 pm (UTC)
Re: Really?

A lot of angry astronomers

Hee hee!!

Astrologers too. Do you know there is an astrology group that has tried to sue NASA for slamming an artificial bolide into an asteroid, because it altered the orbit of the asteroid?

the radius without the atmosphere is important for landing spaceships

The radius with atmosphere is important too, which is why all the landers on Mars have been at relatively low altitude -- so they can use parachutes, which saves a lot of weight.

I participated in a JPL workshop to design a lander for Europa, which has very little atmosphere. One of the concerns was to find a rocket propellant that would not contaminate any attempts to find biosignatures on the surface, because we could not use the tenuous atmosphere to help brake the lander.

Posted by: Who, me? (metageek)
Posted at: February 11th, 2009 04:50 pm (UTC)
Re: Really?

Do you know there is an astrology group that has tried to sue NASA for slamming an artificial bolide into an asteroid, because it altered the orbit of the asteroid?

Sigh. Apart from the Fundamental Idiocy of Astrology, there's the basic question here: do they really believe the orbits of asteroids never change?

One of the concerns was to find a rocket propellant that would not contaminate any attempts to find biosignatures on the surface

Did you come up with anything? Can you brake against Europa's magnetic field?

Oh, and I forgot about the Boston solar system model. I have only seen the part that is inside the museum.

Yeah, I've seen only one part outside the museum, at a nearby mall. It looks like seeing them all would be quite a trek.



Edited at 2009-02-11 04:51 pm (UTC)

Posted by: Hey You (martianfencer)
Posted at: February 11th, 2009 08:50 pm (UTC)
Re: Really?

do they really believe the orbits of asteroids never change?


That is an interesting question. But I suppose they're upset because it was done artificially.

Believe it or not, there is a professional astrologer who is a grad student in my department, working with Lynn Margulis. He started at the same time I did, and they have worked his astrology into his thesis topic: He is studying something about the perturbations to Earth's biota caused by the orbit of Jupiter.

(He's eager to cast my horoscope because, in his words, many athletes are born under the sign of Cancer. I'm guessing about 1/12 of them, in point of fact.)

Posted by: Who, me? (metageek)
Posted at: February 11th, 2009 09:22 pm (UTC)
Re: Really?

Posted by: C. Virtue (cvirtue)
Posted at: February 11th, 2009 09:24 pm (UTC)
Re: Really?

Posted by: Who, me? (metageek)
Posted at: February 11th, 2009 09:25 pm (UTC)
Re: Really?

Posted by: C. Virtue (cvirtue)
Posted at: February 11th, 2009 09:27 pm (UTC)
Re: Really?

Posted by: Hey You (martianfencer)
Posted at: February 11th, 2009 11:07 pm (UTC)
Re: Really?

Posted by: C. Virtue (cvirtue)
Posted at: February 12th, 2009 12:34 am (UTC)
Re: Really?

Posted by: Hey You (martianfencer)
Posted at: February 12th, 2009 01:49 am (UTC)
Cranky about planets

Posted by: C. Virtue (cvirtue)
Posted at: February 12th, 2009 10:17 am (UTC)
Re: Cranky about planets

Posted by: Hey You (martianfencer)
Posted at: February 12th, 2009 08:28 pm (UTC)
Re: Cranky about planets

Posted by: C. Virtue (cvirtue)
Posted at: February 12th, 2009 08:44 pm (UTC)
Re: Cranky about planets

Posted by: Who, me? (metageek)
Posted at: February 12th, 2009 01:51 pm (UTC)
Re: Cranky about planets

Posted by: Hey You (martianfencer)
Posted at: February 12th, 2009 08:29 pm (UTC)
Re: Cranky about planets

Posted by: C. Virtue (cvirtue)
Posted at: February 12th, 2009 09:38 pm (UTC)
Re: Cranky about planets

Posted by: Who, me? (metageek)
Posted at: February 12th, 2009 09:39 pm (UTC)
Re: Cranky about planets

Posted by: Who, me? (metageek)
Posted at: February 12th, 2009 01:46 pm (UTC)
Re: Really?

Posted by: Hey You (martianfencer)
Posted at: February 11th, 2009 08:55 pm (UTC)
Re: Really?

Oops, forgot to respond to the propellant question. Yes, there was a propellant that wouldn't affect the readings of the kind of instruments we were trying to launch. Unfortunately I can't remember what it was, and its name doesn't seem to have found its way into our final presentation.

Posted by: C. Virtue (cvirtue)
Posted at: February 11th, 2009 05:08 pm (UTC)
Re: Really?
Bird Brain

because it altered the orbit of the asteroid

"No matter how thin you slice it, it's still baloney."

Posted by: Hey You (martianfencer)
Posted at: February 11th, 2009 04:24 pm (UTC)
Re: Really?

Oh, and I forgot about the Boston solar system model. I have only seen the part that is inside the museum.

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