h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

How should we define "planet"?

I suggest that it should be an object orbiting a star (or co-orbiting a star with an object no more than 100 times its mass) massive enough that its shape is determined solely by gravity and rotation.

This definition gives the solar system at least 15 planets:

1.) Mercury
2.) Venus
3a.) Earth
3b.) Luna
4.) Mars
5.) Ceres
6.) Jupiter
7.) Saturn
8.) Uranus
9.) Neptune
10.) Ixion
11.) Pluto
12.) Varuna
13.) Quaoar
14.) Sedna

Planets 1-5 are terrestrial, planets 6-9 are jovian, planets 10-14 are KBOs (I propose the name plutonian to describe them).

Your suggestion of terran, jovian and plutonian categories makes a great deal of sense. Especially if they're going to keep finding KBOs.

I am surprised to see that 'Luna' qualifies. I hadn't realised that our moon was that much significantly larger than the 'moons' of other planets.
The way you've listed it with Earth, as 3a and 3b, is quite striking and causing me to rethink the solar system of my mind. Perhaps old Luna is what makes our earth unique. Certainly its gentle rocking of the cradle of life, the oceans, has contributed to earth's many distinctive life forms.

~jwf~

Well, Ganymede, the largest moon in our solar system, has a 1:12837 mass relationship with Jupiter. Calisto and Io are also bigger than Luna, but since they orbit Jupiter, they are equally tiny by comparison.

Titan, the second largest moon in the solar system, has a 1:4207 mass ratio with Saturn.

Luna, the fifth largest moon in the solar system, has a 1:81 mass ratio with Earth. Charon, Pluto's moon, has an even smaller mass difference, and should probably be counted as a planet as well (if it is big enough).

Luna is the 5th largest of about 100 moons in the solar system, but that isn't what makes it so special. What

Sorry--I hit send too soon.

What makes Luna special is that it is so close to the Earth in size.

The bigger moons of the Jovian planets are big--six of them, plus Luna, are bigger than Pluto and Ganymede and Titan are bigger than Mercury--but they're tiny compared to to the giants they orbit.

On the other hand, Luna is about a quarter the diameter of Earth and is rediculously large for a terrestrial planet's moon. Mars' Phobos and Deimos are more too scale.

I've read that the moon is actually too far from Earth for the Earth to keep hold of it in a tug-of-war with the sun; it actually co-orbits the sun with the Earth. Supposedly, it's orbit is never convex with respect to the sun, while the orbits of all other satelights except Charon are.

Aliens who came into the solar system with no preconceptions about what it looked like would surely see Earth-Luna as a double planet system.



Also, Luna's formation be impact of a Mars-sized body (sometimes caled Theia, after a moon goddess), is partly responsible for Earth's density. Earth got Theia's core along with its own, but some of the mantles of both bodies ended up in Luna. So Earth has more heavy elements than it would otherwise. Perhaps the extra heavy elements have meant more radioactives and contributed to Earth's natural background radiation enough to have sped up evolution here. If so, who knows what we may find on Europa, bathed in Jupiter's magnetosphere.

Simialarly, Mercury's high density has been suggested to be the result of a colition that knocked off much of its mantle.

Wow.
That's a bit scary.
I mean just knowing just how fragile the balance was/is/will be.

I had never heard that formation theory before and my imagination was recalling all sorts of billiards and curling memories of collisions and tangents and I suddenly realised that if what you say is true, then we are very 'lucky' that the moon ended up where it did.

And I thought it was only good for tides, romance, werewolves and lunatics.

~jwf~

Hey I just remembered that this is the Great Debating Society forum.
I'm supposed to be arguing with you not agreeing with you, let alone admitting how impressed I am by what you're saying.

~jwf~

"I had never heard that formation theory before and my imagination was recalling all sorts of billiards and curling memories of collisions and tangents and I suddenly realised that if what you say is true, then we are very 'lucky' that the moon ended up where it did."

Of course it took two tries. From what I've read, the Theia impact probably disloged a bunch of material that hit the Earth again, ejecting the moon. The early solar system would not be a fun place to be.

You've certainly stimulated my imagination.

Knowing that there a zillion bits of rubble floating around out there is always a bit scary, but realising that the formation of the solar system was like a 'break' in billiards, with planet sized balls scattering every which way, really puts the 'odds against life on earth' in perspective. That delicate balance of the living earth and its barren moon.

I was aware of a theory that our oceans came from a huge ice comet that collided with a still semi-molten earth. A subsequent version of that theory goes on to suggest that 'life' was frozen in that ice and 'revived' when it melted to become the oceans. Is that the 'Theia' impact?

peace
jwf

You've certainly stimulated my imagination.

Knowing that there a zillion bits of rubble floating around out there is always a bit scary, but realising that the formation of the solar system was like a 'break' in billiards, with planet-sized balls scattering every which way, really puts the 'odds against life on earth' in perspective. That delicate balance of the living earth and its barren moon.

I was aware of a theory that our oceans came from a huge ice comet that collided with a still semi-molten earth. A subsequent version of that theory goes on to suggest that 'life' was frozen in that ice and 'revived' when it melted to become the oceans. Is that the 'Theia' impact?

peace
jwf

"I was aware of a theory that our oceans came from a huge ice comet that collided with a still semi-molten earth. A subsequent version of that theory goes on to suggest that 'life' was frozen in that ice and 'revived' when it melted to become the oceans. Is that the 'Theia' impact?"

I don't think so because I don't think Theia is thought to have been icey--if it was it would have had to be enourmous. I think it was supposed to be a body a lot like Mars, but sharing Earth's orbit.

Anyway, the Theia collision would produce so much heat that most of the water in any comet involved would boil off into space. THe Earth would need to get its oceans afterward, or get them back from more comets.

>> most of the water in any comet involved would boil off into space <<

My understanding is that a moon-sized ball of ice spit out the moon when it collided with a thin-crusted earth. The moon was just the right size to be caught in earth's orbit and to cool into a solid ball of rock.

The beauty of where it ended up - balanced between heaven and earth - creates a bubble where the earth and sun's gravitational fields are neutralised.

So the moon's gravitational influence was strong enough to keep enough water trapped below a heavy layer of gaseous residues, ash and dust, created by the impact. All of this would have boiled into space but the moon sent it circulating round the core magma like a three dimensional cooling soup, because of earth's rotation and mass.

The constituent chemical compounds heaved upward by the impact were eventually pulled down, thus creating the oceans, the new crust and several filtered layers of gases which we generally call our atmosphere.

Life, if it came then, was the result of lightning hitting something that was melting in the soup of the sky. Some of it would have landed on the new crust, fallen into the seas or been consumed in the magma. Ah, the more things change, the more it's the same ol', same ol'.


~jwf~

That last smiley should have been .


~jwf~

<>

Huh? You're saying that something twice the size of the moon hit Earth, it spit out half of its mas as ice, which mad eEarth's oceans while the other half made the moon?

If you're suggesting that the moon was caught without a collision, that theory has been mostly abandoned because it is very hard to set up a situationiin which it would be captured without one.

I am confused, please explain.



<>

The L1 point, but it isn't very stable. Things don't stay there for long.




<>

<>

I really don't understand what you're discribing; it doesn't sound like anything I've ever heard about. Could you give me a source I could read?, maybe then I'd understand.

>> I am confused...<<


I did not mean to confuse and I do hope it's not a worrisome or painful confusion.

I am working on a way to explain it and will post later, perhaps over the weekend, but RL keeps me from exploring such things at the moment. At least this will keep the thread near the top of my most recent and I'll try to dis-confuse you later.


~jwf~

Ok. I'll be waiting.

Oh dear.

I have been reading and re-reading post #11 and have come to the conclusion that perhaps I probably should have said nothing.

It is said that if you understand something you can explain it. In fact, the best test of understanding is an attempted explanation.

Now I see that Post 11 is proof that what one thinks one understands isn't always the case. Even though I wrote it thinking I was explaining myself clearly I realise I don't understand it now either.

So now we are both confused. Or at least you would be if it wasn't now clear to you that I am a lazy befuddled git of the species half-wit.

I don't even remember writing: "Life, if it came then, was the result of lightning hitting something that was melting in the soup of the sky."

It has something to do with 'circumference' being the 5th dimension and the density of a semi-liquid earth splashed into various comet-flung particulants 'orbiting' well beyond the current dimensions of a cooled and settled 'solid' earth. But I can't explain it in two or even three dimensions.

I'll just get my coat and go hang out with the gas giants until things cool down enough for the spark of life to take root without bursting the balloon of a liquid earth.


~jwf~

We have to consider the future in deciding what makes a planet.
Consider the future solarsystems yet to be investigated.

The word planet should begin with designating bodies which consist of the same material as the sun for said system. Any bodies of foreign material should be designated as such.

where you draw the size line im not sure.

Hi jimj,

I hope LB finds you a better conversationalist on this subject. I tend to get 'cosmic' when I start trying to imagine how the solar system was formed.

But there may be hope.
The latest issue of National Geographic has informed me of a new term - "the Mohorovicic discontinuity", which is apparently the warm slushy layer between the cold hard crust and the hot liquid center of our globe.

Run the clock backward a few billion years to when the ice comet hit the earth and I bet the earth's surface was pretty slushy with perhaps a wafer thin crust forming here and there.

A huge impact into this slush and a great deal of the early 'mohorovicic discontinuity' (aka: hot soup) would have splashed into space along with chunks of crust from the impact zone.

One big blob cooled and became the moon. Because it was 'slushy', gravity made it round. And while a lot more stuff might have flown off into space creating some of the 'near earth' asteroids that just keep missing us, I was thinking about the stuff that didn't quite reach escape velocity and stayed up in a soupy sky for aeons and aeons, the heavier bits falling back as they cooled.

It's just so hard to think about liquids and semi-liquids in three dimensions but this big splash must have caused the 'relative size' of our semi-liquid planet to grow considerably until things settled back down to reform the crust.

I guess my point was that even gas giants must have liquid centers.


Anyway, god bless Mohorovicic and all who sail in her!

~jwf~