Planetary Distribution

[Pariah]

Although I've had to postpone my Outworlds campaign indefinitely due to a new job, I'm still ruminating over things to include in it...one day. I still have details and such that I'm trying to figure out.

 

Here's one such detail: In our solar system we have nine eight planets. The four closest to the Sun are all made of rock. The four outer planets are all gas giants. And then there's former planet Pluto, most likely a captured object.

 

My question: Is this the normal arrangement? Do we know enough about extrasolar planets to know whether other solar systems share this structure (terrestrial planets in close, Jovian planets farther out)? Do the mechanics of stellar- and solar system formation favor this arrangement? Or can there be gas giants intermixed with the smaller rocky planets? (Okay, so that's four questions, but you get the idea.)

 

Any help would be appreciated. Thanks!

[Clonus]

Re: Planetary Distribution

 

Although I've had to postpone my Outworlds campaign indefinitely due to a new job, I'm still ruminating over things to include in it...one day. I still have details and such that I'm trying to figure out.

 

Here's one such detail: In our solar system we have nine eight planets. The four closest to the Sun are all made of rock. The four outer planets are all gas giants. And then there's former planet Pluto, most likely a captured object.

 

My question: Is this the normal arrangement? Do we know enough about extrasolar planets to know whether other solar systems share this structure (terrestrial planets in close, Jovian planets farther out)? Do the mechanics of stellar- and solar system formation favor this arrangement? Or can there be gas giants intermixed with the smaller rocky planets? (Okay, so that's four questions, but you get the idea.)

 

Any help would be appreciated. Thanks!

 

We know for a fact that gas giants are 900 pound gorillas. They sit anywhere they want, although the ones that orbit more closely than Mercury to a hot sun will be very large indeed. It is probably more likely though to see rocky planets inward and gas giants farther out. The epistellar gas giants we have detected, have been because they are easy to detect, not because they are all that common.

 

For a game resource that comes closest to a plausible up-to-date world generation sequence, you can't do better than GURPS Space. The book is dominated by star system generation rules that are game-system independent (although the back quarter of the book is an alien generation system which would require some free adaptation to be usable with any other game system.)

[L. Marcus]

Re: Planetary Distribution

 

I second GURPS Space for the fourth edition. I love it.

 

Love it to bits!

[Nyrath]

Re: Planetary Distribution

 

My question: Is this the normal arrangement? Do we know enough about extrasolar planets to know whether other solar systems share this structure (terrestrial planets in close' date=' Jovian planets farther out)? Do the mechanics of stellar- and solar system formation favor this arrangement? Or can there be gas giants intermixed with the smaller rocky planets? (Okay, so that's four questions, but you get the idea.)[/quote']

The original conventional wisdom among astronomers was that gas giants only form in the outer solar system due to the heat and radiation from the central sun driving off the hydrogen which is a gas giant's main component.

 

Then they started detecting lots of extrasolar gas giants that were exceedingly close to their primary star. Now the question is open.

 

I saw one theory that while gas giants may only form in the outer solar system, the chaotic process of solar system creation might nudge a gas giant into a closer orbit.

 

Bottom line is: nobody knows.

[Drhoz]

Re: Planetary Distribution

 

altho it seems likely that a gas giant migrating inward would completely screw up the orbits of any inner rocky worlds it passed on the way, if not eject them out of the system entirely or into the sun.

 

plus quite a few of the exo-planets have staggeringly elliptical orbits

[Cancer]

Re: Planetary Distribution

 

Catalog of exoplanets

 

Several warnings apply to our knowledge of other planetary systems.

1. There are strong selection effects in our surveys so far. No way around this.
   
2. Dynamic evolution is IMPORTANT in planetary systems. It seems clear that all the ones we see today (including our own) have evolved substantially since their formation.
   
3. This goes as part of point (1), but we cannot now detect Earth-mass planets except in rare, peculiar circumstances.
   
4. Evolution of the star, not just the planetary system, is important for life. This effect is diminished for lower-mass (fainter) stars, but it's important even in our own system.
   

 

Now, it's generally agreed: the presence of a "hot Jupiter" in a system indicates that any Terrestrial planets that used to be in that system are gone. Whether ingested (by the jupiter or the star) or ejected, they aren't there any more. The process of drawing the jovian planet to small orbital radii is completely destructive to little planets between where the jovian started and where it ends up.

 

The conventional picture of planet formation has little rocky guys forming inside the "frost line" (the point where it's cold enough for water to condense out as ice crystals), and Jovian planets for some distance beyond that. Planets only accrete out of solids, until they hit some threshold mass (believed to be about 5 earth masses) at which point they have enough gravity to capture and retain hydrogen & helium. Inside the frost line all you have in the way of solids are silicates and metals; those make for a much smaller proportion of the available matter, so you only get small rocky planets in close. Further out, the ices (water, methane, ammonia) condense out and that boosts the available mass of solids by almost a factor of 10, so the Jovians grow big and fast and get above that gas-capture threshold.

 

What makes for the cutoff in the Jovians is a combination of two things: the mass density in the protosystem disk (from which everything forms) drops off with increasing distance from the forming star ... so you are running low on mass ... and the dynamical processes that lead to accretion of big planets slow down out there (the orbital velocities get smaller proportional to the square root of distance from the central star) so things don't "get organized" and accrete in the few by 10^7 years that you have before the early strong stellar wind starts up and clears the dust out of the system, which cuts off the supply of material from which planets are made.

 

Beyond the poorly-defined outer edge of big planet formation you do make lots of "little" (up to sizes comparable to Earth's Moon, which is pretty large, actually) icy bodies which never accrete into a single large planet because their accretion time is too slow. These are the Kuiper Belt objects (KBOs).

 

Once the Jovians are formed, their tidal influence strongly alters the KBO populations, accreting some, ejecting some, scattering the rest; it is clear that the KBOs that still exist in the Solar System today is a tiny and much-modified remnant of a much larger original population.

 

That's the conventional picture. Lots of squishyness to this picture (and questions about why no other system we've seen looks like it ), but I gotta run off to lecture.

[L. Marcus]

Re: Planetary Distribution

 

Cancer, have you seen any figures on the relation between the metallicity of a star and the likelihood of it developing planets?

[Cancer]

Re: Planetary Distribution

 

Yes, there does seem to be a correlation in the sense that planets are preferentially found orbiting stars that are more metal-rich than the general population. Exceptions are known (that is, there are planets orbiting stars that are about as metal-poor as you find in the Disk), so it's not an absolute. There's a decent little figure here; look at page 6, and note that there's lots of stars to the right of the [Fe/H] = 0 line; that's well in excess of the proportions normally seen if you just take a sample of stars randomly and figure out their metal content.

 

{Apologies: "metallicity" means content of of all elements heavier than helium, which is dominated numerically by O, C, Ne, and N; those are hard to measure. Iron is easier, so it is most often cited as a number for metallicity. For the meaning of the "[Fe/H]", see http://en.wikipedia.org/wiki/Metallicity#Calculation ... it's kinda arcane. Positive values have more iron atoms per hydrogen atom than the Sun does; negative values have fewer.}

[L. Marcus]

Re: Planetary Distribution

 

As I've understood it, our Sun is a bit on the rich side in 'metals' than stars of similar age and luminosity. Is that right?

[Cancer]

Re: Planetary Distribution

 

It's higher than average for its age, but not grossly so. The distribution is fairly broad.

[L. Marcus]

Re: Planetary Distribution

 

Gotcha, thanks.

[Cancer]

Re: Planetary Distribution

 

The May 2009 issue of Physics Today has an article on detecting exoplanets which is not behind the subscriber wall (it says "free"). (The HTML version of the article is here.)

 

Not much there about the planets themselves, though it's a pretty good discussion of the detection strategies and in-progress/soon-coming search programs.

[austenandrews]

Re: Planetary Distribution

 

Catalog of exoplanets

 

Cool link, thanks. I can't wait to see that catalog in ten or twenty years.

[Nyrath]

Re: Planetary Distribution

 

Cool link' date=' thanks. I can't wait to see that catalog in ten or twenty years.[/quote']

Hopefully by then the list will be called "Space Probe Target Systems"

[Badger]

Re: Planetary Distribution

 

Although I've had to postpone my Outworlds campaign indefinitely due to a new job, I'm still ruminating over things to include in it...one day. I still have details and such that I'm trying to figure out.

 

Here's one such detail: In our solar system we have nine eight planets. The four closest to the Sun are all made of rock. The four outer planets are all gas giants. And then there's former planet Pluto, most likely a captured object.

 

My question: Is this the normal arrangement? Do we know enough about extrasolar planets to know whether other solar systems share this structure (terrestrial planets in close, Jovian planets farther out)? Do the mechanics of stellar- and solar system formation favor this arrangement? Or can there be gas giants intermixed with the smaller rocky planets? (Okay, so that's four questions, but you get the idea.)

 

Any help would be appreciated. Thanks!

 

Pluto is still a planet! i dont care what they say! Long live Pluto!

 

*gets taken away in straitjacket*

 

Interesting questions, though. Have to think on it.

[Pariah]

Re: Planetary Distribution

 

Thank you all so much for your help. Like I said, I won't be using any of this info in a campaign any time soon, but it is nice--and frankly, pretty darned cool--to read as you share your expertise.

 

Hats off to all of you!

[Cancer]

Re: Planetary Distribution

 

This field isn't changing quite as fast as it once was, but new important stuff does come in at a good clip. I imagine that necromancy on this thread in a year's time would you good new stuff.

[L. Marcus]

Re: Planetary Distribution

 

. . . If I order a planet, how long until FedEx delivers?

[Cancer]

Re: Planetary Distribution

 

Dunno, I never tried. Out of my price range.

[Pariah]

Re: Planetary Distribution

 

Besides, Magrathea was still shut down, last I heard. Maybe when the economy gets better....

[BlackSword]

Re: Planetary Distribution

 

. . . If I order a planet' date=' how long until FedEx delivers?[/quote']

It depends, custom orders have a long assembly time.