Re: [FT] Jovian formation

On Sun, 3 Oct 1999, Laserlight wrote:

> So, you astronomer blokes, once again we're going to call on
> your erudition:
> a) place orbits randomly?
> b) place major gas giant at Jovian insolation distance?
> c) other?

again, i'm not an astronomer, but here's my 0.02 E's worth
(incidentally,
anyone got any suggestions for doing the euro sign in ascii?).

<arrogance>
Bode's law does not represent any physical truth - it's a mathematical
curiosity, as is the fact that the sun and moon have the same apparent
size when seen from the earth. in fact, it illustrates the principle
that
if you look hard enough, you can find a mathematical correlation between
any set of things!
</arrogance>

i seem to remember reading about orbital resonances affecting stability
(in Nature, iirc); it seems that if you have two planets in orbits with
highly resonant periods (ie, with a low lowest common multiple, eg in a
ratio like 3:2 or 4:7), then interactions between them destabilise the
orbits. i'm not entirely sure about this, but it would suggest that if
you
have a massive planet at some distance D from the star, you would not
expect to see planets ate distances of D/2, D/3, 2D/3, 3D/2, 4D/3, etc.

mind you, i could be utterly wrong about this. i've got a memory like a
sieve of Eratosthenes - there's no way of predicting what it'll catch.

anyway, this doesn't help you with placing the massive planets in the
first place :(. i'd suggest that heat from the sun would mean that
close-in planets would be rocky, intermediate ones gassy and far-out
ones
icy. from the distribution of elements in the system (lots of hydrogen
and
helium, quite a lot of oxygen, nitrogen and carbon, not much silicon and
iron), you could guess at the mass distribution: all that H/He goes into
gas giants in the middle, the O/N/C and some of the H into ice
sub-giants
(water, ammonia and hydrocarbons) at the edge and the silicon and iron
into little rock worlds near the sun. thus, if you have more silicon,
you
get more rock worlds; more nitrogen, more ice worlds.

as for massive planets in very close orbits, they're either staggeringly
huge rocks, or the astronomers have muffed it up again. i know which
option my money's on :).

tom