How did gas planets form? (question from my 13-year old son)

I attempted to impart to my 13-year old son my limited knowledge of how
planets form. I told him, basically, that solid planets were formed
when solid matter (asteroids) coalesced by gravity, and gas planets
were formed in like manner with gasses instead of asteroids. I believe
this explanation is basically correct.

My son had no problem understanding how this works for solid planets.
Asteroids vary in mass, from mere dust to large boulders. The larger
bodies exert a greater gravitational attraction, which draws smaller
bodies to themselves. Eventually these larger bodies accumulate
additional mass, and perhaps are attracted to other nearby larger
bodies, until a planet is formed.

He had a harder time understanding how this principle applies to gas
planets. After all, gasses in space are widely dispersed with great
uniformity (I believe there are gas laws that describe how gas pressure
equalizes over a certain volume). Gas molecules don't possess
sufficient gravitational attraction to "retain their own atmospheres.
There are no particularly large bodies of gas (as there might be
particularly large asteroids) to be the genesis of a gaseous planet.

I was stumped by his question. What am I missing? Do gas planets have a
solid core that is sufficient to retain an atmosphere, and this
solid/gas body managed to attract a large amount of additional gas?

Can anybody help me answer my 13-year old son?

Thanks!

wrote:
I attempted to impart to my 13-year old son my limited knowledge of how
planets form. I told him, basically, that solid planets were formed
when solid matter (asteroids) coalesced by gravity, and gas planets
were formed in like manner with gasses instead of asteroids. I believe
this explanation is basically correct.

[rest deleted for brevity]

The condensation process believed to have occurred early in the
formation of our solar system has solid material scattered throughout
the solar nebula, generally consisting of particles of dust, much like
what is observed in molecular clouds. In the rotation process that
developed as the solar nebula shrank, this material was swept into a
disk and collisions could take place between the dust particles, slowly
building them into larger bodies (much like building a snowball).
Eventually, these were big enough to draw bodies to each other by
gravity, further building their size.

At the same time, the Sun was slowly turning on. Its heat would drive
away the gases and volities from the inner part of the solar system, to
a distance of about 2 to 4 times the current Earth-Sun distance, leaving
just the heavy material that could condense at these higher
temperatures. Out beyond this distance, it was cool enough not only for
the heavy stuff to condense, but the gases and volities as well. Since
there was much more of this stuff than heavy, rocky-metallic material,
it would eventually dominate the formation process, with the
rocky-metallic material being the "seed" of that larger planet growth of
ices and volitiles.

For a more complete explanation, you might Google something like "solar
nebula".

Just remember though the data that we are starting to get in from the
discovery of exo-planets that are found close in to their primaries.
Also the data from that comet probe
(forgot the name at the moment) that is showing that it's material
isn't as "pristine" as originally thought.

There seems to be more to this concept of planet making but of course,
the more of these comets and asteroids that we visit the more complete
the data will be.

wrote:

I attempted to impart to my 13-year old son my limited knowledge of how
planets form. I told him, basically, that solid planets were formed
when solid matter (asteroids) coalesced by gravity, and gas planets
were formed in like manner with gasses instead

and there you have it! Keep it simple. The kid aint going to Harvard
tomorrow?

Patrick Edward Murray wrote:
Just remember though the data that we are starting to get in from the
discovery of exo-planets that are found close in to their primaries.
Also the data from that comet probe
(forgot the name at the moment) that is showing that it's material
isn't as "pristine" as originally thought.

There seems to be more to this concept of planet making but of course,
the more of these comets and asteroids that we visit the more complete
the data will be.


Close large planets have been explained via migration toward their star.
It is not likely that they started there. Such migration could occur
in a denser dust disk acting as a breaking mechanism on the planet in
question. Such migration might also turn out to be detrimental to
smaller planets formed in closer to the star, sweeping them up or
slinging them out of that area.