Expert: Jayendra Upadhye - 11/5/2006

Question
My understanding of how most large objects in space are formed is that they start with a huge rock presumably with a solid core, and with enough weight (gravitational attraction) to attract other rocks to itself. Eventually it may become big enough to be considered a planet. I feel that in a large planet, the mass near the center would not be gravitationally attracted to the center of the planet (since there is little mass in that direction) but to a point along the planet's radius that corresponds to the actual center of gravity.  This actual center of gravity would not be the exact middle of the planet as many people suppose, but a roughly spherical plane inside the planet.
Accordingly, since logically there would always be outwards gravitational pull from rather than inward attraction to the center of large planets, isn't it possible that many planets, including perhaps Earth, could have hollow cores?  

Answer
Hi,
You know marty..
I actually like people like you.
Who hypothesize on their own strength of convictions.
It does not matter if these may be wrong or right.
Thinking for oneself is important.
Therein lie seeds of discovery.
Most of such hypothesising throws up rubbish no doubt.
But from tiny acorns...(to use a much maligned phrase that is..:))

Now to your hypothesis.
sadly it is a fallacy.
The resaon is, that wise guy newton spoiled that chance by inventing integeral and differential calculus !
He also did research that stymied thinking along this line.
He proved that ..
1 - All gravitational force (non-point bodies too) "appears to point to their "point center of mass".
[Not a spherical thin shell etc).
2 - [this is very important], all mass outside of a spherical volume of a given radius, within a body, DOES NOT contribute to gravitational intensity percieved at surface of that sphere.

Consequences:-
1 - Gravitational acceleration g is greatest at earths surface, and falls off exponentially as one goes away from it, but this inverse square law appears to see all of the earth's mass concentrated into a point at its center!

2 - As one goes inside the earth, (resides on an imaginary sphere of radius r such that r < earth's radius), the gravitational acceleration falls linearly to zero.
zero at the earth's center.

In large gaseous planets, the sun's gravity, and the fluid nature of gases and liquids (cooled liquid gas), cause tidal interference from the sun to shift the actual physical center away from the center of mass of the planet.

But still, at any given instant, "outsiders" will always percieve the planets mass to be concentrated into a point at its center of mass.

Gravitational force and acceleration will linearly fall as one goes towards the center of such planets.
[that is, wherever it may be with respect to the physical center of that planet].

Anyway these two facts preclude formation of hollow planets.
However all hope need not be lost!
Have you studied gothic arches? Often a building is seen in ruins, but the arches are seen intact.
[The famous arch at Ur, the biblical city, was intact even after 1000's of years!]

Well, if during the formation of a planet, enough silicate rocks whirled around it to form a loose ring, (stable spheres never form, as gravity disposes things towrds rings..), which consolidated into a solid thick rocky circle a few 100 miles thick, we could have a planet with this ring fortress embedded in its gaseous core!

For astronomers to see and marvel some time in the distant future.

That would be biblical Ur in space!

Who knows. that ring would dictate the dynamics of convecting gas flows within the planet.
Check that out with any astronomer friends.

They would be certainly shocked!

Last but not the least, do rate this answer..
Jayen