Another Stupid Science question: Could there be a double planet system with...

[Razgovory]

I didn't have enough space in the subject box to put the whole question.  Sorry about that.  It was idly browsing the net, and I found a video on Youtube http://www.youtube.com/watch?v=usYC_Z36rHw which showed what it would look like if the solar planets were in the spot the moon was.  An interesting little exercise, that made me wonder:  Could double planet system exist with a small gas planet and a terrestrial planet?  Obviously small moons can orbit large gas planets, but what about smaller one co-orbiting with a terrestrial planet in the way that Earth and the Moon do?  I found a small exo planet this likely made up of gas and believed to be in fairly close orbit of it's primary http://en.wikipedia.org/wiki/Kepler-11f and thought it would be a good model for my hypothetical gas planet, and I suppose Earth or Venus would could be our terrestrial planet.  Could these planets form a double planet system?  If they could what would it be like?

Just Idle wondering, no need for Viking to call me a creationist or some other nonsense.

[Viking]

Actually this is the case right now. Both Saturn and Jupiter have moons which have a greater diameter than Mercury (and Pluto, but Pluto no longer counts), one of which has 75% of of the diameter of mars, or about half of earth.

BTW, the earth might be special in this sense. To the best of our knowledge, the earth is the largest solid object (as in not fluid or liquid) that we know about. Earth seems to have formed in a collision of two mars sized objects which re-coaleced after colliding and obliterating everything on the planet over 6 billion years ago (this is when the moon was formed). It might just be that the special conditions in that collision made our large solid planet possible and the late timing of that collision in accretion meant that the gas giants and the sun had hoovered up all the hydrogen before we went from being two small planets to being one big one.

The earth's moon is also seemingly unique since it seems to be the only large moon in the solar system which seems to have been made from the same stuff as it's planet.

So, yes, Raz it is not only possible, but is happening right here in our solar system. There are over half a dozen moons orbiting jupiter and saturn which would be counted as planets if they orbited the sun.

[jimmy olsen]

BTW, the earth might be special in this sense. To the best of our knowledge, the earth is the largest solid object (as in not fluid or liquid) that we know about. Earth seems to have formed in a collision of two mars sized objects which re-coaleced after colliding and obliterating everything on the planet over 6 billion years ago (this is when the moon was formed). It might just be that the special conditions in that collision made our large solid planet possible and the late timing of that collision in accretion meant that the gas giants and the sun had hoovered up all the hydrogen before we went from being two small planets to being one big one.

The earth's moon is also seemingly unique since it seems to be the only large moon in the solar system which seems to have been made from the same stuff as it's planet.

So, yes, Raz it is not only possible, but is happening right here in our solar system. There are over half a dozen moons orbiting jupiter and saturn which would be counted as planets if they orbited the sun.

Proto-Earth was nearly as large as it was now before it got hit by Theia (mars sized object)

Venus is only marginally smaller than Earth, it's nothing special. Plenty of terrestrial type planets have been found by Kepler.

A double planet would have to be created by unusual orbital mechanics resulting in a capture. Satellites don't typically more than 1/100th of the mass of their parent body and are usually much smaller. 

[Berkut]

I think the odd thing about the Earth (or potentially odd) is not so much it's size, but that it has a moon that is 1/3rd it's size, which is probably rather unlikely given what we know about planetary and solar system formations.

It is one of those things that makes things like the Drake equation so hard to really quantify. What if the abnormally large moon (and the resulting abnormally large tides that are very large but very consistent) have some direct impact on the likelihood of life and intelligent life forming? It is very hard to say, of course, since there isn't any to replicate the experiment, and there isn't even any other data available of course.

Did our unusual moon dramatically change evolution by creating greater churn at the convergence zones between water and land? Did the moon stabilize the Earth wobble enough to prevent the radical climate change that would occur if the Earths axis shifted by as much as 85 degrees over time, and would that result in considerably more difficult evolution of complex life?

These are still pretty open questions in astrobiology these days, or at least they were several years ago when I was working tangentially in the field and hence somewhat exposed to the questions.

[The Brain]

I'll expose you to abnormally large if you ask nicely.