Wow.

I kind of lament the 'made up science questions' clause, myself. I've seen that they are sometimes noise, but on the other hand, that sort of thought experiment is interesting to read, and has a longstanding tradition in the world of science.

Me too, Miko (hah, as if that wasn't already clear). I've advocated in various MetaTalk threads that a line be drawn between hypothetical questions on which all that can be said is completely random speculation, and hypothetical questions which actually allow informed, rational speculation, and that the latter should be allowed. But I've accepted that that is not the decision of the moderators, and I'm not going to change their minds, so I won't press my case any more than I already have in MeTa.

Er, I've mostly accepted it. If I had completely, wholeheartedly accepted it, I wouldn't have written the answer above in the first place.

Sweet! I think I now have a science crush on you, DA.

"someone wanted to fill the inner solar system with air"

It's called a planetary nebula.

"an infinite source of mass"

AAAGGGGHHHHHHHHHHHH!!!!!!!!!!!!!

There was no good way to answer that question. Too much confusion in the question.

Adding mass to the Sun in the form of an Earth atmospheric mix to the point of creating an Earth sea-level atmospheric pressure at the orbit of Mars, starting from the assumption that it's possible, is one hell of a lot of additional mass. Just in terms of added mass, the limit isn't black-hole sufficient mass, it's neutron star level. And then below that threshhold is enough to incite a nova that blows it away before you get there. And it wouldn't stay in the Earth mix, anyway. There'd be ionization, separation, chemical reactions, etc.

The question just doesn't make sense. It's Chewbacca astrophysics.

Another consideration, pressure of 1 atmosphere at the martian orbit would likely cause so much friction at earth orbit as to make earth uninhabitable.

"I'm writing a science fiction novel and I want to take the notion of 'terra-forming' one step further. [More inside]

Rather than have individual planets converted so they can support human life, I'm wondering if it is possible to fill a whole solar system with a breathable atmosphere at one ATM of pressure. I want the novel to be at least semi-respectable in terms of the scientific plausability of this sort of conceit.

Could this be done? (I'm thinking of setting my story in our solar system). If there are reasons why such a thing would be impossible, I'd like to hear those too. For example, if the mass of the gass required would generate a black hole, I'd like to consider using that as a plot device itself.

The more scientifically accurate your answers can be, the better, but if the whole premise is 'Chewbacca astrophysics' I'd like to know that too. It might be easier to just set my story on Endor and forget the science."

Think of it this way, GD. The sun is primarily hydrogen, the lightest element and which (I believe) is primarily ionized. What you propose is to fill the inner solar system (a sphere, not just a disc) with a mix of nitrogen and oxygen, both of which occur naturally as molecules (N2, O2) with respective weights 28 and 32 times that of ionized hydrogen. (I think those numbers are correct.)

Now, the sun formed from a hydrogen cloud that once spanned a volume of space much larger than even Pluto's orbit, and its density was far, far smaller than one atmosphere. Through its own gravitational forces that cloud of hydrogen condensed.

Now, you basically want to form a similar cloud of gas, but one whose weight is 15 times that of molecular hydrogen. That air will also fall into the sun. Long before a black hole forms, all that heavy gas would destabilize the sun's chemistry and hasten its demise as a planetary nebula.

That's problem number one.

Problem number two is generating that volume of nitrogen and oxygen. Strictly speaking, hydrogen is the only "naturally" occurring element (An argument could be made for helium but that is outside the scope of this mind experiment.) Every other element was formed at a star's core and blasted out into space when that star exploded.

Collecting that amount of nitrogen and oxygen would be far too expensive; to be honest, I doubt our entire Milky Way galaxy has enough nitrogen and oxygen to perform this stunt. You would need something resembling a stellar nursery to provide the fusion, and that nursery would need massive shielding due to the X-rays generated.

Problem number three (and this is a biggy): solar heat. All that nitrogen and oxygen would be absorbing solar energy. In a vacuum, virtually all of the sun's energy is blasted out into space. Few people realize this, but the engineering marvel of manned space flight is not so much getting off the ground, but insulating the spacecraft from the tremendous outpouring of solar energy.

OK, these are just off the top of my head and I may be missing an even more glaring problem.

Probably your best bet is to invent a planetary system that formed in a gaseous mixture that was not large enough to condense into a star but large enough to provide enough heat through pressure to sustain lifeforms. This would probably have to occur in a damn near ancient universe that had already seen ten or more generations of stars. Your biggest enemy then in terms of internal logic would be entropy, but hell, the existence of both entropy and life as we know it is a contradiction of divine proportions.

I just realized that the storm systems in such an environment would be absolutely colossal. Also, without a central sun, planets would be "revolving" about their central point of mass. Those revolutions would have to be slow, because since those planets are immersed in a ubiquitous atmosphere, their passage through that atmosphere would be much like sticking your hand out the window of a speeding car.

Actually, I was just trying to re-write the original question in such a way that the rules of AskMe would (in my opinion) allow it to stand.

Mischief's answer is a great one though, as was the discussion generally, both there and here.