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I've been pondering for some time an alternate-history story where the Earth as we know it is replaced by a Rocheworld, of the canonical variety introduced in the eponymous Robert Forward novel where the two lobes share a common atmosphere but the solid and liquid surfaces do not touch, and in which the "New World" (i.e., the Americas) is literally a different world--the other lobe of the Rocheworld, inaccessible by land or sea from Eurasia, Africa, and Australia.

The two lobes are separated by approximately 150 miles. While that's well above the conventional edge of space, the lower gravity and correspondingly larger scale height of the atmosphere at the interior poles should result in an atmosphere that remains dense enough to breathe all the way through the gap.

So, I figure the occasional bird every few hundred, or maybe thousand, years will manage to fly or get swept from one lobe to the other, along with plenty of airborne microbes, flying insects, and whatever seeds or other small animals the birds might carry with them... but just how much viable biological material, of what maximum complexity, should I expect to be regularly exchanged? And as a result, just how weird should the flora and fauna of the New World be expected to be?

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    $\begingroup$ I hope you are aware just how difficult this question is to answer. You would have to model weather patterns and parallel evolution on 2 planets that cannot exist in reality over an unspecified time frame. You would also have to model how that 150 miles changes over billions of years. I think this is one of those cases where nobody can prove you wrong and you can just build your world in whatever way you like. My guess is, the older the system, the less likely that any exchange will do anything because of vastly different biochemistries, but nobody can prove that right or wrong $\endgroup$ – Raditz_35 Oct 16 '17 at 8:15
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    $\begingroup$ @Raditz_35 Look it's plainly ridiculous to expect anyone to model billions of years of evolution & climatology of a Rocheworld. A Big Picture answer is needed. So how likely is 150 mile gap to have an exchange of biological material? $\endgroup$ – a4android Oct 16 '17 at 10:49
  • $\begingroup$ @a4android Of course it is. At the very least, the status quo needs to be defined and then one could look what happens if a bird flies to the other world. An answer that is meaningless because it introduces massive error bars is as good as no answer at all or any random answer. That's a lot of work put into something for nothing - which is why I adviced to just make stuff up. Speaking horses? I could define a computer model that would yield that one world absolutely has to have speaking horses. $\endgroup$ – Raditz_35 Oct 16 '17 at 10:51
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In terms of biological transfer there are a number of possibilities

Smooth still world with high pressure bridge
The remoteness would be similar to that between America and Europe on our Earth. Airborne seeds, spores, birds and anything that was small enough to hitch a ride on a bird such as insect eggs and bacteria etc would be able to make the journey.

Smooth still world with low pressure bridge
The remoteness would be greater than that between America and Europe. Birds would not be able to survive the low pressure environment, but airborne seeds, spores and bacteria should be able to make the crossing depending on how much weather we allow on “smooth still” world.

Real world
Real worlds don’t tend to be smooth or still. They tend to have mountains and weather and both of these would cause problems. The presence of mountains would create also sorts of tiny gravitational anomalies which would help to destabilize the Roche world. But the weather would be the real killer. The presence of Earth sized storms migrating across the planet into an area of low gravity will cause chaos.

In an area with little net gravity, gravitation would not play the leading role. Instead other forces such as storm fronts would. Even at 1g on the Earth the sea can be whipped up to 10’s of meters high and rocks and boulders from the sea floor can be thrown into the air. Such a storm moving into the air bridge area under perhaps just 0.05g would carve up the sea into enormous mountains and the higher the seas got the lower the gravitational attraction would be. It is highly likely that sea water at some level would be transferred. So some lower forms of sea life might also make the journey.

Best guess world
Given the propensity for things that can move freely to end up spinning when a force is applied (like stars, planets and storm systems). I suspect the air bridge area itself would suffer the most horrific perpetual circulating storm which would suck up water and seafloor materials and fling it out into the bridge. Some of the smaller solid and liquid particles could easily be transferred between worlds.

So in summary in best guess world seeds, spores and bacteria for sure. Other microscopic life forms such plankton very probably. Larger life forms such as insects, possibly especially if carried aloft inside a small branch or similar in egg form. Macro life forms such as fish and birds: I doubt it due to the violent turbulence and the unpleasant mixture of mud, water and air that would span large parts of the bridge.

Of course it is entirely likely that something would evolve on one of the worlds to live in such an environment. But that’s another story.

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Good grief, um okay part of this question can be, somewhat, answered, the rest of it is on you, it's too big to answer outside of a complex history which you are going to have to pen. The answer that I can give you is about the amount and complexity of transfer which sod-all and most of it very simple, fruit eating birds don't tend to migrate and so don't find themselves in a position to be "blown off course" the main migratory species are seabirds and grazing species neither of which would carry a major payload of viable material. So the transfer is limited to aeolian transfer of dust, pollen and microbes, there could be a lot or very very little of this activity depending on the atmospheric interface at the air bridge, I'd guess the latter but have no way of knowing or modelling a definite answer.

Even if there was regular transfer of airborne material like pollen there's no guarantee that any crossover could result, if the two worlds have a history of life that precedes their current shared situation then they could in fact share no structural biology in common at all. Planet A may in fact be composed of left-handed proteins while Planet B uses a right-hand twist instead and life could never bridge the gap. If only one planet had life while the other was lifeless then the lifeless world's new ecosystem is going to be built on material blown in from it's lush partner but will be badly behind due to the minimal transfer of material.

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  • $\begingroup$ I would have though that at the very least bacteria would have been able to make their way across $\endgroup$ – Slarty Oct 16 '17 at 16:15
  • $\begingroup$ @Slarty If you're talking about the right/left thing then they can but only if they're primary producers and get their nutrients from inorganic environmental sources only. The nutrients in lifeforms with opposed chirality are pretty inaccessible until the proteins degrade to a very great degree. $\endgroup$ – Ash Oct 16 '17 at 16:25
  • $\begingroup$ I suppose it depends on when the world became a Roche world and how long it had to evolve. Roche worlds, if they exist at all, would be highly unstable so evolution is a moot point. But if life did evolve on one world, I would have thought sufficient materials would have transferred each way to ensure commonality in things like the chiral basis of life. In other cases it’s hard to be definitive. The question is based on a scenario that is both inaccessible and not clearly enough defined. $\endgroup$ – Slarty Oct 16 '17 at 16:51
  • $\begingroup$ @Slarty Like I said it depends on the relative age of any existing ecosystem(s) in the system, it may be that you'd get species with a seasonal migration from world to world for some purpose, if the situation was favourable. There could be total co-evolution of the combined life-system but my money is on not a lot of interaction, 150 miles is a long way for most species with the access AKA fliers. The biggest thing is that the rules for "life as we know it" were set in the oceans a long long time before anything living got airborne. $\endgroup$ – Ash Oct 16 '17 at 17:01
  • $\begingroup$ 150 miles is a long way in 1g, but 150 miles where gravity changes from 0.05g > 0g > -0.05g its a bit different. $\endgroup$ – Slarty Oct 16 '17 at 17:52

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