In my world there are two planets within the same solar system, which are connected approximately every seven cycles through a portal that opens for a few days.

  • The planets are very similar.
  • The atmospheres are breathable.
  • Both people and animals can move from one planet to another without problems.
  • Gravity is the same (don't touch this).
  • The fauna is different; different kinds of plants, flowers and animals exist on both sides.

Is it possible that these openings affect the flora and fauna, taking into account that they only remain open for a short period of time?

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Aug 30, 2018 at 19:47
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    $\begingroup$ Is the portal occurrence a very old phenomenon, or has it only started happening recently (in evolutionary terms)? Did life evolve separately on both planets, or on one planet and then spread to the other via the portal? $\endgroup$
    – Ben
    Aug 31, 2018 at 5:12
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    $\begingroup$ @Ben The life developed separately. But just 100 Cycles ago the Portals opened for first time. $\endgroup$
    – Hedufigo
    Aug 31, 2018 at 7:14
  • $\begingroup$ How big is this portal? Just big enough for people and animals to walk through, or a km-radius circle, or…? $\endgroup$
    – abarnert
    Sep 3, 2018 at 8:51
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    $\begingroup$ edit your question to include the relevant details. A question that isn't answerable (and whose answers aren't understandable) without knowing how to seek out and find some chat history isn't a good question. $\endgroup$
    – abarnert
    Sep 3, 2018 at 9:11

13 Answers 13



  • Look at the concept of invasive species. Just a few pregnant rats or rabbits, a few seeds can start the process which imbalances an ecosystem, especially if their natural predators don't come along immediately.
  • Look at the concept of virgin field or virgin soil epidemics. When a plague hits a population without immune defenses, the results can be devastating, as in the contact between Europe and America.
  • Farmers (or their rulers) might deliberately bring crops because they taste better, or simply to add variety to the dinner table.
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    $\begingroup$ +1 for the virgin soil epidemics, very good point. i didn't even think about immune defenses for my answer $\endgroup$ Aug 30, 2018 at 8:52
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    $\begingroup$ Another important thing is how long this has been going on - even an open window every couple of years is like 'bascially always' on evolutionary time scales: if the portals have 'always' been active you will have substantial mixing of flora & fauna from the two planets over time (and probably get to similar but related species on both planets). If you already exchanged some of the first bacteria It may even be hard/next to impossible to say if life evolved on one plant or both independently. $\endgroup$
    – Nicolai
    Aug 30, 2018 at 11:58
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    $\begingroup$ Well, if the phenomen is long-running, and it seems so, 7 years don't seem long enough to cause lack of immune defenses. It might be slightly diminished though, maybe, at least for the newest mutations. $\endgroup$ Aug 30, 2018 at 23:21
  • $\begingroup$ if they only opened a hundred years ago the influence should still be local at least. $\endgroup$
    – John
    Sep 1, 2018 at 18:14

Unless the two atmospheres are exactly balanced in pressure (including day to day weather) you will get a massive wind coming from side to the other portal.

Imagine a 1020 bar atmosphere (hot summer on one side) to 980 bar (cold and wet on other) suddenly having no barrier. Over a few hundred miles this creates significant weather systems.

Over no barrier at all this is probably cyclone forming. At least a very large tornado.

As far as biology is concerned ... anything large is dead, but seeds will be scattered very far and very wide.

  • $\begingroup$ One imagines that if the portal activates semi-regularly (or is permanently active) that the air-pressures would equalise over time, effectively the atmospheric pressures of both worlds would rebalance until the air pressure levels out. during that time you'd get incredible cyclone-strength wind forcing through the portal but after a while it should stabilise. It might take a long time though, atmospheres are big things. $\endgroup$
    – Ruadhan
    Aug 30, 2018 at 13:25
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    $\begingroup$ Agreed that massive pressure differences would balance out over historic time scales. The issue here is achievable with simple day to day weather being in disagreement between the two sides though. You would get the same effect opening a portal over central Asia in summer (very high constant pressure) against some cold wet place at night elsewhere on Earth. $\endgroup$
    – Windlepon
    Aug 30, 2018 at 13:36
  • $\begingroup$ Good point certainly, you'd want to situate the portal in underground caverns at both ends to help mitigate the extremes of temperature and some pressure. $\endgroup$
    – Ruadhan
    Aug 30, 2018 at 13:39
  • $\begingroup$ After overall equalization, high winds would not last long, because it would be a small high-pressure area blowing into a small low-pressure area. $\endgroup$
    – WGroleau
    Aug 30, 2018 at 15:00
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    $\begingroup$ Those area's aren't that small. The mass exchange isn't significant compared to the size of air pressure area's in the larger weather systems, unless the portal is very large as well. And then im talking about 100 metres diameter if it wants to equalize before closing. Or of course the high/low pressure area's pass. $\endgroup$
    – Gloweye
    Aug 31, 2018 at 8:13

The fact that they're different planets doesn't matter compared to the environments in which they open. if as you say the planets are very similar in terms of gravity etc.

Envirnomental Effects

If one side of the portal was in the polar ice cap and the other somewhere near the equator then the movement of hot and cold air between them would cause some significant weather patterns to develop while the portal is open, possible wet environment to dry environment could cause a small "oasis" type environment to form around the portal on the dry end. its unlikely there would be much difference on the wet end.

If However we assume that opening a portal would require a truly huge amount of energy and that when the planets get too far that energy would have to dissipate somewhere, then that alone would likely cause other effects that such massive bursts of energy would cause, fires around the portal could from grounding of the energy in the forms of lightning would be very likely. if the ground had a high silica content then that same lightning or bursts of energy would cause the formation of crystaline glass. best to look up How Lightning Effects the Ground to give you ideas on what to expect


Plant life, if similar to that of Earth's but different form each other (perhaps depending on environmental changes) would likely cross through the portal on the wind, meaning that plant life around the portal sites would appear vastly different to the surrounding areas...

However there's a caveat to this. if the portals had been opening for a substantial amount of time, say a century or maybe less, depending on how invasive the species of plantlike are, then if you approached you won't notice such a difference. because the plants would spread naturally around further increasing their borders unless local flora (being already adapted to the local environment) beat out the invading species in terms of access to nutrients from the ground, water and light.


Wildlife may cross over through the portal... however it depends on the type of portal, if it is a borderless plain "window" in a small rock tunnel then animals might not notice and wander through... after the portal closes however then most likely not enough of the species made it through to provide enough genetic variety and the species would either die out from lack of species all together, or eventually die out due to gene mutations from too much inbreeding.

There will be exceptions, when either enough went through or if a couple went through on each opening and they managed to meet on the other side and bred. Insects and rodents are likely to be examples of these exceptions, as they tend to travel shorter distances and many individuals travel the same area. so its plausible that many could make it through at a time... and likely die shortly after unless the climate was very similar to that of their home side.

However if the portal was more energetic than that, think the portals from Marvel's Doctor Strange, they are a ring of energy that offers a stable portal between locations, that ring of energy which gives off noise and light irractically is likely scare most wildlife off, not draw them in. again insects might be the exception (moths to a flame)


I won't even get into the effects if the portal emits radiation...

Simply put if you want a species to have crossed for the sake of your story then its plausible but it should be done so carefully, and definitely not all animals everywhere

I made such a point of the plant life as it that resource that wildlife would follow initially before finding alternates on the other side.

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    $\begingroup$ I discounted weather effects because the planets have a roughly similar atmosphere. If not, there might be a constant storm through the gates. Even if air moves through the gates at gale force strength, you won't bring enough energy through to affect the regional climate. Imagine a jet engine running 24/7 in antarctica -- it won't melt a large ice shelf. $\endgroup$
    – o.m.
    Aug 30, 2018 at 8:58
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    $\begingroup$ @o.m. Similar atmosphere yes, but not necessarily similar climate, one coastal, one deep forest, it would effect the local climate temporarily in the small area around the portal if the temperature difference was significant, as for the jet engine in the arctic... did you get a RyanAir flight? $\endgroup$ Aug 30, 2018 at 9:03
  • $\begingroup$ The point is that heat transfer would affect only a small area around the portal. A tiny area, on a planetary scale. $\endgroup$
    – o.m.
    Aug 30, 2018 at 9:08
  • $\begingroup$ @o.m. Agreed, which is what i meant in my answer $\endgroup$ Aug 30, 2018 at 9:16
  • $\begingroup$ If the portal joins regions with very different temperatures then you can generate power by running a heat engine connecting the two sides of the portal. $\endgroup$ Aug 31, 2018 at 14:47

It's not only possible, it's very probable that plants spread to the other planet.

Humans have a tendency to take animals with them wherever they go. Be it the loyal guard dog, the fleas living in said dogs fur, the horse pulling the cart or my herd of livestock I would take with me if I wanted to start a new life on another planet.

People and animals crossing the portal are not sterile. They can be pregant (as suggested by o.m.), but they also carry seeds in their fur, intestines and on the soles of their feet.

Many bushes and trees are specialized on dissemination by birds. They produce small fruits that are eaten by birds, including their seeds. These seeds withstand any attempt of being digested and pop out the other end of the bird, fully intact and ready to sprout. All you need is one sparrow with a full stomach flying through the portal to start spreading the seeds on the other planet.

Other plants like thistles and burrs have specialized seeds that entangle themselves in the fur of animals to be carried great distances from the mother plant. These could easily be carried by humans and their livestock to the other planet.

The simple fact that air is exchanged between both planets enables air-borne pollen to fertilize genetically related plants on the other planet.


Living species will travel across the portals, and this will definitely impact the receiving world.

How big the impact is going to be depends on how much similar are the two environments where the portals are open.

To make this clear, imagine that the portal is on one side open on an artic tundra, while on the other ends it opens on a tropical forest. An artic fox can cross it, but it will have a hard time to fairly compete in the tropical forest. Same for an elephant stepping into the tundra.

Same consideration if you consider plant seeds.

If the climates are similar, the newcomers and the locals will start a fierce competition, and the fittest will survive.

A bit more tricky can be for microscopic life forms: they could stay dormient if the environmental conditions are not suitable for them, else they could quickly spread. Also, while winds and waters cannot easily move around large species, bacterial spores and virus can more easily be transported. So a microscopic invasion of the world cannot be avoided.


Others have addressed the effects of air pressure and invasive species, but...

Depending on how you design your portal, you might have a problem with the portal itself

As the two planets orbit their sun, the portal will elongate, contract, rotate, and pass through the sun. As the planets rotate, the portal will pass through each and both of the two planets. Let's have some fun with this.

It becomes a pump As it lengthens and shortens the volume available for mass inside the portal increases and decreases. That's a pump. Which direction the pump will push air will depend on the air pressure on each side of the portal. Note that environmentally, that's going to change as climatic zones of high and low pressure come and go. That means the "wind tunnel" will in turn push in either direction. What's breathtaking is their delta-V. Earth books along at 67,000 mph. Mars (for example), books along at 54,000 mph. Max delta-V: 13,000 mph. that's a wind force that would strip plant life down to bedrock for hundreds if not thousands of miles in front of the portal entrances.

And then there's the heat The kinetic energy involved with the "motion" of the portal through space (call it the "wiggle" at the portal entrances) is well beyond non-trivial. I'm not going to try to calculate it, but it would easily vaporize water and probably vaporize rock.

Finally, there's two planets and a sun to contend with How gravity and the fusion forces of the sun affect passengers travelling through the portal during those periods when it must pass through the celestial bodies is, well, it's unknown, but it would cause a pretty bumpy ride. Worse, it could either super-heat the air as it resides inside the passage (which means it could be a 13,000mph plasma when it exits the portal) or simply create high/low pressure zones during the passage. Combined with the gravity "bumps" it could be like riding a rollercoaster that moves both above and below ground (hot/cold) and up and down (high/low gravity).

And that's all in a fraction of a second

Creating portals between two points in space-time that have no "apparent" distance is, basically, magic. Or, perhaps more scientifically, we don't really know what "distance" means when you warp space-time. We think we do, but it's all theoretical. As the author, you'll need to decide what happens between "in" and "out."


These give you some ideas of the complications of non-magical portal travel. Your story certainly needn't be as extreme as I've described (not by a long shot), but it can be made more interesting due to these "effects." For example:

a) There may be certain times of the year where portal travel is unavailable (e.g., due to passage through the sun).

b) The huffing and puffing of the wind as it's pumped through the tunnel could be used during a daring flight from danger as your characters must brave near-hurricane-force winds from the tunnel during their dash to safety.

c) The temperature changes can create unique environments near the portal entrances. In other words, during the middle of winter the portal entrance may be a lush, summery condition.



Invasive Species

Unless the portals have been open for a looooong time in history, chances are neither side has ever seen the other planet's flora & fauna. Given that they are similar (plants and animals), it's very likely that at least one organism which is perfectly suited for the other world (that will make it across).


This is a big problem. A portal between planets with different atmospherical pressures would cause some trouble. At every portal-opening, high speed winds blast through the portal until it closes. The pressures will equalize over a long period of time, but atmospheres are big, and it will take quite a while.

Assuming two atmospheres at 900 and 1000 milibars and a 1-meter opening, bad things will happen, because ~50 cubic meters of air will be flowing through at once

The good thing is, if the atmospherical difference is high enough, this will also solve your invasive species problem by destroying them. [Initially.]


If these portals have been open for a long time, both of these issues won't be problems, as the atmospheres will have balanced, and the ecosystems in each planet will have adapted to each other's species.

A Fun Sidepoint

If there are no constraints to where the portal can be, we can have a lot of fun, e.g. a portal in the bottom of the ocean (or deep in the planet's core) connected to one on the surface may cause all sorts of havoc.

For more information on portals and oceans and Mars, I suggest reading this What-If article.

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    $\begingroup$ It will solve the invasive species one way only. $\endgroup$
    – Gloweye
    Aug 31, 2018 at 8:17

You have just created the plantary leaf blower.

This site: https://www.tlv.com/global/TI/calculator/air-flow-rate-through-orifice.html

if I plug in values for .1 psi difference at 20 C for a 50 m portal, I get about 3 million cubic meters per minute. A 50 meter diameter portal has an area of about 2000 square meters. This would give a air speed of 1500m/min or about 25m/s -- about 40 mph. Moderate gale. A tenth of a psi corresponds to about 7 mb. Such variations are common and within the daily weather cycle. To first order I expect that the wind would be linear with pressure difference. Bad day when you get a .5 psi difference. (Edited: cmaster points out that the calculation results I initially posted are cubic meters per minute, not per second. This answer is corrected for that figure.)

This also ignores that two arbitrary planets are likely to have matching atmospheres. Consider the results with one portal opening on Venus and one on Mars. Might make both planets closer to habitable. No mere 40 mph breeze this.

Doesn't mean you can't use your device, but it does mean you need to come up with a means of dealing with this.


  • The portal only opens when the pressure is equal on both sides.
  • The portal only opens when the pressure is highly unequal -- and uses the pressure differential to power it.

Other problems:

Is the other planet a 'mirror earth' with same continents? Same erosion? Going through a portal and finding that the other side is 10 km above the surface could wreck your morning, and make you late for lunch. A portal that has one end at the bottom of an ocean trench could make you miss supper.

Do portals lock to their local geography? E.g. if this week a step through the portal from the foreign planet lands me in Paris France, will it do the same next month? This may be one of the conditions for portal creation: You have to have some degree of matching of conditions, and anchoring bedrock to keep things still.

  • $\begingroup$ Oops. Edited. Sigh. $\endgroup$ Sep 2, 2018 at 2:19

A note on biological compatibility

First, I want to make a note on biological compatibility. It is by no means clear, that life forms evolved independently on different planets will use the same biochemical base. Even considering life based on RNA/DNA, proteins, sugars, and fats (like the life on Earth), there are always incompatible choices possible: A general choice on the chirality of the molecules, the selection of RNA/DNA bases, the selection of preferred sugars, the selection of aminic acids used in proteines.

Incompatible life forms will compete on resources like sunlight, space, minerals, etc. and they may even kill each other, but they cannot feed on each other, forming one consistent ecosystem.

Effect of the portal

Of course, life forms will pass the portal in either direction. There will also be a permanent wind through the portal: Depending on the momentary atmospheric pressure, there will be winds towards the lower pressure. These winds are very effective in transporting seeds of plants, microbes, dust, insects, and birds between the planets.

We soon see invasive species

in both of the connected planets.

When there is no biological compatibility, the invasive species will be plants (autotroph) in the first wave, because other species will not discover digestible food. They are inedible or even poisonous to the domestic animals of the planet. In the worst case, some weed is able to shadow out the domestic plants and takes over the planet by a catastrophic breakdown of the old ecosystem.

When there is biological compatibility (either by magic or by panspermia through ancient portals) the situation becomes more earthy: The invasive species will blend into the ecosystem and become food for some domestic animals. Among the invasive species are now also animals of all kind.

In the long run, a mixed ecosystem with the most robust species from both planets will establish itself.


You state that the gravity is the same - but the altitude of the portal can be different on the 2 planets; and as long as the atmosphere is breathable then we can change exactly what it's composition is.

  1. "Human breathable" doesn't mean "any living thing breathable" - so you can eliminate creatures that can't tolerate the different composition for whatever reason.

  2. One can get some fairly significant pressure differentials between the 2 portals by having one at high altitude, and one at low altitude - which will draw in content from the high pressure into the low pressure.

  3. You can then use this to explain how the invasive species was able to spread so far so quickly (high differential - so more wind - so more stuff spread more places) or so slowly (low differential), or why only some things survived.

  4. following on with different altitudes for the portals - consider the case where one side is at the top of a mountain, and the other side is at the bottom of the sea... Given enough time, the other planet could easily lose most of it's water and the other planet would drown .... (and have the gravity change)


If you are asking for the effects of the portal itself on both worlds, it may release high levels of radiation to anything that crosses between the worlds. Lastly, gravity may cause problems for structures and environments around the portal because gravitons may flow through the portal also. Chirality can cause problems, as organisms of differing chiralities or biochemistries are not edible to each other.


An extinction event across both planets.

– Leading to a single biosphere.

A portal that connects two planets effectively turns them into one planet.

First of all, all airborne and droplet waterborne life will cross both ways.
Then all the spores/larvae/pollen: fungal, plant, and animal will cross over.

Larger life forms will swing by and scatter more of the above, as well as parasites, seeds, and other larger biota.

The two ecospheres will be forced into re-balancing. It will be a mess. It's not just an 'invasive species' scenario - it's full on biosphere battle leading to about 50% species becoming extinct on both planets. Adding more portals increases the extinction percentage:

(100 * 1 - 1/(number of joined planets))

Ten or more planets joined would be equivalent to the P-T extinction across all ten planets. 90% of all species would be eliminated.


Given that all previous answers concentrate on the catastrophic effects that different pressure or height or gravity levels could generate, I thought that your portal might have a security device preventing this.

what's a portal

In some theories, distance in space can be defined as "if you're strongly quantum entangled, you're close, if you're weakly quantum entangled, you're far". So you could dream up two strongly entangled doorways which are brought to different planets by handwave and continue to function, generateing all the above mentioned problems.

security functions

So, the reason for opening and closing is weather or incompatible locations (tunnel through the sun anybody?). Probably you will have a weather depending, season-depending open/close cycle. Maybe it is even a door, like a real physical world treasury metal door, that closes and opens for security. One possible plot point could be if the mechanism fails one day, and then the weather changes on one or both sides. If you have a few dozen or hundreds of those portals around the world, including in the maritime area, you could let one fail and the catastrophic desert in the area spreading slowly being a part of the story.

had been there for ages

If it is there since ages, animals could have adapted to it. They could regularly visit it like a waterhole; waiting in the area until it opens to the other side. Probably there will even be species adapting their entire life cycle, like moving to the rain season of one planet for breeding, moving to the spring season of the other planet for the dry period... and all this water-hole behavior might bring up predators as any waterhole does. It would also explain why both environments are compatible - if you have spread all the time.


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