In my story there are multiple planets in a solar system. And each of them are inhabited with different species. The main inhabitants of one planet are "bird+human" things. Is it possible for those things to fly from one planet to another?
The fastest bird on Earth is the peregrine falcon, accredited of a maximum speed of 320 kmh. Orbital speeds or escape speeds are on the order of magnitude of 10 km/s, or 36000 kmh.
There is no way for a bird to reach those speeds counting on its wings. And even if they were able to reach them, lacking air in between the two planets they would have no means of steering/adjusting the trajectory. But that would be less relevant, because they would be dead for the lack of air, and the unprotected exposure to star radiation would char them.
But humans have reached those speed despite being wingless, thanks to devices called spaceships, having steering capabilities and a breathable atmosphere in them. If your human-birds have those, they can, too. Else not.
Let's ignore the laws of physics for a moment.
The Earth's atmosphere is approximately 480 Km thick. Let's assume two planets.
- They are equal in mass.
- They have matched their orbital speeds such that they are always next to one another.
- Their surfaces are separated by 700 Km.
In this condition the gravity between the two planets would be pretty darn close to zero, meaning that escape velocity is no longer an issue. Their atmospheres are mingling, and although at the halfway point the air would be precious thin, there would be air all the way through (Please note that Mt. Everest is only 9Km and air tanks are pretty much required, so you avioids will need air).
Would such a planetary pair meet the level of suspension of disbelief? For the vast majority of humanity, yes.
How realistic is it?
Not in the slightest. Planets that close together would very quickly be drawn together (big boom...). But they must be that close to provide the lower gravity and necessary atmosphere for your avioids.
It's also unrealistic that they'd pace one another (the outer planet moving just faster than the inner planet). If they moved normally, the passage of one across the other would create so much atmospheric friction that it would burn up most of both atmospheres. Everywhere else would experience breathtaking (literally) storms.
And this ignores the fact that the universe is far from precise. Planets vary in their orbits in many ways. Ignoring everything else, the two planets would sometimes be further apart, sometimes closer together, sometimes one above the plane of the other, sometimes vice-versa, blah, blah, blah. it would never be a consistent or permanent condition.
But, you're writing science fiction. Don't forget the fiction. I think it'll make a great story. Have fun!
You can write a story where aliens flap their wings to travel through interplanetary space.
But since that is generally considered impossible as other answers show, you might have to work to make it more possible, or less impossible. For example, the aliens wouldn't have any air to breath in interplanetary space. If you find a plausible and possible way for them to survive the trip, that impossibility will disappear. It would still be impossible for their wings to propel them on the trip, but that would be one impossibility instead of two.
I believe that Larry Niven wrote novels The Integral Trees (1984) and The Smoke Ring (1987) set in a sort of interplanetary zone of breathable air.
And I suppose that it might be possible to have a double planet with two planets orbiting each other just outside their Roche limits and with some column of air connecting their two atmospheres. Thus bird like aliens or aliens like air squids might be able to survive travelling along the air column.
I believe there is a similar concept in Rocheworld/Flight of the Dragonfly (1982, 1984) by Robert L. Forward.
You should look up other questions which mention such concepts.
Or the aliens might have tough integuments that prevent them suffering from decompression during interplanetary flight, and might be able to store a lot of oxygen internally.
As far as I remember, the last time any science fiction story had beings flying between different planets and moons were in H.P.Lovecraft's "The Whisperer in Darkness" Weird Tales, August 1931, and At the Mountains of Madness, Astounding Stories, February, March, April 1936 (82 years ago). And in children's fantasy Dr. Doolittle in the Moon, 1928. In the Lovecraft stories the interplanetary flying was done by aliens partially made of exotic forms of matter and their wings interacted with the hypothetical luminiferous aether that has been more or less disproved by relativity instead of with the extremely rare particles of matter in the vacuum of interplanetary space.
Unless I haven't heard of any later examples the last published science fiction story with interplanetary wing flapping flying was published 82 years ago. And the story was published because the author was a genius and the story a great classic, despite the implausible interplanetary flying.
I tend to have a "head canon" that the One Ones in At the Mountains of Madness didn't actually fly through outer space flapping their wings but traveled in spaceships shaped a lot like their own bodies for aesthetic reasons. When Danforth and Dyer saw images of those spaceships they misinterpreted them as Old Ones flying through outer space.
I don't know when the last science fiction story using balloons or airplanes to travel from the Earth to the Moon or between planets was published. Scientists became convinced that interplanetary and interstellar space is basically a vacuum long before science fiction became a genre in the 20th century.
It seems "more plausible", to use the phrase loosely, to have aliens - winged or not - with some form of biological anti gravity space drive. That is about as plausible as telekinesis or the way that Superman flies in air and in outer space.
It would be more plausible than that to have aliens - winged or not - who are cyborgs, with built in machines that generate anti gravity forces under mental control by the aliens. And maybe those machines also provide air for them to breath in outer space.
That dangerously addictive site TV Tropes, has a page called "Mohs Scale of Science Fiction Hardness".
So as far as I can tell the following:
1) Interplanetary travel by flapping wings working on the incredibly thin "air" in outer space.
2) Interplanetary travel via airplane.
3) Interplanetary travel by flapping wings working on the luminiferous aether.
4) Interplanetary travel by aliens - winged or not - who use telekinesis to move their bodies through interplanetary space and to surround those bodies with enough air to survive the trip.
5) Interplanetary travel by aliens - winged or not - who have biological organs in their body that generate anti gravity forces and use those forces to move their bodies through interplanetary space and to surround those bodies with enough air to survive the trip.
6) Interplanetary travel by cyborg aliens - winged or not - who have mechanical devices artificially implanted in their bodies that generate anti gravity forces and use those forces to move their bodies through interplanetary space and to surround those bodies with enough air to survive the trip.
7) Interplanetary travel by cyborg aliens - winged or not - who have mechanical devices artificially implanted in their bodies that generate anti gravity forces and use those forces to move their bodies through interplanetary space and who wear spacesuits to survive the trip.
8) Interplanetary travel by aliens - winged or not - who use spacesuits with built in anti gravity drive for the trips.
9) Interplanetary travel by aliens - winged or not - who use spaceships for the trips.
Are progressively more plausible and possible.
The first one is definitely One on the scale, and number seven might be considered Two, Three, or Four on the scale by various persons. Number nine is used in countless stories ranging from One to Five on the scale.
If you want to write a science fiction story with aliens flying between planets by flapping their wings or by using squid like air siphons go ahead and write it. It would certainly be unusual and distinctive. And if you are as good a writer as H.P. Lovecraft it could turn out to be as good a story as "The Whisperer in Darkness" or At the Mountains of Madness.
But you might wind up marketing the story as a fantasy story or a children's story.