Double Planet World

[Vaporo]

The story I'm working on at the moment takes place on "twin worlds." I've been working with the idea that each planet has its own solar system, each with its own Sun and moon, but recently I've grown to like the idea that my world is a "double planet," where each world is essentially the other's moon. With such similar size, both planets would have to always have one side that always faces towards the other planet. This means that either each planet takes up a huge amount of the sky of the other, or both planets have day/night cycle that are several days to over a month long. Even though a world with month-long days would be interesting to write, it's not what I want for this story.

The problem is that my plot revolves around the idea that the two worlds used to be connected by a bridge, and that the existence of the other world has largely been forgotten or dismissed as legend. With the other world so close, things like weather and landforms would be easily discernible, and astronomers would be able to see the other world "shift" across the sky is they traveled, revealing that both their own world and the one in the sky are spheres. It wouldn't take too great of a logical leap for them to connect the numerous stories involving the other world with the giant ball floating in the sky. With all that, I'm not certain that I can convincingly say that the second world has been reduced to legend across the entire world.

I say "world" so many times. Sorry if this is confusing to read.

 

[Jerseydevil]

I know little about astronomy, but I am under the impression that two worlds of similar size would have gravity that would attract each other and collide. Centripetal (centrifugal, whatever, I'm a historian, not an astrophysicist) force may keep them apart, but that would only work if one body is much smaller than the other. Also, orbits are not a constant distance, and even if the tech is available, a bridge would break easily. It's a cool concept, one that I can suspend my disbelief for, but there are people who are way more knowledgeable than me who would have problems taking something like this seriously. Just a thought.

Perhaps the second world had a civilization, and now that they are separated, has become something of a legend. The world itself is real and known, but no one knows what is there. After Rome fell, travel and communication effectively ground to a halt and Europeans had no idea about sub-Saharan Africa and were shocked to discover dark skinned people living there. Maybe an Atlantis like legend sprang up after the worlds split.

Just my three cents (used to be two, inflation is a terrible thing).

 

[Penpilot]

I'm not an a physics expert, but I do have a science degree. It's in computer science, but I had to do all the first and some second year science classes, so I understand enough to get myself into trouble. But anyway.

Your idea is cool, but to my understanding any system containing more than three stars is going to be highly unlikely and pretty unstable, with unlikely and unstable increasing exponentially with each star. Check out this game called Super Planet Crash. It's an offshoot of software that astronomers use and will show you in a simplistic manner what happens when large bodies and their gravity mix.

Here's a link that describes what it is. Orbital physics is child's play with 'Super Planet Crash' And here's a link to the actual game. Super Planet Crash - Can you feel the gravity?

BUT, this doesn't have to stop you from creating your universe they way you want. There are many-many great stories that ignore science for the sake of cool. So don't let this deter you. Just be aware of it.

As for your binary planet system, the planets wouldn't orbit each other. They would orbit the binary system's center of mass, which given similar sized planets, would be at roughly the halfway point between the two planets. If you imagine a barbell with the two round spheres at the end, and you balance that barbel by the connecting bar and spin, that's how the binary planet system would orbit.

One way for you to obscure one planet from the other is with distance. Make their distance such that they appear as nothing but small dots.

Another way is place something opaque between them at the center of mass like a gas cloud. And maybe from time to time every hundred or thousand years, something happens to that gas cloud that makes it possible to see the other planet.

As for your bridge, what kind of bridge are you thinking about? If your thinking a physical bridge, without magic or hand-wavium, I don't think that's likely no matter how close the planets are.

If you're using magic or magic-like technology, you can do what ever you like with bridges. Like I said, you don't have to be scientifically accurate.

 

[ThinkerX]

There are many stable multiple star systems. Some of these systems have planets. Usually, the smaller star has a highly eccentric orbit about the first. In some cases, the orbital period can be measured in centuries or even millennia.

As to the double planet idea, that is feasible - barely. I read an old SF series featuring such a pair where the inhabitants traveled between the worlds via balloon.

 

[pischtoph]

I have no degrees, but here's my opinion. First there are tons of manga and anime that have setting closely to this and few are quite popular so don't worry. Second, for why they can't see it maybe some force slowed one planet or even stopped it for a few, which is why the star bridge faded! Anywho... Good luck!


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[Vaporo]

Well, my bridge isn't a physical bridge. It's more like a magical portal that instantly transports people from the surface of one world to the other. I should have made that clear.

As for the physics within my double planet system, I'm aware of how barycenters and tidal locking work. My question here isn't really about physics, but whether I could get away with saying that the inhabitants of my world have forgotten about the other world when it is so near and constantly visible (and with how tidal locking works, they have to be really close together). It is a fantasy story, so I'm open to magical solutions.

Speaking of which...

Another way is place something opaque between them at the center of mass like a gas cloud. And maybe from time to time every hundred or thousand years, something happens to that gas cloud that makes it possible to see the other planet.

Part of my worlds' history involves the gods creating an unbreakable prison for their enemies. One location that I've considered for the prison is at the barycenter between the two worlds. If that's where I decide to put it, there could be a massive black fog surrounding the prison to help dissuade mortals from finding and opening it by accident. So this would actually work really, really well.

 

[K.S. Crooks]

Each planet will take up little of the sky for the other, view Earthrise from NASA.

https://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=4129 The larger the planets are the further they need to be from each other. They don't need to have the same sides always facing like our moon.

Mass is the main factor to consider. If a planet has more mass than the other (more rock and less water or gas than the other) then it could be dominant and the other could be orbiting it. Set up what you want and it will probably be fine for everyone who reads your story. Everyone loved when Luke Skywalker gazed at the twin suns of Tatooine, and this was before astronomers had confirmed their existence.

 

[jm.milks]

Dual planet systems exist (there was a time when scientists around the world tried deciding whether or not the earth and the moon would be classified as one, but decided the moon was just a bit too small).

A dual planet system where both worlds are large enough to sustain liquid water without it evaporating is likely to be in tidal lock with each other (how the moon is with us) so each world would only ever be visible on half of the other one in most cases). I read that the bridge you mentioned was more like a portal, but with my last point, a form of physical bridge would be possible if constructed by an advanced race, but difficult, because orbits aren't perfectly circular... Like ever.

Also with tidal lock, it means a day only lasts as long as it takes for the two to revolve around eachother. This would have a large impact on the ecosystem, greatly affecting the evolution of the flora and fauna.

But most fictional worlds don't take evolution and physics in account, so go hog wild if you don't care too much about it.

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