# Research, I Guess

BuddingAuthor
Hi All, I Am Looking for a bit of information, as you might note my name i am At the moment attempting to write a book, and am researching for said novel, although it is Sci-Fi/Fantasy in Genre.

For The Field of Astrophysics I am asking all those with knowledge of or theories on the behavior of gravity well when, or more importantly the plausibility of two same sized masses (eg. Two Earth Sized Planets) existing in the same gravity well without collision and at a close enough range so that their atmosphere's can at some points join, even if this is not possible at such a close range information on two same or very similar mass planetoids existing in orbit of each other.

My second question is what effects would the introduction of such a mass to say earths gravity well have have on the tidal flow ie. how much higher would they become if such a a thing happened.

thank you for all your feedback, anything is greatly appreciated, i hope this is the right place to put this thread thanks again

JaredJames
Well you're talking about a binary type system. At the ranges you are talking about I doubt it's possible.

dacruick
The moon's tidal effect on the Earth is fairly substantial. Bringing an Earth sized body near the atmosphere of the Earth would be catastrophic. Depending on how fast they are rotating, probably really fast since you are talking about small distances, I would assume Earth would be completely covered in thrashing water in a day.

There would also be gravitational ramifications. I'm going to make a whole bunch of loose approximations for you here.

The Earth is about 100 times the mass of the moon. If we assume that the Earth's atmosphere is 13 times the Earth's radius, that means in a situation where gravity is amplified by the "other earth", a person is about 29* the radius of the earth away from the "other earth".
29*radius of earth is 184 964 900 metres.
The average moon distance is 382 500 000 metres, roughly twice the distance.

So we have something 100 times the mass, half the distance.

acceleration due to gravity = GM/r²
I will neglect the 'G' since it is constant, and put the ratios of your fictional scenario in comparison to the moons effects.

gravitational effect compared to the moon = 100/(1/2)². = 400.

Therefore your scenario would have tidal effects that are 400 times the severity of the tidal effects currently caused by the moon.

I hope you understand what I did, but you can ask me for clarification, or take my word for it.

BuddingAuthor
so that would be catastrophic to the the possibility of the survivors, well that does help alot thanks heaps for that would there be anyway at all that i could alter the size of the other mass to not have such catastrophic events or maybe expand its atmosphere to a distance that would be far enough away that it the gravity would not have such greatly negative tidal effects, i wish i had studied more at school lol science and all its little facets intrigue me so and i have the brain for it if i would just have the drive to apply myself to it but i am an escapist i guess reality just isn't my thing =)

dacruick
something that might work to your advantage is that there is no real way to decide how far the atmosphere goes out. You could easily say it goes out 20 times the radius of the Earth, but the results would still be extreme.

However, there is the problem that this "other earth" would never orbit around the Earth. They would both orbit around the sun. Especially as the distance from the Earth increases.

BuddingAuthor
can you think of anyway i can alter it at all to make it work as i would like to have it work what i am looking for is a two planets, both capable of supporting life, with a single moon in orbit where nothing drastic happens if i can't make it plausible i may just cry lol, only joking but you get what i am saying right, the two planets have to have at least one moment where their atmospheres connect even just long enough for it to transfer a bacterium carrying a retrovirus to enter the original planets atmosphere and then i would like them to orbit one another so once every week or so there is a long night where the sun is blocked by the "other earth" =) its sci-fi i can stretch the truth a little but i don't want to go all psychedelic and create my own reality lol

JaredJames
It's sci-fi, you can do whatever you like.

The best thing with sci-fi is to not explain anything.

dacruick
can you think of anyway i can alter it at all to make it work as i would like to have it work what i am looking for is a two planets, both capable of supporting life, with a single moon in orbit where nothing drastic happens if i can't make it plausible i may just cry lol, only joking but you get what i am saying right, the two planets have to have at least one moment where their atmospheres connect even just long enough for it to transfer a bacterium carrying a retrovirus to enter the original planets atmosphere and then i would like them to orbit one another so once every week or so there is a long night where the sun is blocked by the "other earth" =) its sci-fi i can stretch the truth a little but i don't want to go all psychedelic and create my own reality lol

yeah I understand what you're saying. its a tough one. The blocking out the sun is definitely plausible, so you don't have to worry about that. I'm just going to go through everything you said and tell you what I think has the OK and what I think is sketchy. You will obviously be the final decision in what you want to do.
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If you want two life sustaining planets that are a binary system, that means that the planets must rotate around each other. For that to happen they probably have to be similar in size. And if they are similar in size, the catastrophic events outlined above happen.

You might be better off having Earth get hit by an asteroid and then ejecting rock into space, and that rock making its way to the "other earth".

A week orbit seems pretty quick though. I'm not sure if the physics of that is possible, I would have to do some calculations to find out exactly how out of the question it is. The orbital time and the distance between the planets are dependent on each other. I guess if you give me the maximum number of moon effects you're willing to have (my above calculation gave 400), then I could figure out a period of orbit roughly.

Staff Emeritus
Gold Member
can you think of anyway i can alter it at all to make it work as i would like to have it work what i am looking for is a two planets, both capable of supporting life, with a single moon in orbit where nothing drastic happens if i can't make it plausible i may just cry lol, only joking but you get what i am saying right, the two planets have to have at least one moment where their atmospheres connect even just long enough for it to transfer a bacterium carrying a retrovirus to enter the original planets atmosphere and then i would like them to orbit one another so once every week or so there is a long night where the sun is blocked by the "other earth" =) its sci-fi i can stretch the truth a little but i don't want to go all psychedelic and create my own reality lol

With an orbit of one week, It works out that the "other Earth" would block out the Sun for a maximum of ~1.5 hrs.

Also, the distance apart the planets need to be for a one week orbit is ~ 1/2 the present Earth-Moon distance. Since tidal forces decrease by the cube of the distance and increase directly by the mass involved, and the Earth is 81 times the mass of the Moon, the tidal forces would be 81x2^3 = 648 times stronger than what we presently experience.

This causes a problem when one talks about the planets "getting close enough for their atmospheres to touch". Practically speaking, the upper limit of the atmosphere is the top of the Exosphere, which extends to an altitude of ~1000 km. For two planets to exchange atmospheres, the planets have to pass close enough for the exosphere to extend past what is called the Roche Lobe. For two Earth massed planets, this means that the centers of the planets would have to pass about 3.5 planet radii apart. But such a close approach would cause immense stresses on the Earth's crust ( a little over 1 more planet radius closer and the planet would start to break apart.)

Another problem is going from such a close approach to an orbit such as you mentioned.

dacruick
Also, the distance apart the planets need to be for a one week orbit is ~ 1/2 the present Earth-Moon distance. Since tidal forces decrease by the cube of the distance and increase directly by the mass involved, and the Earth is 81 times the mass of the Moon, the tidal forces would be 81x2^3 = 648 times stronger than what we presently experience.

Janus is right about the tidal forces, I forgot it is an inverse cube. so make that 400 that I had earlier into an 800. I don't see a way that you could have two earthlike planets that close to each other without seriously breaching the laws of physics. I understand what you are trying to do. If I were reading your book and you made claims like that, you would probably lose some credibility.

BuddingAuthor
okay i know this is probably beginning to annoy some of you as i am probably sounding like an ignorant fool...

but i have another crazy idea lol

solving the tidal problems i hope, a transfer of water and such at the topmost and bottom most points depending on the season or would this just be something even more stupid than before, thanks to all the replies and help

dacruick
okay i know this is probably beginning to annoy some of you as i am probably sounding like an ignorant fool...

but i have another crazy idea lol

solving the tidal problems i hope, a transfer of water and such at the topmost and bottom most points depending on the season or would this just be something even more stupid than before, thanks to all the replies and help

yeahhhh thats not going to work. It would be an inconceivable amount of water, and it would have to be transferring all of the time considering the period of rotation of the binary system being about a week.

JaredJames
Look, it's sci-fi. You can literally do whatever you like. So do it.

Now, the worst thing you can do with sci-fi is try to explain the physics. The moment you do that things get messy. Avoid detailed explanations (well, any explanations really) and just say what occurs.

If the atmosphere's meet at some point, so be it. Just ignore the 'how' question.

Attempts at explaining the science behind sci-fi have killed films in the past.

BuddingAuthor
thanks again your right, the only reason i wanted to do it is so that people can pick it up and feel it could really happen but i guess thats what sci-fi is lol, tanks again for all of your help