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Author Topic: User tutorial: Understanding Planets in Binary Systems.  (Read 10045 times)

Omnigeek6

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User tutorial: Understanding Planets in Binary Systems.
« on: May 01, 2011, 11:05:25 PM »
This isn't as much a tutorial as some helpful information that will let you more accurately figure out what life would be like on that planet you made with two suns. I will not tell you how to create a stable binary system: that has already been covered in multiple places, so read some threads and ask around.

At this point, I assume you know how to make a binary star with planets.

All right, there are two basic types of binary planets: ones that orbit both stars in a system (This is called a "P-type" or circumbinary planet), and those that only orbit one of the stars, with the other much farther away (This is called an "S-type").

For P-types, there actually isn't that much to think about. Angularly, the two stars will be very close together in the planet's sky, and day-night cycles will be normal. However, the stars may eclipse each other. Also, make sure you use the combined luminosity of the stars to determine your planet's temperature.

S-type binaries are much more complex. I will tell you right off, you will not get a planet where the sun never sets. Conditions on an S-type binary will be dependent on the relative orbital positions of the planet and the more distant star. For example, when the second star is in opposition, the planet can theoretically experience continuous daylight. However, because the planet's axis will probably be tilted, and it will have a different orbital inclination and argument of periapsis, in practice this will most likely only occur in one hemisphere. If the planet is tidally locked to its star, of course, the "dark" side of the planet will get constant sunlight in some parts of its year.
Now: This constant daylight will not last forever. For example, assuming no axis and orbit tilts, a planet would only get perfect 24-hour light for one day (let's call it the summer solstice, since for inhabitants of this planet summer would be the season of longest daylight.) However, on the equinoxes, the planet would get the equivalent of 18 hours of daylight. In the "winter" both stars would be visible at the same time, and daylight would be similar to that experienced on a planet orbiting a single star.
Note: The chances of one star eclipsing the other in an S-type binary are virtually nil.

Now, here's something else to consider: the comparative sizes and distance of the two stars.

If the planet's parent star is the brighter of the pair, or the other star isn't that much brighter, the more distant star will actually provide only a very small amount of the total light and heat the planet receives. For example, if Jupiter was magically replaced by a star as hot and bright as the sun (Jupiter is at about the minimum distance for a secondary star the same mass as the primary if the planet is to have a stable orbit), Earth would only get as much light from Jupiter as the real Jupiter gets from the sun right now. Consider the temperatures on Jupiter's outermost large moon, Callisto (which doesn't get as much tidal heating as Io or Europa). Pretty cold, right? That's how much a second sun could warm Earth. If the second star was a red dwarf, or was further out (Quite common), it would heat Earth even less. 

However, while this extra light wouldn't be enough to raise planetary temperatures, or allow photosynthesis during what would normally be night, it might be enough to see by, meaning that diurnal animals could become active for a much longer part of the day, while nocturnal animals might have a tough time of it. Thus, animals on an S-type planet might have circadian rhythms with seasonal variation.


Now, while most S-type planets might be like this, there is one special case.

Consider a star the size of the sun, with a 0.25 solar mass red dwarf orbiting it at about 1.2 Au in a roughly circular orbit.

Now have a planet orbiting the red dwarf at 15 million km (ought to be plenty stable).

This planet will probably be tidally locked. So, the "light" side will receive about 1 sun's worth of light. The "dark" side will only get light (1 sun's worth, from the larger, but more distant star) during half of its year. Luckily, the planet in this scenario will only have a year 23 days long, so both sides of the planet would be habitable.

atomic7732

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Re: User tutorial: Understanding Planets in Binary Systems.
« Reply #1 on: May 02, 2011, 07:05:44 AM »
Nice informational article thing. But on your last paragraph, I don't think it'd be as much "habitable" on both sides than on just one (the one facing away). Also, what that is assuming is it doesn't have an atmosphere, and if I go much further into this I think someone will have to split it off into "User tutorial: Understanding Planets Tidally Locked to Their Stars".  ;D

Omnigeek6

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Re: User tutorial: Understanding Planets in Binary Systems.
« Reply #2 on: May 02, 2011, 02:01:16 PM »
Nice informational article thing. But on your last paragraph, I don't think it'd be as much "habitable" on both sides than on just one (the one facing away). Also, what that is assuming is it doesn't have an atmosphere, and if I go much further into this I think someone will have to split it off into "User tutorial: Understanding Planets Tidally Locked to Their Stars".  ;D

If the planet has an atmosphere capable of circulating heat, temperatures on the day side will remain in the habitable range, right? Don't forget, analogs of Earth's thermophiles could survive right up to boiling point.

Normally, the "dark" side of the planet would, although possibly warm enough for some organism to survive, have no light for photosynthesis, and thus only be able to rely on chemoautotrophs, but with light coming in from the secondary star, organisms of some type would be able to survive over most of the planet's surface.

Joshimitsu91

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Re: User tutorial: Understanding Planets in Binary Systems.
« Reply #3 on: May 03, 2011, 09:30:27 AM »
S-type binaries are much more complex. I will tell you right off, you will not get a planet where the sun never sets.

I have made a system where there are two equal mass stars (both equivalent to the "sun") with the Earth at the Barycenter, does this count?  :P

Omnigeek6

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Re: User tutorial: Understanding Planets in Binary Systems.
« Reply #4 on: May 03, 2011, 02:02:04 PM »
S-type binaries are much more complex. I will tell you right off, you will not get a planet where the sun never sets.

I have made a system where there are two equal mass stars (both equivalent to the "sun") with the Earth at the Barycenter, does this count?  :P

Sorry to break it to you, but that system is unstable. Any small perturbation would result in Earth falling towards (and possibly colliding with) one star or the other.

Joshimitsu91

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Re: User tutorial: Understanding Planets in Binary Systems.
« Reply #5 on: May 03, 2011, 03:08:55 PM »
S-type binaries are much more complex. I will tell you right off, you will not get a planet where the sun never sets.

I have made a system where there are two equal mass stars (both equivalent to the "sun") with the Earth at the Barycenter, does this count?  :P

Sorry to break it to you, but that system is unstable. Any small perturbation would result in Earth falling towards (and possibly colliding with) one star or the other.

Well yes it is unstable in reality, but in the simulation where there is only 2 stars and a planet it seems to stay in the centre indefinately (as long as the time step is relatively small). Both suns are of equal mass and velocity etc so the RK method will always give them the same trajectories (i think). I also managed to make a quad star system that formed a nice path shape, but i've yet to experiment further with such a system :)

FiahOwl

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Re: User tutorial: Understanding Planets in Binary Systems.
« Reply #6 on: November 25, 2011, 03:49:33 PM »
This inspired me. I'm going to write a ton of these things.

smjjames

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Re: User tutorial: Understanding Planets in Binary Systems.
« Reply #7 on: November 25, 2011, 04:25:08 PM »
You know, I've gotta correct the OP on the eclisping of S-type binaries, unless the planets orbit is inclinated where the second star won't get hidden by the planets primary, you could get a situation where the second star is on the opposite side of the parent star at that point in the planets orbit. Of course though, the positions and inclination have to be just right.

It is true that you won't get an eclipse of the planets primary in an S type.

As far as nocturnal animals living on such a planet, well, they'll have evolved in those conditions.