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.
Topic: User tutorial: Understanding Planets in Binary Systems. (Read 10007 times)
