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I have a question about how would seasons and the moon cycle be affected in a system where one planet orbits Sun #1, and Sun #1 orbits a second sun. Online I found this description:

"Type II: "Close Stars" - The suns may occupy different parts of the sky, indicating that one sun orbits the other farther out than the planet." (I had a sketch, but didn't know how to upload it)

To make it easier to explain I'm going to assume this system's Sun #1, planet, and moon are equal mass and size of Earth, it's moon and it's sun. The planet is in a habitable zone of Sun #1 but outside the gravitational pull of the second sun. So I guess my questions are:

  • How would the moon cycle be affected when the planet orbits between both suns? Would there be times when it seemed like the sun never set or would days be twice as short, because there is a sun on both sides of the planet for 3-4 months out of the year?

  • Would the same situation described above cause a significant shift in seasons(summer, winter,spring) when the planet is between two suns?

  • Does the size of a sun, or it's mass more dictate it's gravitational pull? Is it even possible for a system such as this to survive or would it eventually get stuck in between both suns gravitational pull, or switch to a figure 8 orbit, and eventually be thrown out of both suns orbit.

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  • $\begingroup$ I'm having a little trouble wording it properly. I meant to say the second star was bigger than the first. I thought, it must be in order to hold the other in orbit. When I said "outside gravitational pull" I meant the pull would not be strong enough to pull the planet away. But I now see what you are saying when it would have no real effect on it. $\endgroup$ – SunSpots Feb 19 '14 at 21:12
  • $\begingroup$ post the link for the sketch and someone here can stick it in $\endgroup$ – Jim Apr 21 '14 at 13:25
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I'm going to interpret what you're asking as follows: The moon orbits the planet, the planet orbits Sun #1, and Sun #1 forms a binary star system with Sun #2. The planet is close enough to Sun #1 that the gravitational effects from Sun #2 are small.

I believe this is a stable arrangement, particularly if Sun #2 is very far away. It's quite similar to the way that Jupiter's moons are in stable orbits, even though Jupiter and the Sun orbit one another*. In fact I believe planets have been detected that orbit stars in binary systems. (If anyone has a reference, please feel free to edit it in.)

As for what effect this would have on the seasons, that's strongly dependent on the relative brightnesses of the two stars. Since stars can differ wildly in size from one another (see http://what-if.xkcd.com/83/, as well as endless YouTube videos on the subject), it's entirely possible that Sun #2 could be the brightest one in the sky, even though Sun #1 is much closer.

It's also entirely possible that Sun #2 would be just one tiny point of light among all the other stars, perhaps even invisible without a telescope. This is probably the most likely scenario, but it's also a bit boring, so let's assume that Sun #2 is some kind of giant star, so that it's of roughly the same magnitude as Sun #1, as seen from the planet.

There are several possibilities for how the two Suns might move across the sky, which depend on the configurations of the orbits. One possibility is that all four bodies might orbit in roughly the same plane, like a giant version of our Solar system. In this case, there will be a time of year at which the planet and its two Suns are roughly in a line. At this time of year, the Suns would appear close to each other in the sky, rising and setting with one another, just like the famous scene in Star Wars**.

Half a year later, the three bodies would line up again, but this time with the planet in between the two Suns. Then they would appear on opposite sides of the sky, with one rising as the other sets. Thus it would indeed be daylight all the time, as you say.

The light from the two Suns might be different in quality, though. Since Sun #2 is a much bigger star than Sun #1, it's also likely to be hotter. In this case it might seem warmer when Sun #1 is in the sky, but brighter after Sun #2 rises. The light from Sun #1 would also be noticeably redder and the light from Sun #1 noticeably bluer.

If Sun #1 is a very dim star, it might even be possible to observe light from Sun #1 reflected from its surface. In this case it would have phases, just like Earth's moon. The full phase would occur when the two Suns are on opposite sides of the sky, and "New Sun #1" would be when they're in the same part of the sky. (It would be nice to do some calculations to see if that's actually possible. I haven't done that.)

Speaking of phases, the Moon would reflect light from both Suns. It would typically look like two moon phases super-imposed on top of one another. If two stars were different temperatures (thus giving out different coloured light), I imagine this would look really cool.

It's also possible that the planet might orbit Sun #1 in a different plane from the one in which the two stars orbit one another. Let's take the extreme case and say they're at right-angles to one another. I'll assume that the planet's axis of rotation is more or less aligned with its orbit around Sun #1.

Now we have to consider two "years": the time taken for the planet to orbit Sun #1, and the time taken for the two stars to orbit one another, which will necessarily be longer - perhaps up to 100,000 times longer. I'll call this second year the "long year" and the other one the "short year". We can imagine that a short year is something like an Earth year.

If it's the right time in the long year, an interesting thing will happen. The plane in which the planet orbits will be at right-angles to an imaginary line drawn between the two stars. This means that the planet's axis of rotation will point towards Sun #2: it will appear in the same part of the sky thought the whole (short) year, just like Polaris does on Earth. Thus one hemisphere of the planet will have constant light from Sun #2, while the other has only the light from Sun #1, which rises and sets daily.

This situation will change over the long year. A quarter-long-year later (which might be tens of thousands of short years) the Suns will be more or less aligned with the planet's orbital plane, and the situation will be similar to when they all orbit in the same plane, as described above. Another quarter of a long year and the situation is reversed, with the opposite hemisphere being exposed to constant light from Sun #2.

There are other possibilities, depending on the relative alignments of the planet's orbit, its axis of rotation, and the plane of the stars' orbit. Because these won't all exactly line up with one another, the seasons on such a planet would be complicated. On Earth the seasons are due to the tilting of the Earth's axis of rotation relative to its orbital plane. This would likely be in effect on this planet as well, but the location of the two stars in the sky would likely have a bigger effect. Whether these two types of "season" happen in phase or out of phase with one another will vary over the course of the long year.

*Jupiter is just about massive enough that the Sun can be said to orbit it, as well as it orbiting the Sun. The two bodies orbit their common centre of mass, which is inside the Sun but quite near its surface.

**In that scene the two stars appear about the same size in the sky. I think this is unlikely - as a rough guess, I'd say Sun #1 should appear larger, with Sun #2 being smaller in the sky but shining more intensely, like a tiny ultra-bright LED next to a big incandescent bulb. But it does depend on what kind of stars they are, and the Star Wars scene might be possible for all I know. If Sun #1 is large in the sky it might eclipse Sun #2 once per year when the three bodies line up.

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  • $\begingroup$ Very nice, easy-to-picture answer. However, I think you got the suns backwards in the "If it's the right time in the long year" paragraph. $\endgroup$ – Bobson May 6 '14 at 19:10
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This all hinges on how far away and how big the second star is, which you haven't said. You did say that the planet was "outside the gravitational pull of the second sun", which of course is impossible since gravitational pull extends to infinity. If you are saying that the gravity from the second star is very little compared to that of the first star, then you have a system like earth and the second star is irrelevant. For example, if Jupiter was a small star with the same mass (ignoring that fact that this may be too little mass to actually be a star) as the real Jupiter, then it would have exactly the effect the real Jupiter has on us, which is very little.

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