One way to test it is to lay out some coins, one for each celestial body. Then plan out their rotations.

If moon is orbiting a planet, it has to circle it, and the period is going to be much shorter than one year (as is the case with our moon). If moon isn't a moon but is just another planet behind the first one, as Mars is behind earth, Kepler's law shows that its year must be longer, regardless of its mass.

However, you _can_ have a planet always show the same face to the sun, so that its outer side is always in shadow.

They even showed a model of how the planet or moon (not sure which) ends up in that situation.

Jupiter has moons with orbital periods of over 700 days.

So, if you had a Jupiter-size planet that was as close to the sun as Mars (orbital period 686 days), then it seems it could have satellites with orbital periods longer than the planet's year.

If the planet's orbital period around the sun were equal to the moon's orbital period around the planet, and the moon's orbit was not retrograde compared to the planet's orbit, and the moon's orbital plane was relatively close to the planet's orbital plane, then the moon could constantly be in the planet's shadow.

If you plotted the moon's path relative to the sun, it would look like it was an ellipse just outside the ellipse of the planet's orbit, but this doesn't violate Kepler's Law because the main gravitational influence on the moon is from the planet, not the sun.

Of course, such a setup would be very unlikely, but it seems possible.

I haven't checked your figures, but I'd like clarification on their meaning.

Of course, Taygete is tiny and has a somewhat eccentric, retrograde orbit, so it's not a perfect match. But I think it gives us a ballpark figure on how far away a moon would have to be in order to have that orbital period.

Unfortunately for my theory, it appears that the Sun's gravity would have too much effect on the moon's orbit (as TheoPhileo suggested it might) if you bring the Jupiter-sized planet into a Mars-like orbit.

Survivor is correct about creating a Lagrange three-body system. You'd have to stick the moon in L2 (the only one on the outside), but the proportions of the planet to the moon would have to be pretty big. A Gas giant would NOT develop that close to a star, though. They develop much farther out because they have to be rather cold in order to remain "gas giants." Further in and we'd at least be talking liquids, or (more likely) a smaller binary star that would be consumed by the primary star almost immediately after creation.

In general terms, yes, of course a moon could be eclipsed for a significant period of time. The "Dark Moon" is a staple of Science Fiction. There are hundreds of stories -- and even several movies -- which involve the "extensive eclipse" concept.

Just be sure you've taken temperature into account. Dark = cold, and the further out your planet/moon system (farther = slower orbit), the colder the moon would be. Asking life to survive unassisted gets harder as you move out from the star. And for life to develop without a star's energy? The planet would need heat energy from somewhere. Possibly volcanic life would be possible. Etc.

Many questions, many answers. If this is sociological Sci-Fi, though, you can do whatever you want... as long as the explanation you use makes sense. If this is Hard SF... plot an orbit using known physical laws and tailor your story to work within this framework. If you are deficient in this area (i.e. physics), ask a professor at a local university to help you. They usually will, if you're nice about it.

Good luck to you, and ask for readers when you're done! (I'm already intrigued.)

~MR

[This message has been edited by MichaelCReed (edited July 23, 2005).]

Could it be possible for a planet like Earth to be following a planet like Jupiter around the sun so Earth never has sunlight?

The gravity of Jupiter is just enough to pull Earth along with its orbit but not enough to claim Earth as its moon.

In other words: Langrange points are natural, stationary places created by all orbits where an object of insignificant mass would remain stationary. (Relative to the position of the two significant masses. Relative to everything else, it would be in constant but proportional motion.)

In other words: Yes, an Earth-massed planet could follow a Jupiter-like planet exactly as described above -- if the proportions were correct, and if it were far enough from the star for a Gas Giant to actually exist. (Which would likely make the Earth-massed planet very cold, unless it got its energy from somewhere else.) Really though, the higher the masses, the more problematic this gets.

::Shrug::

Fascinating, like I said.

Well, I guess you could make the planet have a thick atmosphere that allows no light down to the planet, but then how would plants grow? Or maybe it's an alternate dimension where darkness is like light.

Remember, energy radiating from a star is neither created nor destroyed, it can only be refelected or transformed. If the gas giant is really darkin color, it will absorb light and heat from the star and constantly radiate heat in all directions. That could help keep the little moon habitable, but the temperature would not be stable enough to support life.

Here's a suggestion that might give you what you need: put a ring of dark matter around your star right in the plane of the ecliptic. Make your story's rules such that light goes into dark matter but heat comes out. We know very little about dark matter, so readers will accept anything that doesn't violate a fundamental law of physics.

Since 1993 (when we first started discovering extrasolar planets), we've mostly been finding Gas Giants around other stars because extremely high-mass planets are pretty much all we can see (a limitation imposed by how we look for them.) Mostly we use "Radial Velocity" to detect them, essentially just a deduction from the perturbation of a given star's spectral lines (because of the Doppler effect), i.e., how unevenly the star moves toward or away from Earth. The unevenness is thought to be caused by big planets (i.e., Gas Giants), and they have to be somewhat close because we can't see them and because of how much they affect the parent star.

Yes, Survivor, most of the Gas Giants we've discovered have naturally been closer to their parent stars than say Jupiter is (contrary to previous theories of planetary formation) -- but the closer they are, the hotter they get. There is a suggestion, in fact, that they are not Gas Giants at all but a completely different kind of planet that does not exist in the Sol System. One we aren't familiar with. We call them "Gas Giants" at the moment because planets need serious mass to be discovered by Radial Velocity at all, and it is unlikely that a rocky surface could be supported on planet large enough to be detected (though this has been suggested).

Anyway, the closer a "Gas Giant" gets to a parent star, the less, well, "gassy" it gets, and I don't think a true Gas Giant could form close enough to a star for a satellite in L2 to be warm enough to support life. I suppose this depends on the constituent elements, but an atmosphere made up of known Gas Giant elements would almost certainly start to condense at a certain distance (Certainly at a distance close enough to have a Lagrange point at a radial distance equal or near to Earth's.)

But a planet of some kind could, given proper proportions to eclipse the moon.

As for the occlusion point being at L5, well, I'm not really sure what is being said there. There are exactly 5 Lagrange points in any two-body system, and if you look at is as a clock with the star in the center and the planet at 3 o'clock:
L1 = just left of 03:00
L2 = just right of 03:00 (The eclipse point. Naturally, we have an observatory in ours.)
L3 = 09:00 (opposite the planet)
L4 = 01:00 (60 degrees ahead in the orbit)
L5 = 05:00 (60 degrees behind in the orbit)

All points are directly exposed to stellar radiation without interference from the planet except L2. We have a solar observatory in L1, in fact (the obvious place).

You can find pictures of these points all over the place on the web.

Anyway, a backdrop that can cause this much comment and thought is really a nice idea. I like Milieu that elicits discussion, a lot. Hopefully you'll get something out of all this back and forth. Again, if it's an important point I urge you to get professional assistance. Then, if it's wrong... you can blame the professional. lol

Technical mistakes pop up all the time in Science Fiction. Did you know the Earth rotates the wrong way at the beginning of Ringworld? (First edition only, they corrected it in subsequent editions after an irate fan pointed it out.)

~MR

With a Jupiter-sized planet in Mars-like orbit, the L2 point would be about 16 million km. At that distance, the umbra would be even larger than it would be at 22.5 million km, which I used in my estimate above.

The effect of heat on a small gas giant can be significant, but as the gas giant gets larger, it becomes less important. Jupiter actually generates most of it's own heat, for instance. And stars, if considered as special cases of gas giants, manage to hold themselves together quite well.

That gives rise to another question. Why does this planet or moon or whatever have to be in the shadow of another planet? Why not just have it orbiting a very dim star, like an M5 or darker? It could be warm enough for life, but there wouldn't be much visible light even in the middle of the "day", such as it would be.

Of course, talking about visible light as opposed to infrared radiation gets us onto the topic of what qualifies as "dark" to the inhabitants of said planet. But then again we're probably talking about humans that are living on this planet/moon/whatever for reasons of their own.