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Suns are stars. They are orbited by planets, and sometimes other stars. Planets are orbited by moons. Reasonably advanced civilizations may launch artificial satellites into orbits around their planet as well. Light and energy in a planetary system mostly come from its central star, which is powered by nuclear fusion of the lightest elements. Due to revolution of celestial bodies, planets often have regular seasonal and day-night cycles. Life that has developed on a planet depends on those cycles. I’d like to alter one of these fundamental concepts, making the system geocentric instead of heliocentric:

Would it be possible to have a star-like satellite orbiting a “class M” planet?
In other words, how can I light up a moon? (This seems to have been done before, but with handwaving/unobtainium, by Clarke in The Sands of Mars “Project Dawn” for Phobos and by Pohl/Kornbluth in Wolfbane.)

Since the discs of our Sun and Moon appear to be about the same size in the sky, imagine Moon would not reflect but emit light and possibly would revolve quicker around Earth (“1-day month”).

This light-emitting moon would replace a proper sun, if orbiting a rogue planet, or would augment a sun that was too far away, i.e. the planet is outside any normal habitable zone of a usual planetary system.

An array of smaller satellites would also be acceptable and they may be artificial if necessary.

Problems

Regardless of energy source, I expect radiation to be a problem, even with a massive atmosphere.

Heliocentric systems are natural, because the smallest stars are still larger and more massive than the biggest planets: OGLE-TR-122b seems to be the smallest known star, which has about the size of Jupiter but 100 times its mass. The largest discovered exoplanets have 2 or 3 times the radius of Jupiter.

I think the large mass is required to make constant, stable hydrogen fusion possible. So that one is most likely not an option.

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    $\begingroup$ Related: Can a planet realistically have multiple suns? and How can I set a moon on fire? $\endgroup$ – Crissov Jan 30 '16 at 11:26
  • $\begingroup$ See also astronomy.stackexchange.com/questions/8396/…. $\endgroup$ – HDE 226868 Jan 30 '16 at 16:32
  • $\begingroup$ Thanks, @HDE226868, so like I assumed fusion-powered mini-suns are impossible, but maybe there is another way to have a bright, non-reflecting moon. $\endgroup$ – Crissov Jan 30 '16 at 17:25
  • $\begingroup$ To pass the requirements for geocentrism, we need to completely cancel the parallax effect when viewing the rest of the universe, right? Or do we need there to be other planets that also orbit this planet? $\endgroup$ – sh1 Jan 30 '16 at 19:39
  • $\begingroup$ Other stars and planets don’t matter, @sh1. Supporting a geocentric world view on the planet is not the goal of this question. $\endgroup$ – Crissov Jan 30 '16 at 23:28
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While I don't really have the complete physics background to back this up, an interesting thought for a geocentric system might be a binary star system. Imagine a pair of fairly equal mass stars in a very low eccentricity orbit around each other. Both of these stars would actually rotate around a point in between them, where you could emplace your much smaller planet right on the point of rotation, and you effectively have a pair of suns orbiting your (probably very hot) world.

Day on this world would, of course, be effectively endless (barring some other satellite giving you temporary reprieve in the form of an eclipse) so you would probably want to set these stars further away from your M class planet than the normal Goldilocks zone to cool things down a little bit. Depending on the distances in question and whether or not you include other satellites, the planet might also be tidally locked with its suns, giving a band around the middle (effectively the prime meridian) where things are a bit cooler by virtue of having more indirect sunlight

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  • $\begingroup$ I think that's L1 you've used for your planet, right? Does this imply that you can have objects at L4 and L5 that subjectively appear to also be orbiting the planet, or do those disappear when both major bodies are the same size? And can we replace one star with a black hole of the same mass? $\endgroup$ – sh1 Jan 30 '16 at 19:48
  • $\begingroup$ @sh1 sort of, the intent was that the planet be in the barycenter of the system, which I guess in the case of relatively equal masses would also be L1 (can you tell I'm not an astronomer? :) ) along those lines, it does seem that you could have objects at L4/L5, which would be at about halfway along the orbital path between the two bodies. As to the black hole, sure, why not? it's just a mass $\endgroup$ – JRaymond Jan 30 '16 at 22:26
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    $\begingroup$ Unfortunately, it's not a stable orbital configuration... any perturbation from the center, and the planet, no longer in equilibrium with its surroundings, falls straight into the nearest sun. Artificial intervention (in the form of a massive, powerful, steerable moon) would be required to keep it centered, and it's hard to imagine how it ever got to be there in the first place... $\endgroup$ – Joanna Marietti Aug 26 '16 at 3:53
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There are two issues with the reflecting satellite. I'll illustrate them with the Earth-Moon system, which is a good one since Moon is, for what we know, an exceptionally big satellite.

  • albedo is the proportion of light that the body will absorb and won't reflect. Since it is variable, let's say your satellite is a perfect mirror and will reflect all of the received light (the Moon's actual albedo is 0.136).

  • the second one is that your satellite will have only a fraction of the surface of the planet. Even at the same distance from the Sun, it means only a proportional amount of sunlight will be collected. Wikipedia gives us that Moon surface is 0.074 of the Earth one, so the maximum total light the Moon can reflect is 0.074 of the total light that arrives into Earth. Which is not a small amount, but would certainly fall outside the "sun" range.

  • what is worse, the light will be scattered by the moon back towards the space. From the Moon, Earth is a 2°*2° target in a sky that is 180°*180° wide and long. So, about 1/8100 of the light received by the Moon (that remember, is 0.074 of the light received by Earth).

What conditions would improve your moon? A higher albedo, a bigger satellite and closer to the planet (so the planet is apparently bigger) would help, but while they could make nights less dark, I do not think no combination of them would be enough to get a "sun" out of a satellite. The only alternative left would be an artificial object, which could be made as to avoid scattering of the reflected light (but would be impossibly large by our current standards).

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  • $\begingroup$ Um, I would prefer a “moon” that does‘t reflect light, but emits light by itself. It doesn’t have to be fusion-powered, but should be long-lasting. It may require maintenance to do so (if artificial), but preferably was fully autonomous. $\endgroup$ – Crissov Jan 30 '16 at 15:14
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Nuclear light bulb. The idea is that a radioactive plasma is contained within a vessel. The idea was originally proposed as a type of rocket engine (the heat from the vessel being used to drive the propellant), but I think it could live up to its name in a more literal sense - why not? Throwing out the space ship definitely simplifies the engineering.

A simple concept would be to stuff a lot of long-lived radioactive materials inside a large tungsten shell. The radioactive decay will heat up the tungsten - and as we all know from those obsolescent incandescent light bulbs, sufficiently hot tungsten produces a nice, white glow. It's not very efficient, but with the right kind of material powering it, it could still glow for a very long time.

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The most efficient type of energy generation we currently know of is antimatter followed by fusion. As you said the amount of matter needed to make a natural self sustaining fusion reaction would be significantly larger than the planet itself. With a little handwavium if artificial fusion reactors where made in space then you could theoretically have a massive moon that happens to be a power station emitting light and other radiation towards the planet.

Because it is artificial the emitter could by the way of things tidally locked to the planet, so that the "sun" side or emitter side would always be concentrating all of the energy towards the planet not wasting any energy.

Also the moon would not have to be any closer or farther than the real moon because the planet in question would spin. Using the earth system as a example if the moon became like the Sun and emitted the same amount of light as the earth normally does, the day would only lengthen by 50 minutes. This is almost the same as Mars (at 39 minutes more) and would not be all that noticeable to inhabitants.

As an alternate suggestion you could have a massive focusing cluster of satellites by the closest star and beam the power to a moon at L1 relative to the star/planet system. The satellites would ideally be as close as possible to the star to reduce the total size of the satellite(s). This could be a somewhat passive system and would get around energy density problems. The concept reminds me of beam power to something like a solar sail but for a entire planet.

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    $\begingroup$ Antimatter doesn't generate energy since there are no antimatter mines. It can be a concentrated way to store energy but it is not a primary source . And what do you mean «followed by fusion»? Once you anniliate the antimatter you get gamma rays, not something you then have to fuse. $\endgroup$ – JDługosz Aug 26 '16 at 13:23
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in real life, western scientists wondered for centuries what powered the sun. Calculations showed it could burn for just a few thousand years as in biblical chronology if powered by combustion, which was not enough for the eons of the new science of geology.

If the sun was powered by slow gravitational contraction, it could shine for a few millions of years according to later calculations, but by then geologists claimed the Earth had life for hundreds of millions of years. I have read that there was once physical violence at a scientific meeting between an astronomer and a geologist because of that problem.

And finally nuclear fusion was calculated to be the way that stars shone. Now you want to find a way for moon-sized "stars" to light a planet without having the immense mass that makes natural fusion possible. Fortunately what you want is not TOTALLY impossible.

To me a vast, totally mechanized system of power generation powering lights to light the planet seems the "simplest" solution.

IMHO a fully artificial "sun satellite" would probably have to have a vast system of power generators, repair robots and automated factories to manufacture parts for generators and lights, and manufacture the repair robots, and the repair robots would have to maintain and repair everything.

And that seems to be the simplest way to go. if the Earth and moon are ejected from the solar system build a lot of fusion reactors on the Moon to power lights covering the entire surface and all aimed at the Earth to illuminate and heat the Earth.

But if you want a system that runs by itself without machinery the way a sun runs, that is a problem, since you don't want an object with the mass necessary for natural fusion.

If you only want the moon like sun satellite to shine for a measly few thousand years, the creators of it could construct it as a giant pyre in space designed to burn for thousands of years.

One way is to use fusion power to power matter transmuting devices that transmute various elements into radioactive and maybe fissionable elements. If you move enough of such elements to the moon in question they will provide enough energy per second to light and heat the planet. The radiation produced by the elements will have to be turned into electromagnetic radiation (light) and Joanna Marietti's "nuclear light bulb" suggestion shows how to do it.

Of course the more energy a radioactive isotope produces each second, the faster it usually decays to a non radioactive isotope. Thus someone considering such a system will have to calculate which elements to use and how much to have a steady light production for the necessary time since the system was created in your story.

Maybe you want the first explorers to reach the "sun satellite" to discover that the light output will soon fall below sufficient amounts and life on their planet will die, unless they can find a way to replenish the radioactive elements on the "sun satellite".

Since a rocky, Earth like planet is formed by countless collisions of large astronomical bodies, it is molten hot soon after forming. Thus your Earth like planet and any large moons would be molten hot in the beginning. But the smaller moons would cool off and form solid surfaces many millions of years before the larger planets. Thus the planets may have been able to illuminate and heat their moons, and maybe space travelers might seed the moons with life that might survive until the planets cooled.

So the moons in question would cool off and no longer naturally illuminate their planets long before their planets cool off and had solid surfaces where life might need heat and light from the moons. By the time the planets needed light and heat from molten hot moons, the molten hot moons would have cooled and no longer naturally provide heat and light.

So a highly advanced society could have artificially reheated the moon or moons of a planet so the moons would provide enough heat and light, and then seeded the planet with multi celled life forms to take advantage of that heat and light before the moons cooled off again.

It could even be possible for an extremely unlikely natural collision many millions of years after the age when they were common to reheat the moon to molten heat.

Of course reheating a moon or moons to molten heat again would be a very big task for those super advanced aliens. They would probably divert many large asteroids and comets to impact on the moon (and not on the planet, except for any comets needed to provide water and atmosphere) and reheat it. The goal would be for one impart, or many simultaneous impacts, that are almost but not quite energetic enough to shatter the moon, to provide the maximum amount of heating energy.

And the super powerful aliens might want to collide other asteroids and comets with each other to provide a new moon for the planet to replace the one that was turned into a sun.

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