Consider a rogue planet, moving in a region of complete void absent any type of star (they all burned out). It is orbited by an artificial moon which shines and serves as the planet's sun. However, it cannot glow constantly, because the energy it gathers (through unknown means involving alternate dimensions, so basically it's free energy, but with limited bandwidth) is not sufficient.

So I thought about a cycle: the moon starts black and then begins to glow faintly (appearing as a full Moon on Earth) for a few hours. Then the glow rapidly increases (in about one hour) to the apparent luminosity of our Sun. It stays like this for a few hours. It turns off in about an hour, returning to the "moon" state. The entire cycle takes two lunar revolutions. Meanwhile, the planet rotates every 36 lunar revolutions.

The planet should be inhabited by a 20th century level civilization and rich fauna and flora. Could this system work? How should I tweak the details - like moon distance, maximum luminosity, and axis/orbit inclination - to optimize the presence of complex life?

Also, is there any software capable of simulating such a scene?

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    $\begingroup$ You could make the job a lot easier for yourself (as worldbuilder) and for your future readers (who have to understand this setup), if you made the moon's orbit geostationary. That way there would be a circle of land in the center of your continent which get the best sunshine (directly under the moon) which would be used for agriculture, then rings of urban land rippling outward where the light gets increasingly dimmer... and the rest of the planet cold and light-less, but with a breathable atmosphere. A twilight world. $\endgroup$ Dec 2, 2017 at 17:08
  • $\begingroup$ @HenryTaylor Wouldn't that come with many of the same caveats as any other planet which is tidally locked to its star? $\endgroup$
    – user
    Dec 2, 2017 at 17:25
  • $\begingroup$ @FrancescoManzali, you hadn't completed the description of a moon's activity cycle. How long does it stay in a "black" state? It will affect the amplitude of temperature variations and distribution of climate zones around the planet. $\endgroup$
    – A.C.
    Dec 2, 2017 at 17:39
  • $\begingroup$ @MichaelKjörling, absolutely true, but given the shorter distance between light source and planet surface, I think the transition from sunshine to shadow would be significantly sharper. Using our solar eclipse numbers, the "high-noon" sunshine agricultural zone would only be 160km across. I not a good enough mathematician to figure it out, but my gut tells me that a closer light source reduces the size of the lighted zone, leaving more of the light facing hemisphere in a twilight angular relationship with that light. $\endgroup$ Dec 2, 2017 at 17:42
  • $\begingroup$ @A.C. I'm thinking about something like 18h for a full revolution around the planet. So it'll be 1h (transition from "moon" state to "sun") - 17h (sun state) - 1h (transition from sun to moon) - 17h (moon state). I would maintain a simmetry for sun/moon state, but the actual values may change. HenryTaylor It certainly helps for the atmosphere I'm trying to build, so I agree. $\endgroup$ Dec 2, 2017 at 17:48

3 Answers 3


On a website dedicated to wild and crazy ideas, this one is a humdinger. I like it!


  • In the absence of a central star, the world would eventualy have a rotational period equal to the lunar orbital period. (This might be true even with a central star... but at least there's something injecting energy into the system to keep it going.) This results in the geostationary orbit discussed by @HenryTaylor in his comment (what Henry thought of as a simplification is actually what would happen). Here's the twist, if the rogue planet was "spun away" from its parent star "recently" (I'm going to ignore what "recently" means in astronomical terms), then you would still have a difference between the two periods. The longer the rogue is away from its parent, the closer to a geostationary orbit. I'm assuming t's fairly "recent" so the difference still exists. Should the time come when the moon and planet lock up their periods, you'll have daylight on one side, no light on the other, making the other basically uninhabitable (too cold).

BTW, you could have the rogue pick up the moon as debris from its travels, but that begs the question where all the happy earth-like life came from? Could the evolutionary sludge of life have existed in the cold of space for however many eons the rogue wandered the lifeless void? It really complicates the backstory. So, I'm assuming the spin-off backstory.

  • I'm going to hang on to your orbital and rotational periods for as long as I can. The Earth rotates once per day, Luna orbits the Earth once every 27 days. Scaling the issue, your rogue rotates once each day and its moon orbits 36 times per day. That's an orbital period nearly 1,000X faster than our Luna. (Granted, I'm cheating for dramatic effect. Without knowing the difference in rotational period between your rogue and Earth, the calcuation is presumptive. But it is very dramatic....)

However, now that I think about it, this really isn't the primary issue. Without a point of references (big ol' sun in the sky, or at least some stars), the rotation of the planet is entirely irrelevant. If we maintain that a "day" is one period of dark and one period of light, then a "day" is two orbits of your moon at 18H per orbit or 36 hours (which, according to Men in Black can cause the entire population to experience a psychotic episode).

  • For the convenience of this discussion, I'm going to assume the moon has no inclination. This means it's always right over the equator. @A.C.'s answer points out the biggest problem with this assumption: massive polar regions. However, adding an inclination to overcome this has another problem: reducing the overall lighting affect on the planet. This whole situation is just plain ugly and will likely be the cause of saying this can't work, but we'll see.

People would likely die from epilepsy (I'm kidding, but look at that graph!)

  • You now have a rogue world with an equatorial band of life-giving warmth and very large polar ice caps. Your "day" is 36 hours long. Your "great day" is 648 hours long. During most of that time you're going to see pulsating light to one degree or another. Here's a chart of how much light a person would see if they stood on one point along the equator for an entire great day:

enter image description here

Another way of looking at this is that it's "high noon" at one longitude only once every 648 hours. This is nothing more than the product of three sine waves: planetary rotation, lunar orbital period, and illumination cycle. It does not take into account refraction through an atmosphere (which would heighten the backside illumination pulses).

Life would, as at all times, adapt or die. I have no idea how such a light source would ultimately affect anything.

But what about inclination?

Ultimately, what this does is widen the habitable band around the equator at the expense of lowering the average amount of light seen equatorially at one longitude. Your world would be colder, but there's more world (theoretically) to inhabit. I'd almost say crank up the heat and use a high inclination... but during those high pulses the world would be HOT! In the long run, I don't think an inclination does anything but harm the chances of life.

And what about the weather?

I'm not able to easily imagine this, either. You'd have very calm, honking cold weather around the very large polar caps, but you'd likely have serious wind around the habitable region with that pulsing light pushing it along like a combustion engine. If the habitable land has a high percentage of water, you'll get serious storm buildup. This wouldn't be an easy world to live on...


But I think it could be habitable, with complex flora and fauna. But it would be a miserable life compared to the eden of a planet in orbit around a decent sun. Natives would love it and think the sunlings weak wossnames, while the sunlings would never visit such a planet but for a day to "experience" it.

I forgot to explain what back-side illumination is: it's the illumination seen by that one point of longitude when the moon is illuminating the other side of the planet. This is because, starting with high-noon at our reference point, nin hours later the moon is on the other side of the planet at 50% illumination. Nine more hours and it's back over the reference with 0% illumination. Nine hours again and it's the backside with 50% illumination, etc. (This example assumes no rotation of the planet. The graph does.)

Edit: If we're trying to replace the earth, then we need "solar" radiation equal to 1,000 W/m2. The earth is 510 million Km2 for a total energy requirement of 1W/m2 * 510x1012m / 2 = 255 Terrawatts continuous power. (Yes, I didn't bother with the curvature of the Earth reducing the total absorbed power... but it won't the last simplification, either.)

The RMS value of the normalized impact of Sol on the Earth is 71% (I skipped a couple of steps to explain where the % comes from, but it belongs there). The RMS value of the curves in my graph is 25.66%. That means the moon must generate 2.77X the power delivered to the rogue planet the Sol does for Earth.

The problem is, That's outside the max 1.5X habitable zone. (Click here for appropriate newscast.)

So, our fist solution is to reduce the peak solar impact to 1.5X. That gives us an adjusted ambient power delivery of 54% Sol average.

Eureka, the habitable zone!

So, our world will be colder, but still habitable and not suffering from the snowball effect. Deserts are dry, but not hot. The temperate zone is smaller. The rogue planet happens to be the same size as the Earth... but who's counting? But we're alive and enjoying the ride.

Getting the overall energy absorbtion closer to 100% of Earth norm requires changing the duty cycle of the moon illumination cycle. I don't know if you want to go into that much detail (you certainly needn't).

The power needed to move the moon around is trivial compared to the power needed to energize a habitable planet, so you can ignore the problem of geosychronous locking.

  • $\begingroup$ Supposing the moon has some kind of active propulsion (as it's an artificial construct) could the configuration be maintained, avoiding entirely the tidal lock? I imagine this system as a lone vessel in a dead universe, where the last stars have died millions of years before, so that "moon" is the only thing capable of maintaining civilization alive (and it's constructed to do this in an efficient manner). The energy is given from a mechanism involving alternate dimensions, so we can suppose it will never run out. Also, would the rise in albedo from the frozen poles cause a snowball scenario? $\endgroup$ Dec 2, 2017 at 23:04
  • $\begingroup$ @FrancescoManzali, If you power the moon and your society has the tech to both power it and illuminate it, then there's no need to ask this question. Set the moon in an orbit that would duplicate the normal diurnal period and far enough back that you can illumine at least 50% of the earth's surface with adquate heat to avoid massive polar caps. You now have enough Clarkian Magic to do what you want, when you want, negating the purpose of the question. As for albedo, there is always a tipping point. So, "yes, if the ambient heat drops far enough." $\endgroup$
    – JBH
    Dec 2, 2017 at 23:50
  • $\begingroup$ By the way, "tidal lock" isn't the right term. All that means is that one face of the moon is locked toward the planet (the moon's rotation is synchronously locked to the planet's rotation). "Geo lock" or "geosynchronous lock" would be better terms (although it might not be the official term). This is where the orbit (not the rotation) of the moon is synchronously locked to the rotation of the planet. You actually want a tidal lock so that you need only illuminate one half of the moon, thus saving energy. $\endgroup$
    – JBH
    Dec 2, 2017 at 23:55
  • $\begingroup$ I suppose the energy is limited in bandwidth (that's why the moon keeps "blinking", as a mean to save enough energy), so I'm interested in the maximum distance needed to avoid massive polar caps (and subsequent snowball earth scenario) with the minimum luminosity needed to sustain civilization. Basically, there's not enough energy to guarantee a completely Earth-like situation, and I'm interested in the most efficient configuration (less energy consuming) to sustain life in such a way. $\endgroup$ Dec 3, 2017 at 0:05
  • $\begingroup$ @JBH, how did you receive that graph? Due to "moon" phase taking less than moon orbital period, there shouldn't be intervals of time longer that said period when a person on the equator sees only "moon" state. Quite the reverse, there should be days, where he sees only "sun" and transitional states, i.e. "white nights". $\endgroup$
    – A.C.
    Dec 3, 2017 at 4:34

I calculate that the net energy received would be not much more than the amount of energy received at the orbit of Mars from the sun. At this energy level life should be possible, but it could be helped a lot by giving your planet a nice thick atmosphere with a lot of carbon dioxide or methane to provide a nice warm thermal greenhouse blanket.

Fx/Fe = (Re/Rx)^2
Solar flux received at Earth = Fe
Solar flux received at distance x = Fx
Orbital radius of Earth = Re
Orbital radius of x = Rx

Take Fe and Re as 1 and Rx = 1.52
gives 0.42 for Fx or a bit under half the energy reaching earth

I don’t see why a cold but broadly Earth like environment shouldn’t exist. The day night cycle would be similar to that in Earth’s tropics in terms of duration, but with only half of the intensity. It might even be possible to have a tropical environment if the greenhouse blanket is sufficiently good – although I have not done any calculations on this aspect.

  • $\begingroup$ When you say "I calculate" and don't show your calculations, I just can't help feeling that something is wrong here. $\endgroup$
    – A.C.
    Dec 3, 2017 at 4:42
  • $\begingroup$ @A.C. Fair point calculation added $\endgroup$
    – Slarty
    Dec 3, 2017 at 10:05

I will only do a short astrophysical review, without going into biological and sociological part of question.

The idea seems legit, but requires minor tweaks.

  1. Illumination is more or less uniform, if your moon's orbital inclination isn't extremely big. There is one transitional area, where total amount of light depends on how exactly the moon switches between modes, but this zone moves around the planet with a period of 18 days (and by day I mean two moon orbital periods). In this zone only white nights are possible.
  2. Since illumination is uniform, climate zones are distributed according to the angle of incident light, like on Earth. But the moon is very close to the planet (think about Deimos, distance should be 5-7 planet radii). If you choose zero orbit inclination, there will be regions on poles which never get illuminated. This may eventually lead to accumulation of all the water in the world in the polar caps. If you wish to avoid it, make inclination bigger. Just remember - bigger inclination means bigger temperature variations between day and night.
  3. Moon brightness can be basically at any level, but if you wish to keep the planet Earthlike, make sure it is equivalent to 0.5-2 Suns (values for the edges of Solar system's habitable zone).
  4. Moon should be very small, so as to not be destroyed by tidal forces. No tides for your oceans, alas, and one less reason for the planet to have strong magnetic field (may be a death sentence for your planet, since death of all stars should have filled the Universe with lots of radiation).

That's it for now. Maybe I'll remember something else later.

  • $\begingroup$ I'm worried about the radiation problem. Could a magnetic field on the moon mitigate the effect? $\endgroup$ Dec 2, 2017 at 23:10

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