My planet - let's call it Penumbra for now - exists at roughly 1.5 AU from it's sun - which was similar to Earth's - but is about a billion years older.

It is 15% more massive than Earth, but nearly identical in size.

It has around 85% of the atmospheric pressure.

It has an axial tilt of 13.5 degrees +/- 2 degrees, giving it minor seasonal changes.

Its rotation is equivalent to 40 hours, and its orbit is 450 of their days.

In the distant past the planet and solar system were home to a very advanced nonhuman race. As their planet underwent runaway greenhouse effects and their star began heating up, they built a large space construct at the L1 point - a magnetically stabilized plasma mirror with a frequency tuned to reflect all light above indigo-violet – designed to cool their planet. It was built to be fully automated and self-repairing. It’s powered by a ring of solar stations and has arrays capable of beaming light to Penumbra. Currently, they are tuned to emit infrared and ultraviolet light at levels necessary to maintain preset environmental conditions on the planet’s surface.

Due to this, the day/night cycle at the equator is roughly 18 hours of Truenight, 6 hours of Zenith, 10 hours of Falsenight, and 6 hours of Apogee. Zenith and Apogee are the periods where the Sun is not blocked entirely by the Mirror – Falsenight is when the mirror is blocking most or all sunlight and only ultraviolet/violet/indigo/infrared light is reaching the surface.

The race that set this up was also capable of modifying wholly or in part most species of plants, so plants that do most of their photosynthesizing during the 6 hour periods of light or have been modified to utilize UV/indigo.

Question:  Given the fiction of the plasma mirror, is this planet as described realistic?

  • Do not consider moons as I haven't settled on what moon(s) the planet would have.
  • 1
    $\begingroup$ I'd like to be able to answer, at least in spirit, however, as is said in Logic " you cannot prove that a thing does not exist" - We have no means of knowing what you may have 'overlooked'. For my $0.02 - I'd skip the Moon(s) - the orbital mechanics with your mirror would become needlessly challenging. $\endgroup$
    – Joe
    Commented Jul 19, 2018 at 18:09
  • $\begingroup$ The planet itself looks all right, however I think the plasma mirror and its effects should be a separate question (and by the way what is exactly your question?). $\endgroup$
    – Alexander
    Commented Jul 19, 2018 at 18:09
  • $\begingroup$ At first you mention that the plasma mirror reflects all light above indigo-violet, and later you say it passes ultraviolet even in Falsenight? Also, while a solar mirror for cooling makes perfect sense, I doubt that any civilization will significantly alter the day-night cycle with all the biological chaos that would cause (even if their bio-engineering was capable of mitigating it, why create all the extra work?) Much simply to just block XX% of light all the time. $\endgroup$
    – Gene
    Commented Jul 24, 2018 at 20:05
  • $\begingroup$ Originally that was how it worked, the false night and all the crazy genetic engineering that came with it were the results of a later issue. $\endgroup$
    – Marbrand
    Commented Jul 25, 2018 at 3:23

2 Answers 2


The orbital period is about right, although to long by about 10%. The deviation is totally within the reasonable allowance of the question. Cudos to you, OP, for doing your homework.

Actually, in the interests of full disclosure motivated by not wanting to seem too much of a nit-picker, when I first did the calculation (badly) I got a larger error. This led me into trying to write a proper answer, formatted with the Latex package. In doing so, I found my error, but the Latex was so much fun that I'm leaving this answer.

The proposed star is similar to Earth's sun. Let's assume that it is of the same mass.

According to Kepler's Third Law of planetary motion as found in the online Encyclopedia Britannica, "The squares of the sidereal periods (of revolution) of the planets are directly proportional to the cubes of their mean distances from the Sun." The stated distance is about 1.5 AU. The stated year is 450 local days, which are each of 40 Earth hours. The arithmetic says that: $$ Year_{Penumbra} = ((40 EH)/(24 EH/1 ED)) * (450 PD/1 PY/(365 ED/1 EY)) $$


  • ED is Earth Day
  • EH is Earth Hour
  • EY is Earth Year
  • PD is Penumbra Day
  • PY is Penumbra Year

This gives:

$$ 1 Year_{Penumbra} = (40/24)*(450/365) = 2.05 EY $$

By Kepler's Third Law of Planetary Motion, the ration of the Penumbra orbit to Earth's orbit should be: $$ \sqrt(1.5^3) = 1.837 EY \\ which\ is < 2.05 EY $$

I am curious about one other aspect that I find unrealistic because of path dependency in technology development. It feels unnatural, and thus less believable, to have the photosynthetic system of the plants be altered to match the shorter, shielded day rather than continue to match the original day cycle. I would hope to see some reason in the story that describes the reason for and perhaps the difficulty of achieving so great a change.

Many things in the plants would need to change for an optimized organism:

  1. Every pathway involved in the circadian cycles would need to be changed. We are finding that these are complex oscillators involving many genes and regulatory agents.

  2. The day-night cycle runs with a different rhythm. To the plants, there is one longer and one shorter night. This would best be done by adding an epicycle to the circadian system, which could be difficult de-novo bioengineering.

  3. The local stores for inputs and outputs could be smaller. Photosynthesis produces energy when the sun shines, and that energy is stored for when the sun is not shining. Less storage would be required.

  4. Heliotrope is the process of tracking the sun. If the sun appears to be moving faster in the sky, the rate of movement would need to he larger with structural adaptations necessary.

I hope to read your stories from this world.


I would think your scenario is very realistic. The points I would question are:

  1. Density: 15% more mass than earth on same size. Earth is already the planet with highest density in our solar sytem. A rocky planet with a large iron core. I found this on the internet: https://www.universetoday.com/36935/density-of-the-planets/ It is hard to tell how realistic it is to find a planet with an even larger iron core.

  2. Beaming light to the planet: I think for realistic reduction of a runaway greenhouse effect, there would not be a light beam behind the structure, but rather just size the structure down or include holes (with UV filters?) that let some light through.

  3. Mirror lifetime. I think placing it at L1 is totally realistic. Considering pressure from the light phontons, this could easily be compensated by moving a little more toward the sun. Assuming a sufficiently advanced space-travelling civilization, auto-repairing might also work. I would be concerned about micrometeoroids. Micrometeoroids will constantly hit and do their damage over time. See Wikipedia on 'Micrometoroid'. So I would doubt that a self-repair could keep up with this for so long. Once the station-keeping fails, an object will leave L1 soon and fall into some sun orbit. It could be that the propulsion and robot-base is in a very robust think housing, and that the mirror frame is a large and light structure, that just sustains the holes of meteoroids without falling apart. After the long time it would be riddled with holes up to the point of looking like a naked skeleton.


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