In a binary system of artificially immortal stars, each one orbited by a ring of mirrors, a whole line of Earthlike planets orbits the binary within its habitable zone, from 400 to 800 AUs. While some of the planets won't have an axial tilt (meaning no seasons), most will have the same level of obliquity--19.7 to 26.9 degrees on an undetermined cyclic length. All have the same rotation--just 30 hours. But because of the binary's combined mass, the revolution of the inner boundary planet is 12,000 hours and the revolution of the outer boundary is 21,840 hours.

All the habitable planets are barren, with barely a single microbe in sight, perfect for the process of the latest trend in speculative evolution--seeding. And that's where the issue of "alien sunlight" comes into play. Compared to our sunlight, how much would be "too much" for Earth species, and how much would be "too little"? Too much, and the seedlist would have to be strictly marine, as the ocean waters would protect the lifeforms from the intense radiation. Too little, and the germination of plants would take far longer than back home.

So, generally speaking, with our amount of sunlight in the middle of the spectrum, what are the maximum and minimum amounts of "sunlight" that any Earth species would stand?

  • $\begingroup$ Interesting question, although I think you might find it useful to think about what kind of an answer you want and adjust to make that more clear. Do you want a couple of numbers (min and max Watts per m^2 for current Earth plants then a reference) or do you want an "it depends...{what it depends on}" kind of reply? $\endgroup$
    – Dast
    Oct 19, 2021 at 15:57
  • $\begingroup$ Do your planets have ozone layer? $\endgroup$
    – Alexander
    Oct 19, 2021 at 16:39
  • $\begingroup$ @Alexander Yes, they do. $\endgroup$ Oct 19, 2021 at 21:54

1 Answer 1


Something you seem to be overlooking is that Earth plants vary widely in their tolerance for sunlight. There are plants adapted to the intense sun of say Death Valley or the High Sierra, where their growth is limited by factors such as water and cold. Then there are plants which grow in the shaded understory of forests, or in places like the Pacific Northwest, or Britain & Ireland, where much of the sunlight is blocked by cloud cover.

So the question is really how much sunlight is needed to keep the planets from becoming either frozen iceballs like the Jovian moons, or having a Venus-like runaway greenhouse effect. By saying that all the planets are in the habitable zone, you've answered that question.

Yet another factor is that life will alter the planet, and as long as it can survive at all, will evolve to suit the changed conditions. See e.g. https://en.wikipedia.org/wiki/Gaia_hypothesis

  • $\begingroup$ Mars is in the habitable zone, yet has only 40% as much sunlight. That proves that light in a habitable zone cannot realistically be uniform. $\endgroup$ Oct 19, 2021 at 21:56
  • $\begingroup$ @JohnWDailey Mars' primary issue isn't a lack of sun light, but ironically a bit too much. Lacking a molten core and thus a magnetosphere, the atmosphere doesn't have enough of a magnetic field to hold a dense enough atmosphere because solar wind basically blows it away. The thin atmosphere further lacks an ozone layer, which basically leaves the entirety of the Martian surface permanently bathed in radiation. Even if something can survive on the sparse nutrients and atmosphere, which I'm sure something could, the little sun that Mars does see would burn away any substantial life form. $\endgroup$ Oct 20, 2021 at 9:02
  • $\begingroup$ @JohnWDailey: I'm not sure why you think that's relevant to anything I wrote. What matters to the plants is not the light intensity outside the atmosphere, but how much reaches the leaves after it passes through air, clouds, and maybe several layers of forest canopy. Then there's angle: what's relevant is intensity parallel to incoming rays. Intensity at Mars' equator is about the same as at Fairbanks, Alaska, and plants grow perfectly well there. Given a bit of greenhouse effect, Earth at Mars' distance would be habitable. $\endgroup$
    – jamesqf
    Oct 20, 2021 at 17:31

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