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The world I'm building, Qi'raad, orbits a stellar-mass black hole. How would life best adapt to survive? Also, the black hole, named Halku, emits a relatively thin radio jet, which has carved a large canyon on Qi'raad's ecliptic. The planet isn't tidally locked yet.

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    $\begingroup$ The ecliptic is an imaginary line in the sky, corresponding to the intersection of the planet's orbital plane with the celestial sphere. How did the radio jet carve a canion on it? $\endgroup$
    – AlexP
    Commented May 21, 2021 at 20:46
  • $\begingroup$ It spews matter as well, which collides with the planet at high speed, knocking material into space. $\endgroup$
    – Pycoder
    Commented May 21, 2021 at 21:02
  • $\begingroup$ The ecliptic is an imaginary line in the sky. Is the canyon also imaginary and in the sky? $\endgroup$
    – AlexP
    Commented May 21, 2021 at 21:10
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    $\begingroup$ Also, an intermediate-mass black hole has an event horizon radius of ~$5\times10^6$ km. Radio jet diameters are expressed in multiples of the gravitational radius of the emitting body. So you wouldn't be carving a canyon, you'd be obliterating the planet. $\endgroup$
    – jdunlop
    Commented May 21, 2021 at 21:49
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    $\begingroup$ I'm still skeptical about that carved canyon idea. With the science-based tag absent, I guess you can just make it magic, but a comparative stellar-mass black hole exhibiting a radio jet has a jet energy of $10^{41}$ ergs, or ~$10^{35}$ joules. Given that the binding energy of Earth is about three orders of magnitude lower than that, even scaling back to a sun-mass black hole would result in delivering enough energy to liquefy the entire surface of the planet. $\endgroup$
    – jdunlop
    Commented May 21, 2021 at 22:29

2 Answers 2

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Black hole's accretion disk burning rate is nowhere as constant as Sun-like star's, and its spectrum is not life-friendly (as @Demigan had already noticed).

Intelligent life at or little above the tech level of modern day humanity would have to dig underground and shelter there. A more advanced civilization might be able to construct some kind of planetary shield to protect itself from harmful radiation and periods of extreme luminosity.

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  • $\begingroup$ I'm assuming that life evolved underground and learned how to use radiation shields eventually. Or, it might be just UV-resistant in the first place. $\endgroup$
    – Pycoder
    Commented May 21, 2021 at 21:41
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    $\begingroup$ @Pycoder Evolution on such a planet would be very problematic. On Earth, it took billions of years under our very stable Sun. Black hole planet, I assume, would oscillate between the periods of "snowball" and Venus-like greenhouse, losing all of its water relatively early in its history. $\endgroup$
    – Alexander
    Commented May 21, 2021 at 21:45
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    $\begingroup$ @Pycoder - it's hard to imagine life (as we know it) evolving at all in an environment that hostile. $\endgroup$
    – jdunlop
    Commented May 21, 2021 at 21:46
  • $\begingroup$ My imaginary black hole's accretion disk is relative uniformly spread out, scattering energy around. $\endgroup$
    – Pycoder
    Commented May 21, 2021 at 21:49
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I'm ignoring how the planet got there in the first place or what orbit it might have.

Your life will need light and warmth to survive at all. A BH accretion disk could provide this. The time warping effects on the accretion disk apparently shifts light into higher energies causing more harmful UV radiation.

Your life forms will need more resistances to radiation and UV light at minimum. Possibly adapt to weird season cycles as the light of the accretion disk might not be as constant as you would want.

Edit: for some more information this short article might give a bit: https://news.berkeley.edu/2019/07/25/milky-ways-central-black-hole-puts-einsteins-theories-to-the-test/

It talks a bit about the ludicrous size of the black hole (more than 30x the size of our own supermassive black hole) and its spin. Also that it has to be an old galaxy so the matter feeding the accretion disk doesn't rip the atmosphere away and other such unfortunate things.

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    $\begingroup$ Light from near a black hole tends to be redshifted, rather than blueshifted - the emission of high-energy radiation is mostly thermal and comes from the extremely hot gas in the disk - I think it's in the range of a few million Kelvin for the objects the OP's describing. You'd also likely see x-ray emission from the inner regions, rather than just UV emission. $\endgroup$
    – HDE 226868 ♦
    Commented May 21, 2021 at 22:08
  • $\begingroup$ @HDE226868 You seem to be right. I had read it in an article that is now gone from my search history. I would then suspect that it is the x-rays redshifted to ultraviolet. $\endgroup$
    – Demigan
    Commented May 22, 2021 at 8:01
  • $\begingroup$ In my story, it's a rogue planet that got captured, which explains its weird orbit. $\endgroup$
    – Pycoder
    Commented May 22, 2021 at 14:48

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