Habitable Zones around Supermassive Black Holes

Abstract

We analyzed the thermodynamics of hypothetical exoplanets at very low Keplerian circular orbits in close vicinity of rapidly spinning supermassive black holes. Such black hole exoplanets are heated by strongly blueshifted and focused flux of the incoming cosmic microwave background (CMB) and cooled by the cold part of the local sky containing the black hole shadow. This gives rise to a temperature difference, which can drive processes far from thermodynamic equilibrium in a hypothetical life form inhabiting black hole exoplanets, similar to the case of a planet heated by the radiation of the parent star and cooled by the night sky. We found that for a narrow range of radii of very low Keplerian circular orbits and for very high spin of a supermassive black hole, the temperature regime of the black hole exoplanets corresponds to the habitable zone around standard stars. The thermodynamics of black hole exoplanets therefore, in principle, does not exclude the existence of life based on known biology. The peak of the multiblackbody spectral profile of the CMB heating the exoplanet is located in the ultraviolet band, but a significant fraction of the flux comes also in the visible and infrared bands. The minimum mass of a black hole ensuring the resistance to tidal disruption of an Earth-like exoplanet orbiting in the habitable zone is estimated to 1.63 · 10[SUP]8[/SUP] m[SUB]⊙[/SUB].

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Source: Habitable Zones around Almost Extremely Spinning Black Holes (Black Sun Revisited) - The Astrophysical Journal, Volume 889, Number 1, January 2020 (free preprint)

The paper suggests that it is possible for planets to support liquid water on its surface if it is in a very close orbit to a rapidly spinning supermassive blackhole. The heat being generated by the extremely blue-shifted CMB would heat the surface of the planet, and the cooling being generated by the blackhole shadow as the planet rotates. Considering the orbit of the planet to have an Earth-like heating (1,365 W/m[SUP]2[/SUP]) from the CMB the exoplanet would have to have an orbit 1.0006 radii of the event horizon of the blackhole. Furthermore, the blackhole would have to be a minimum size of 163 million solar masses, or 36.2 times the size of our supermassive blackhole Sgr A* at the center of our Milky Way galaxy, for it not to rip apart the planet as a result of tidal forces. The planet would also be orbiting the supermassive blackhole at a substantial fraction of the speed of light. There is also the problem of objects, like other stars, asteroids, and other debris being drawn into the supermassive black hole's gravitational influence and colliding with any planets in orbit.

I thought this was a very interesting paper and opened some new possibilities for life that hadn't been considered before. If life did evolve on such a planet the life-forms would appear to us as extremely old. Time dilation would make it appear to us, viewing from a distance, that life on such planets lived for thousands or even hundreds of thousands of years.