Caption: An artist's conception of a planetary system orbiting a stellar mass black hole (mass ≅ 5 to tens of solar masses).

Habitable Black Hole Planets

As a jeu d'esprit, we present the idea (almost certainly NOT new since nothing is) of habitable black hole planets.

1. Stellar mass black holes probably mostly originate from core-collapse supernovae with masses ⪆ 20 M_☉. The core-collapse itself leaves the black hole as a compact remnant.

2. The expanding supernova ejecta with mass ⪆ 15 M_☉ is typically moving with velocities extending from a few thousands to a few tens of kilometers per second (km/s).

   However, there can be slow-moving ejecta due to turbulent flows in the supernova explosion event and some of this ejecta can remain gravitationally bound to the black hole.

   The gravitationally bound matter can form an accretion disk orbiting the black hole. Then planet formation can create planets.

   Note, even a fraction of a solar mass of ejecta left gravitationally bound is sufficient to create many planets.

   Pulsar planets (known to exist since 1992) form in something like the way just described.

   Pulsar planets are likely to mostly uninhabitable: see Wikipedia: Habitability of neutron star systems.

3. The idea of black hole planets (sometimes called blanets) orbiting supermassive black holes has been considered for some time.

   However, here we are considering planets forming around stellar mass black holes.

4. Black hole planets formed as described could be habitable.

   Here, we take habitable to mean having surface conditions (temperature and pressure) where liquid water can exist since liquid water is necessary for life as we know it.

5. The non-escaping ejecta from the supernova will be enriched in metals needed for life since these are produced in the inner layers of the ejecta in the supernova explosion event itself. The layers could have something approximating the cosmic composition (meaning inside modern galaxies: fiducial values by mass fraction: 0.73 H, 0.25 He-4, ∼ 0.02 metals) with all the elements elements needed for life as we know it: most importantly carbon (C,Z=6) (needed for complex molecules) and oxygen (O,Z=8) (needed for water (H₂O)). This cosmic composition would be present even for black hole planets formed in the era of early Population II stars or maybe even the era of Population III stars if any are small enough to create stellar mass black holes. In this early era, the interstellar medium (ISM) out which ordinary planets form could be very low in metallicity which might prevent the formation of terrestrial planets (i.e., rocky planets) altogether which are the only ones suitable for life as we know it. If black hole planets are habitable at all, they might have been the first habitable planets in observable universe.

6. Primordial-radiogenic heat resulting from heat of planet formation and long-lived radionuclides (probably only potassium-40 (K-40, Z=19, half-life 1.251(3) Gyr), thorium-232 (Th-232, Z=90, half-life 14.05 Gyr), uranium-235 (U-235, Z=92, half-life 0.7038 Gyr), and uranium-238 (U-238, Z=92, half-life 4.468 Gyr): see Earth file: radiogenic_heat.html) from the supernova explosion (produced in the supernova explosion event itself (probably only K-40) or pre-existing in the progenitor star from earlier neutron star mergers: see Wikipedia: File:Nucleosynthesis periodic table.svg for the origin of the elements) would give the black hole planets primordial-radiogenic heat geology. This might be necessary for continents and islands: without these black hole planets could become ocean planets (AKA water worlds). An ocean planet can be habitable and can have intelligent beings, but without dry land fire, and so all advanced technology, may be impossible for the said intelligent beings.

   Note, we are assuming the black hole planet is massive enough for gigayears (Gyr) of primordial-radiogenic heat geology and to maintain a planetary atmosphere against atmospheric escape.

7. Now an isolated stellar mass black hole far from an active galaxy nucleus (AGN) is probably in a very inactive environment. There would be NO deadly ionizing radiation from space that would prevent life as we know it.

8. But what would keep the surface of black hole planet in the temperature range for liquid water (assuming there was enough planetary atmosphere for pressure).

   Primordial-radiogenic heat geology might NOT do the job. If that is strong enough to heat the surface, it might have activity too great for intelligent life to evolve.

   The other source of heat is from space.

   The black hole planet would have to be close enough to black hole for cosmic microwave background (CMB) and other perhaps other diffuse extragalactic background radiation (DEBRA) to be gravitational blueshifted (i.e., negatively redshifted) to warm the planet, but NOT too close to prevent liquid water.

   To be close enough to the black hole planet for significant gravitational blueshifting means that the black hole planet will almost certainly be tidally tidally locked to the black hole. However, the incoming gravitationally blueshifted electromagnetic radiation (EMR) is probably NOT very anisotropic, and so the black hole planet is probably fairly evenly heated given that atmospheric circulation will tend to even out heating imbalances. Since the gravitationally blueshifted CMB comes from all directions, it probably needs to be gravitationally blueshifted to peak somewhere in the infrared band (fiducial range 0.7 μm -- 0.1 cm) in order to have the right total intensity to allow liquid water. Any life as we know it, if able to see, probably sees in the infrared band.

   Note, Qu & Zhang (2026) have investigated extreme gravitationally blueshifted CMB which would overwhelmingly prevent liquid water on the black hole planet featured in Interstellar (2014 film).

9. Since CMB cools with cosmic expansion and other DEBRA evolves too, probably in a non-monotonic way, and black hole planet orbit will evolve also, the time span when the black hole planet has the right conditions for life as we know it is probably finite. But if that span is order gigayear or longer, then life as we know it could evolve and even technologically advanced intelligent life.

   Note, at cosmic present t_0 (equal to the age of the observable universe = 13.797(23) Gyr (Planck 2018)), the CMB T = 2.72548(57) K (Fixsen 2009)).

10. In the conditions suitable for technologically advanced intelligent beings, the said technologically advanced intelligent beings could probably know as much about the observable universe as we do.

    They would discover quantum mechanics.

    They would directly see their black hole's black hole shadow and its gravitational lensing. If there were other black hole planets and black hole moons, they would see them too. They might discover general relativity very easily.

    They could probably see gravitationally blueshifted stars and galaxies in some band of electromagnetic radiation against the gravitationally blueshifted CMB and DEBRA that comes from all directions.

11. It's almost certain that simple calculations would rule out habitable black hole planets. Their parameter space is probably zero or super minute.

    However, if habitable black hole planet technologically advanced intelligent beings have their own arXiv (astro-ph), they might iterate to speculating about life as they know it on star planets. But they would probably conclude that the parameter space for habitable star planets is so small that they NEVER happen.

    Credit/Permission: User:Levis_K._Ellingswort, 2021 / CC BY-SA 4.0.
    Image link: Wikimedia Commons: File:Système blanétaire.png.
    Local file: local link: black_hole__planet_habitable.html.
    File: Black hole file: black_hole_planet_habitable.html.
    

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