In the dark and immense vastness of interstellar space, there can be lone planets that do not orbit around any parent star. Such planets do not receive warmth from stars and any surface inhabitant will experience perpetual night. It appears very unlikely that these dark and seemingly frigid worlds may support life and sustain alien ecologies. However, a combination of mechanisms such as radiogenic heating, tidal heating or having a thick hydrogen atmosphere that is very effective at trapping heat, can sufficiently raise the surface temperature of such a planet to a point where liquid water can exist on the planet’s surface. In this article, I will consider another possible source of heating which can contribute to raising the surface temperature of a ‘sunless’ planet and that source of heating comes from the annihilation of dark matter particles.
All of the dark matter in the known universe contains a total amount of energy that is on the order of 10 thousand times greater than all of the energy that could be released through the fusion of all the hydrogen in the universe into helium. Unlike normal matter, dark matter has a scattered nature and does not interact at sufficient rates to meaningfully contribute to heating a planet. An exception is when dark matter particles are gravitational captured by a planet, whereby interactions with the matter making up the bulk of the planet can cause the dark matter particles to lose momentum and become gravitationally bound to the planet. This causes dark matter to accumulate in the planet’s interior and the annihilation of dark matter particles produces high energy secondary particles which are then absorbed and deposited as heat into the surrounding bulk of the planet, thereby providing a source of internal heat.
For the Earth, the capture and annihilation of dark matter particles in the planet’s interior does not produce any significant amounts of energy and even in the most optimistic scenarios, the energy contribution from the annihilation of dark matter particles is billions of times less than the energy the Earth receives from the Sun. However, the density of dark matter is expected to be hundreds to thousands of times greater in the central regions of the Milky Way galaxy and in the dense cores of dwarf spheroidal galaxies than it is in our solar system. This means that the energy contribution from the annihilation of dark matter particles for planets located in these regions can be very different.
Furthermore, dark matter residing in this unique regions have extremely low relative velocities and this greatly increases the capture rate of dark matter particles by a planet that is located in such a region. This is due to the fact that the low relative velocities of the dark matter particles make them more efficient in being gravitationally focused toward the planet or becoming gravitationally bound to the planet following collisions in which the particles lose just a small amount of momentum. This enables dark matter particles to accumulate in much greater quantities in planets located in these regions, such that the annihilation of dark matter particles can become the dominant source of energy to the extent of providing sufficient warmth for liquid water to exist on the surfaces of these planets even in the absence of warmth from a parent star.
The energy released from the annihilation of dark matter particles can enable rouge planets that do not orbit around any parent star to become potentially habitable and sustain an alien ecology. Around the center of the Milky Way galaxy, Earth mass planets with very low atmospheric emissivity can efficiently trap the energy released from the annihilation of dark matter particles to maintain surface temperatures that are possible for liquid water to exist. For atmospheres with higher and more Earth-like emissivities, super-Earths with a few times the mass of the Earth will then be required to trap sufficient annihilation energy to maintain surface temperatures that are capable of sustaining liquid water. This is due to the fact that although high emissivity atmospheres are less efficient in trapping heat as compared to low emissivity atmospheres, super-Earths can accumulate more dark matter than Earth-mass planets due to their more massive bulk.
The timescale over which a rouge planet can maintain sufficient warmth to have liquid water on its surface solely by the energy released from the annihilation of dark matter particles is on the order of trillions of years. This surpasses even the exceedingly long lifespans of low mass red dwarf stars. Due to the rarity of very high density dark matter environments, planets that are heated by the annihilation of dark matter particles are expected to be very rare. Nevertheless, such planets can provide the energy required to sustain an alien ecology over trillions of years, even in the absence of warmth from any parent star! Given their exceedingly long lifetimes, these rare alien worlds may prove to be the ultimate cradles of life in the universe.