Dark matter could help maintain liquid water on the surface of large exoplanets, making it possible for life to emerge, evolve, and survive, even in the absence of additional energy from starlight or other sources.

For a planet to be able to maintain liquid water on its surface, scientists assume it has to be located in the habitable zone of its parent star: the planet lies far/close enough to its star for starlight to provide the energy needed to maintain liquid water. There are also a few other sources of energy that could possibly warm up a planet: a thick atmosphere driving a greenhouse effect, or radioactive elements decaying in rocks.

Now, in a paper they submitted to the Astrophysical Journal, Dan Hooper and Jason H. Steffen, from the Center for Particle Astrophysics, Fermi National Accelerator Laboratory, Batavia, have considered another energy source for planetary heating: the annihilations of dark matter particles.

Dark matter is the invisible stuff that makes up more than 80% of the matter in the Universe, and almost does not interact with anything. Its nature is still unknown, but many theories give possible descriptions. The most popular one suggests dark matter is made of weakly interacting massive particles (WIMPs), that interact only through the weak force and gravity. WIMPs have an interesting property: as they are their own antiparticles, they annihilate each other, releasing energy.

Because WIMPs can be gravitationally bound and captured by stars or planets, if enough dark matter is accumulated in a planet’s interior, dark matter particles can subsequently annihilate to produce energetic particles that are then absorbed by the surrounding material. This could warm the planet enough to sustain liquid water on its surface.

For their calculations, the researchers considered two different phenomenological models of WIMPs: one that is about 300 times heavier than a proton, and another one 7 times heavier. Then they calculated the capture rate of these particles in Earth-like and super-Earth planets. Finally, they determined the resulting surface temperature of those planets that would come from dark matter annihilations.

On Earth, they found that dark matter does not make any difference: its contribution is negligible compared to sunlight . This is mainly due to the fact that our planet lies in a region of the Milky Way with a relatively low amount of dark matter.

However, the density of dark matter is much higher in the central region of galaxies: a rocky planet lying within about 30 light-years of the galactic center, and with a mass 10 times greater than that of Earth could maintain liquid water on its surface. This does not even require any additional energy from starlight or other sources.

Hooper and Steffen’s model relies on WIMPs only: if dark matter turns out to be anything else, their model is no longer valid. Finally, if such planets exist, they would have an incredible advantage over planets like ours: they would have an extremely long lifetime, literally trillions of years, outliving even the smallest, and longest-lived, main sequence stars.

“Such planets may prove to be the ultimate bastion of life in our universe,” concluded the authors.


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