Dark Matter Heat Could Make Exoplanets Habitable

Dark matter collecting inside exoplanets could heat some cold worlds enough to support life, even without the warm glow of starlight. Alien-hunting astronomers generally search for planets that lie just far enough from their stars to keep from boiling off or freezing any liquid water, which is thought to be a prerequisite for carbon-based life. […]

Dark matter collecting inside exoplanets could heat some cold worlds enough to support life, even without the warm glow of starlight.

Alien-hunting astronomers generally search for planets that lie just far enough from their stars to keep from boiling off or freezing any liquid water, which is thought to be a prerequisite for carbon-based life. But other heat sources could potentially warm up a chilly planet that is outside this habitable zone.

One possibility is radioactive elements decaying inside rocks, which already give the Earth about 0.025 percent of its geothermal energy. Another is a thick atmosphere to drive a greenhouse effect, which renders Venus an inhospitable hot house. Some have even suggested that planets that have been kicked out of their solar systems could still support life beneath a thick atmosphere or a shell of ice.

In a new paper posted on arXiv.org and submitted to the Astrophysical Journal, physicists Dan Hooper and Jason Steffen of Fermilab in Illinois suggest an exotic internal radiator for cold, rocky planets: dark matter. In certain parts of the galaxy, they say, dark matter could effectively outshine the sun.

"It’s not something that’s likely to produce a lot of habitable planets," Hooper said. "But in very special places and in very special models, it could do the trick."

Dark matter is the name given to the mysterious stuff that makes up about 83 percent of the matter in the universe, but generally ignores regular matter. No one knows exactly what dark matter is, but one of the most popular theories says it's made of hypothetical particles called WIMPs -- weakly interacting massive particles -- that interact with regular matter only through the weak nuclear force and gravity. WIMPs are also their own antiparticles: Whenever one WIMP meets another, they annihilate each other in a burst of energy.

If those explosions happen inside a planet, they could warm the world enough to melt ice, Hooper and Steffen suggest.

Physicists are still waiting for WIMPs to show themselves by colliding with detectors in deep underground mines. But the fact that the detectors haven't seen anything conclusive yet puts limits on how heavy and large the particles can be. If WIMPs were bigger or heavier than a certain theoretical limit, physicists reason, the particles would have shown up by now.

Hooper and Steffen considered two possible model WIMPs that interact as often as they possibly can while still being consistent with the experiments, one particle that's 300 times heavier than a proton and one that's just 7 times the proton's mass. Then they calculated how much energy the explosions from colliding these hypothetical dark matter particles would contribute to the planet's overall warmth.

On Earth, they found, dark matter doesn't make a difference. Earth lies in a part of the Milky Way where dark matter is relatively thin, so it contributes at most one megawatt of energy to Earth's internal thermostat. By contrast, the Earth absorbs about 100 petawatts, or 100 quadrillion watts, from the sun.

But in the dark matter-rich centers of galaxies, WIMPs could be a contender. The researchers considered rocky planets that lie within 30 light-years of the galactic center, and found that planets with masses 10 times greater than Earth's could scoop up enough dark matter to generate 100 petawatts of energy. That could be enough energy to keep liquid water on their surfaces, even without the aid of a nearby star.

"This is a fascinating, and highly original idea," said exoplanet expert Sara Seager of MIT, who was not involved in the new study. "Original ideas are becoming more and more rare in exoplanet theory."

She points out that the idea is limited to WIMPs, though -- if dark matter turns out to be something else, it won't work. She also notes that these planets would be too far away for followup observations, a point on which Hooper agrees.

"I don’t foresee any way of detecting such planets any time in the near future," he said.

If dark matter-heated planets exist, it's not clear that they would resemble Earth at all. They may not have solid, rocky surfaces for liquid water to pool on, or a molten mantle to drive plate tectonics.

"It’s very possible that this would look like a very different type of planet than the ones we’re used to," Hooper said.

But dark matter-heated planets have one advantage over planets that are tied to a star. Halos of dark matter can sit undisturbed at the centers of galaxies almost indefinitely, much longer than the lifetimes of individual stars.

"You can imagine planets being heated in this sort of way for literally trillions of years," Hooper said. "In the far future when all the stars have burnt out in our galaxy, all the surviving civilizations may find themselves migrating to these sorts of planets. They'll be the ultimate bastion of civilization."

Image: An artist's rendition of the planetary system around the star 55 Cancri. Credit: NASA/JPL-Caltech

See Also: