Materia oscura podría hacer planetas habitables

Dark matter could make planets that would otherwise be hostile to life habitable, a new study suggests. It suggests that in areas rich in dark matter, particles of the stuff could collect inside free-floating planets that have no star to warm them, heating them enough to maintain liquid water on their surfaces.

No one knows what dark matter is – astronomers merely detect its gravitational pull on normal matter, which it seems to outweigh by a factor of five to one. But many researchers believe it is made of particles called WIMPs, which interact only weakly with normal matter but annihilate on contact with each other, creating a spray of energetic particles.

This annihilation could produce heat if the particles were absorbed by surrounding matter. Now, Dan Hooper and Jason Steffen of Fermilab in Batavia, Illinois, have calculated how much heat would be produced inside planets in different dark matter environments.
Gravitationally captured

For decades, theorists have calculated that dark matter could be gravitationally captured by planets and stars. Dark matter surrounds galaxies in so-called halos, and dark matter particles, feeling the force of gravity, orbit their galaxy's centre of mass.

When these orbiting dark matter particles pass through objects, such as planets, they occasionally slam into atoms, losing energy and speed. If they lose enough energy after one or more collisions, they could become trapped by the planet's gravity, ultimately settling in its core. There, they are likely to hit other trapped dark matter particles and annihilate.

Is annihilating dark matter heating up the Earth? Not appreciably, say the authors. The Earth lies about 26,000 light years from the centre of the galaxy, far enough away that the concentration of dark matter is too low to have much of an effect.

The power produced by annihilating dark matter "is something like a megawatt – small by power-plant standards", says Hooper. "One power plant in the middle of the Earth isn't going to matter very much." By contrast, the Earth absorbs about 100 billion megawatts of power from sunlight.

'Sustainable energy'

But closer to the centre of the galaxy, the concentration of dark matter is much higher – so this heating could approach the heat that Earth receives from sunlight. The researchers found that a planet weighing a few times the mass of the Earth, lying within about 30 light years from the galactic centre, could be heated enough by dark matter alone to maintain liquid water on its surface.

That would mean that any planets that had broken away from their host stars in gravitational tussles with neighbours could be habitable, even though they were floating through cold space.

And the heating would continue for trillions of years, since dark matter would continually be captured by the planets' gravity. "This is the ultimate form of sustainable energy," Hooper told New Scientist.

Last bastion

That could come in handy far into the future, when stars have exhausted their nuclear fuel and died. "Trillions of years into the future, you might have some remnant advanced civilization that is looking for a new home because their star shopped shining," says Hooper. "Where would they go? They might choose to migrate to dark-matter-heated planets. They might be the last bastion of life in the very distant future of our galaxy."

That may be so, but Dave Stevenson of Caltech in Pasadena, California, points out that planets where dark matter would provide significant heating are extremely rare.

"You have to go to a place where the dark matter density is 10 million times larger than the usual estimate [in Earth's neighbourhood]," he says. "They're talking about a tiny fraction of possible cases."

Not testable?

"It's a very creative idea," says James Kasting of Pennsylvania State University in University Park. "But it's not really testable."

That's because these dark-matter-rich areas are so far away – 26,000 light years – that even if the presence of planets could be detected, current and planned telescopes would not be able to image them, looking for signs of water.

"We're most interested in stars within about [65 or 100 light years] of the Earth because we think we can build a big telescope sometime within the next 20 years to try to image planets around them," Kasting told New Scientist.

But Hooper says dark matter detection experiments on Earth could provide a test of sorts. His calculations are based on WIMP candidates that interact as strongly with normal matter as is allowed by current observations. If experiments fail to detect dark matter in the next five to 10 years, then that will suggest dark mater does not interact strongly enough to produce much planetary heating, he says.

If dark matter does interact as Hooper and Steffen have calculated, though, it could produce too much heat for life to survive on some planets, frying those that are also getting heat from their host stars. "If the planet was getting the same amount of starlight that the Earth does, and you added an equivalent energy coming from below, you'd go into the runaway greenhouse case," Kasting says. "You'd lose the water and end up with a planet like Venus."

Gregory Laughlin of the University of California, Santa Cruz, agrees with Kasting that these planets would be extremely difficult to detect: "I think it's going to remain strictly a theoretical curiosity for quite a while."