When searching for life on other planets, we tend to look for conditions similar to Earth—after all, that's the only place we've found life in the universe so far. But an astrophysicist recently showed that life on other planets might not need all our Earthly luxuries, like an atmosphere and a sun, to survive.
Instead, just like a deep-mine-dwelling bacterium right here on Earth, tiny organisms could get their energy by "eating" the cosmic rays that bombard the universe with radiation.
"This bacterium in a South African gold mine, about two miles deep, was powered by radioactive substances," Dimitra Atri, a researcher scientist at the Blue Marble Space Institute of Science, told Motherboard. "This was something new and it changed our view on what energy sources could be."
Atri recently published a paper in the journal Royal Society Interface where he demonstrated that the conditions in the mine that allow that bacterium to thrive are similar to the conditions created by cosmic rays on planets and moons where there's little or no atmosphere.
The bacterium Candidatus Desulforudis audaxviator was discovered back in 2008, 2.8 kilometers underground in a South African gold mine. Along with being the only known example of a single-species ecosystem, the bacterium was the first example of radiolysis in nature.
While the rest of life on Earth gets its energy from the sun—either through photosynthesis or by eating the products of photosynthesis (plants, and animals that eat plants)—this little bug gets all its energy from the decaying radioactive uranium that surrounds it in the mine.
"For radiolysis you need two things: some kind of nutrients, so here we had different irons, sulphur, oxygen, nitrogen, different types of compounds," Atri explained. "Sitting there by themselves, they're not useful for life at all. But then you also have a source of radiation, in this case it was radioactive substances, and that radiation breaks those molecules and life can utilize those ions that are produced."
This got Atri thinking. He researches cosmic rays: charged particles that shower through space, but from which we are protected by Earth's atmosphere. If life on Earth could use radiation to get the building blocks it needed to survive in its environment, could galactic cosmic rays have a similar effect on planets where the particles reach the surface? In his paper, he demonstrates that—at least theoretically—indeed they could, as long as the planet had a thin enough atmosphere, and some trace amounts of water and other nutrients.
Testing his theory through simulations and calculations, Atri showed that cosmic rays could reach several feet below the surface of a body such as Mars, where bits of nutrients and pockets of water would be broken apart, allowing for similar, simple life forms to survive. This opens up the possibilities for where we might find life in our solar system and beyond. Rather than sticking to Goldilocks planets—not too hot or cold, similar size to Earth, with a similar distance to a star, and a similar atmosphere to allow for liquid water—we could consider stars with no atmosphere, and even bodies with no star.
"Generally when we talk about looking for life elsewhere, we are looking for life exactly like ours," Atri said. "But this is a completely different type of energy. Our atmosphere protects us from radiation, but for radiolysis to happen the planet needs lots of cosmic rays [and little protection]. It's actually the opposite of what we think of finding life in other places."
Atri would like to test his theory out with a practical experiment, but the only way to recreate these conditions would be with a particle accelerator. Since there are only a handful of these on the planet, and a lot of researchers with ideas they'd like to test, it could take several years before he can put his ideas in action. But in the meantime, it might be worth looking at some less likely candidates as we comb the universe for signs of life.
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