Planets walk a thin line between nurturing life or killing it off altogether, blasted as they are by high-energy particles spewing from stellar flares. While these particles can bombard a world, causing biological damage and chipping away at its vital atmosphere, they might also kick-start the stuff of living things.
By simulating a solar flare blasting young Mars with high-energy protons, at Harvard University and his colleagues found that these mighty bursts might create the building blocks of life.
To forge simple atoms and molecules into more complex organic compounds, you need a source of energy. There are a few different ways for a promising young world to get that energy, including heating from impacts, radioactivity, volcanoes and even simple ultraviolet (UV) radiation from a star.
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Stellar flares, for all their danger, can also provide the vital injection of energy that can turn simple ubiquitous molecules like carbon dioxide and nitrogen gas into amino acids, aromatic compounds 鈥� and the nucleobases that make up DNA.
Double-edged sword
For young Mars and Earth, several different sources of energy were at play in the model. Loeb and his colleagues found that while protons from solar flares could have formed some of the building blocks for life, most of them probably came from the sun鈥檚 ultraviolet radiation and other pathways.
鈥淥n Earth or Mars, this is a relatively small contributor to prebiotic molecules, 10 or 20 per cent, but on planets orbiting other stars it could be a major contributor,鈥� says Loeb.
Many of the most promising exoplanets we鈥檝e found orbit smaller, cooler stars called red dwarfs. Because these stars emit less UV radiation but flare often, worlds like Proxima Centauri b could be on the receiving end of blasts that build organic molecules.
鈥淚n some situations, life doesn鈥檛 exist, and then this bombardment creates all these organic compounds that can turn into life as we know it,鈥� says at the Blue Marble Space Institute of Science in Seattle, Washington. 鈥淏ut if there are already organisms there, this same bombardment can kill them. It鈥檚 a double-edged sword.鈥�
How these high-energy particles affect existing life also depends on whether the planet in question has an atmosphere. Air could stop 鈥渟tarter鈥� particles from reaching the planet鈥檚 surface but once life is there, air can stop water on the planet鈥檚 surface from sublimating away into the vacuum of space.
Alternatively, the same particles can strip away a world鈥檚 atmosphere. 鈥淥n Mars, for example, there might have been life prior to stripping the atmosphere,鈥� says Loeb. 鈥淚t could be one after the other: the energetic particles first allow life to form, and then destroy it.鈥�
Life from lightning
Luckily, there is a gentler way solar flares can help complex molecules assemble on a world with an atmosphere. Loeb and his colleagues suggest that the flares鈥� particles could pour energy into the air, triggering electron 鈥渁valanches鈥� that give rise to lightning.
When lightning reaches the ground, it discharges a burst of energy that can fuel the formation of organic molecules. 鈥淭he triggering of lightning could require much less energy than the phenomenon itself,鈥� Loeb says, so it may be fairly easy to seed life in this way.
Still, the planet鈥檚 immediate environment is the most important factor in figuring out whether it can host life. If a planet has a sweltering surface and no atmosphere, chances are not high, regardless of how many amino acids it has.
Read more: Enceladus鈥檚 hot, gritty core may cook up ingredients for life
The Astrophysical Journal
