By showering the sky with x-rays for thousands of years.
If you were standing on Earth six million years ago, the sky would have shone with the light of two suns. That is, it would have if you had x-ray vision.
That's because Sagittarius A*, the supermassive black hole at the center of our galaxy that carries the weight of four million suns, was throwing a fit. It had swallowed something—a massive plume of gas, perhaps even a star—and was belching radiation all over the Milky Way. Here on Earth, the eruption lit up the night with an x-ray shower that lasted up to a hundred thousand years.
Astronomers know that once in a while, supermassive black holes flare up as bright as the entire galaxy. But few have paused to consider how these events could impact life on Earth. In a forthcoming study, astronomers Xian Chen and Pau Amaro-Seoane show how the eruption six million years ago might have been powerful enough to rip a giant hole in Earth's ozone layer. If similar x-ray bursts occurred frequently over the past several billion years, then the singularity 26,000 light years away may have directly influenced the evolution of life on Earth, and shaped the habitability of other planets throughout the galaxy.
"The main reason there has not been much work in this direction is that astronomers used to think black hole flares of this intensity were very rare—once every hundred million to billion years," Chen told me. "But thanks to very new theoretical and observational progress, this picture has to be revised."
The early universe was a tumultuous place. Stars were sprouting to life and collapsing into black holes—infinitely dense, massive objects from which nothing, not even light, can escape. In those early days, supermassive black holes were voracious things, sucking up blobs of interstellar gas and stars, and spewing electromagnetic radiation across the cosmos.
But as our universe aged, these monsters quieted down. Nevertheless, astronomers now know, both from theory and observation, that flare-ups still occur—for some black holes, they do so as frequently as every ten thousand years. While solar flares from our Sun produce lots of visible and ultraviolet light, Sagittarius A* flares spew out loads of high-energy x-rays—a form of radiation that can be incredibly damaging to life, but which our ozone layer helps deflect.
In the new study, the authors focused on a Sagittarius A* flare-up that occurred roughly six million years ago. Their calculations show that this particular eruption showered Earth with an intensity of x-rays comparable to an epic X-class solar flare. For the thousands of years that the eruption took place, Earth's atmosphere swallowed a massive dose of this radiation.
High-energy x-ray photons can set off a chain of chemical reactions in the upper atmosphere that shred ozone apart. According to the authors' calculations, the flare up in question could have been powerful enough to eat away a significant percentage of Earth's ozone layer, forcing life to contend with additional radiation for millennia. (For comparison, the several percentage point depletion of Earth's ozone layer in recent decades has already impacted a wide range of biota, including juvenile fish, crustaceans and amphibians.)
Interestingly, there's another cosmic event that doses our planet with x-rays: Supernovae. The radiation from exploding stars only lasts for months to years, but can, in that time, pack the same punch as a black hole flare.
"The radiation dose of a supernova is similar to that of a black hole, but the timescales are very different," Chen said.
It's unclear which type of cosmic explosion—a black hole flare or a supernova—might affect the Earth more. The short, powerful burst of radiation from a supernova may hit our atmosphere harder, leading to more ozone damage and more damage to life on Earth. But the same dose of x-rays spread out over a much longer time period could give the biosphere a chance to adjust, so to speak. A thinning of Earth's ozone layer for thousands of years might be a powerful enough environmental change to act as a selective pressure, and shape the evolution of life itself.
"There are some analogies we can borrow from supernova, but we can't expect the two phenomena to have the same effect," Chen said. "The effects of a long duration x-ray burst on our atmosphere haven't been explored yet."
Still, the notion that the evolution of Earth's biosphere might be guided by a black hole thousands of light years away is a profound, and slightly unsettling one. The more we peer into the vastness of the cosmos and unlock its secrets, the more, it seems, we find ourselves humbled by the power of its darkest elements.