By Shannon Hall
The center of any galaxy is a hazardous home. There, supernovae explosions shower nearby planets with x-rays, gamma rays and ultraviolet photons that obliterate any ozone layer present. Gamma-ray bursts hurtle even more damaging shock waves, blasting any biosphere into oblivion. Even encounters with nearby stars knock planets around, driving them out of their habitable zones. “We don’t expect life to be easy within the inner kiloparsec of the Milky Way,” says Abraham Loeb from the Harvard–Smithsonian Center for Astrophysics. But now we can add one more menace to the list that tops the rest: supermassive black holes.
Every large galaxy’s center hosts a supermassive black hole that is wont to throw wild tantrums in its youth. Although many astronomers have speculated these behemoths, called quasars when they are active, would likely wreak havoc on any nearby planets, no one had taken a quantitative look at those effects—until now. A new study, posted on the preprint server arXiv and submitted to Monthly Notices of the Royal Astronomical Society, provides the first calculations that show when, where and how planets are harmed by quasars. And the quota alone is surprisingly high. Loeb and his postdoctoral researcher, John Forbes, found half the planets throughout the universe have lost the equivalent of Mars’s atmosphere, 10 percent have lost the equivalent of Earth’s atmosphere and 0.2 percent have lost the equivalent of Earth’s oceans—all thanks to quasars alone.
Although quasars are known to drive strong winds and jets of relativistic particles that can be dangerous in their own right, Forbes and Loeb looked at the damage caused by their light alone. The accretion disk of debris that orbits the black holes, funneling gas and dust in, are so bright they can outshine all the stars in their galaxies, which are 100,000 times larger. And when that light illuminates the atmosphere of a planet, the high-energy photons transfer energy to those atmospheric particles, giving them the boost to escape the planet’s gravitational pull altogether. Outside experts like Duncan Forgan from the University of Saint Andrews in Scotland are quick to point out the amount of loss depends greatly on the atmosphere’s composition and the planet’s mass. This finding is one piece in a several-million-piece, three-dimensional puzzle, he says, but nonetheless it is still that crucial first piece.
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