Thursday, October 01, 2015

A Rough Neighborhood

Once every century or so, a supernova occurs somewhere in the Milky Way, blasting out as much energy in one event as a sun-like star emits over billions of years. According to a paper recently accepted for publication in Physical Review Letters, the level of antimatter in the vacuum of our solar system makes it look like one of these supernovas happened pretty close to home, and not too long ago.

A supernova in the nearby Large Magellanic Cloud heats
gas to millions of degrees, causing the X-ray glow shown here in blue.
The new paper suggests that our sun lies inside such a bubble.
The data that led to this conclusion comes partly from the PAMELA detector, the first-ever space-based cosmic ray detection experiment, which was launched in 2006. With equipment that allows it to differentiate matter from antimatter, PAMELA has given us our first glimpse of what the interstellar medium near Earth looks like, and it’s turned up some pretty surprising results.

The interstellar medium—what we typically think of as the “vacuum” of space—is really full of photons and charged particles streaming out from the sun, as well as cosmic rays from more distant sources. This cosmic ray flux, which seems to come from all directions equally, contains an unexpectedly large antimatter component, and it’s this observation that’s led researchers to suspect that our home planet is nestled within the ghost of a supernova.

If a supernova happened within ten light years of Earth, the immense burst of high-energy particles would likely scour the planet’s surface clean, according to the study’s authors, so we know that it can’t have been TOO close. (The galaxy is about a hundred thousand light years across, for scale.) At the same time, it couldn’t have been too far away, either—rare iron isotopes in the earth’s crust, which can only be made in processes like supernovae, suggest that we were showered with a good amount of matter from the event.

The combination of isotope and cosmic ray data suggests that roughly two million years ago, around the time the genus Homo first showed up in the fossil record, a star within a few hundred light years of Earth went supernova. Since then, the paper postulates, we’ve existed in the “cocoon” of high-energy matter and antimatter created by this explosion. If this is the case, the authors state, the course of life on earth has been irrevocably altered by the supernova. “High-energy particles from a further, hundred(s) of light-years away supernova would still produce a noticeable (beneficial or harmful) effect on the biosphere”, says Michael Kachelriess, one of the study’s three authors.

How much influence does “space weather” like this have over events here on Earth? It’s impossible to know for sure, but the old saying about butterflies and hurricanes makes it seem like a shower of radioactive nuclei could have some serious consequences.

Check out the upcoming edition of PRL for the full story, or you can read the Arxiv preprint version here!

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