By Seth Shostak, Senior Astronomer and Director of the Center for SETI Research

There’s plenty of real estate for extraterrestrial life, right?

Well, maybe not. An article recently published in the journal Physical Review Letters argues that much of the universe could be off-limits to complex biology.

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The reason isn’t a lack of worlds. Scientists analyzing data from the Kepler spacecraft bumped the tally of exoplanets to 1,000 this week, with thousands more awaiting accreditation. Most of them will get it.

There’s a simple take-home message from that: The cosmos is packed with planets. A rough estimate of the number afloat in our galaxy is a trillion, and a crude stab at the count of worlds that might be similar to Earth is 10-100 billion.

Big numbers all. Consequently, it’s a fair and frequent conclusion that worlds with life could be as plentiful as commuter-lane cheats.

But two astrophysicists, Tsvi Piran and Raul Jimenez, the former from Israel and the latter from Spain, think that environmental catastrophe on a massive scale may greatly constrict just how many planets will ever beget multicellular life. They say that most of the universe will be devoid of any critters bigger than a paramecium.

The catastrophe in question is called a gamma ray burst. This may sound like an energy drink, and in a sense it is. When black holes or highly condensed stars collide, the result is a sudden flash of high-intensity, high-energy light — a gamma ray burst. Ditto when certain kinds of massive stars burn through the last of their fuel, and go ungently into the night with a glorious, gamma ray bang.

Astronomers detect about one such GRB each day. Because of their extraordinary brightness, we can see these dramatic eruptions even when they occur in galaxies billions of light-years distant.

There are flashbulbs going off in the cosmos. But the danger is this: A 10-second blast of gamma rays could be bad news for any inhabited planet lying within a few thousand light-years. The rays would cause chemical changes in a planet’s atmosphere and quickly destroy its ozone layer, assuming it had one. The effect would be similar to spritzing a gazillion cans of chlorofluorocarbon-laced hair spray into the air.

With the ozone taken out, ultraviolet light from a planet’s home star would mercilessly rain down on any life below, causing fatal mutations. This death-by-ultraviolet would last until the atmosphere rebuilt its ozone shield, a matter of months or even years.

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The two researchers worked out the details of this unhappy scenario, and in particular decided where the danger from GRBs is greatest. Their conclusion is that you don’t want to be too near the center of your galaxy, and “too near” means closer than 10 thousand light-years. Otherwise, the chance is high that, even if your planet is on its way to producing trilobites or tyrannosauri, it’s not going to get there. Their predecessors will be fried by a GRB. This woeful computation suggests that we might not have very much clever company in space.

But I’m inclined to see the glass as still mostly full.

To begin with, recall that life is tough. I’m not talking about personal experience, but the fact that life on our world has exploited nearly every possible environment, no matter how unattractive. Everyone thinks they know the type of conditions required by terrestrial flora and fauna – access to oxygen, a variety of minerals, and temperatures warmer than freezing and cooler than boiling.

It’s not so. Extremophiles — a category of mostly microbial species that are the bread and butter labor of many astrobiologists — are happy to work their metabolic magic a mile underground, in a nuclear reactor, an aircraft fuel tank, a flask of acid, or a pot of boiling water. True, these aren’t the sorts of complex life that Piran and Jimenez believe are endangered by GRBs, but they are a good object lesson in avoiding too much conservatism when judging the conditions under which life might thrive.

And there’s this: Even if you rule out the inner 10,000 light-years of the galaxy, you’ve only cordoned off five percent of its volume, more or less. Yes, stars are more prevalent near the center, so this “no man’s land” might include 20 or 30 percent of all stars. But you know, even with 30 percent of all fast-food restaurants gone, there are still plenty of places to get a quick meal.

The idea that certain regions of many galaxies might be sterile — or at least limited to life that requires a microscope to see — is interesting and sobering. But pessimism about habitats for life has a bad track record. We once thought that in our own solar system, only Earth had the right conditions for life. Today, we know of a half-dozen worlds within the reach of our rockets that could support some biology.

My advice is to do the experiment. The theoreticians may point to the sky and say “it’s dead, Jim.” But unless you look, you’ll never know for sure.

Guest contributor Seth Shostak is the Senior Astronomer and Director of the Center for SETI Research in Mountain View.

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© Copyright 2014 by Seth Shostak and SETI Institute and CBS San Francisco. All rights reserved. This material may not be published, broadcast, rewritten or redistributed