Meanwhile, other wild interactions are taking place in the dense star clusters. When a cloud of hydrogen forms stars, it does so because it contracts and begins to spin. Clumps of greater density form here and there, Bally explained, and these clumps gain spin and collapse to form stars.

"Nearby interactions with multiple stars in a rich cluster can truncate and even completely eliminate protoplanetary disks," Bally said.

All the while, the rotation and gravitational forces can fling stars hundreds of light-years from their birthplaces. After a few million years, many of the stars escape the worst radiation environments. And the large, bully stars pay a price for all their energetic activity -- they typically die within 40 million years.

In an interview, Bally said his research and that of others shows that around most stars, there is a tight time constraint on when planets must form before the star's dust disk is blown away.

"I'm not saying planets can't form," he said. "But you have only 100,000 years to a million years."

However, if a planet can form quickly -- and theorists are not sure just how long this process takes -- then the radiation is irrelevant, Bally said, and his constraints would be largely lifted.

Our own Sun may have been born in a cluster and later tossed out, but Bally said it's not yet possible to figure out if that was the case.

If a planet finds itself around a star that fortuitously plans to hang out for a long time in a habitable zone of the galaxy, its odds of supporting life -- especially any kind of intelligent life -- are still slim. One only needs consider our own solar system, where intelligent life exists on just one out of nine planets.

Most planets probably do not end up in habitable zones around their stars -- slim orbital paths where radiation from the star is just enough to support life but not so much that it evaporates the oceans away. [The terms "habitable zone" and "Goldilocks zone" were originally devised to describe these favorable swaths around stars.]

Problem is, planetary habitable zones shift, too.

Kevin Zahnle, an astrobiologist at NASA's Ames Research Center, said our Sun has gotten significantly brighter during its roughly 4.6 billion-year life. It emits 30 to 40 percent more radiation than when Earth was born. Luckily, and possibly because life is present and moderates the change by evolving and modifying the atmosphere, Earth's surface temperature has remained about the same, he said.

Until other possible Earth-like planets are found and studied, no one can say whether it is commonly possible to preserve such a delicate balance.

Long before we fry, another asteroid or comet will strike Earth. Every 100,000 years or so, leading experts agree, an impact large enough to threaten civilization occurs. If other planets are anything like our own, they too would face this peril of bombardment.

Christopher Chyba of the SETI Institute has theorized that only the smart can survive. A civilization must evolve to the point that it can detect and then either detour or destroy threats from space, lest it be rendered extinct or, at best, plunged back into a Dark Ages existence.

"There is a kind of selection effect for long-lived civilizations," Chyba said in comments to a group of reporters during the STScI conference. "If you want to be long lived, you need to become technical because you need to be able to observe the impact environment around you and respond to that environment in some way to mitigate its effects on your planet."

Chyba pointed out that it took 700 million years or so for life to begin on Earth. The planet had to cool down after its initial formation, and then it weathered a barrage of asteroid impacts. The largest objects might well have evaporated the oceans, he said, preventing the origin of life or resetting it if it had already occurred.

"Our solar system tells you that life isn't going to be much younger than a billion years," Chyba said. "It's going to take that long for the planet to be capable of supporting life at all. It couldn't be 10,000 years. It couldn't be a million years."

Even in an ideal world, there are hazards and limits to life at both ends. Along the way, it is no picnic. Life is tough, these theorists all recognize. But it is not impossible. At least one planet has proved that.