It took 4.5 billion years for Earth to generate and evolve a life form that could think, reason, and finally fly off the planet. That's a long time, even by cosmic measures. Perhaps too long.

At a time when the only known sentient species has earnestly and optimistically begun to search for life on other planets, several scientists within that species have found a host of reasons to guard the optimism. Throughout the galaxy, hazards to planet formation and sustained evolution are so serious and varied that life may be exceedingly rare. Intelligent life, presumably, would be the rarest of all.

Guillermo Gonzalez, an Iowa State University expert in stellar evolution, says there are relatively small bands and patches of the Milky Way Galaxy that he considers to be habitable regions. There are places where conditions are just right for the formation of planets and where things stay calm enough, long enough, to allow the evolution of anything but the lowest forms of life.

Our Sun happens to be in one of these Goldilocks zones. For now, at least.

Gonzalez has examined the structure of the galaxy and the amount of heavy elements distributed through it. The central region of the galaxy, he says, is far to cramped and chaotic to expect Earth-like planets to have much chance of developing and remaining stable.

Planetary systems, if they are like ours, are expected to include outer belts of comets, like our own Oort cloud which extends beyond Pluto. Near the center of the galaxy, which all astronomers agree is more densely packed with stars, close encounters between stars would gravitationally boot more of these comets into the inner reaches of a solar system, where the planets would be.

Further, because there is a greater concentration of heavy elements -- carbon, iron and other stuff that weighs more than hydrogen and helium -- near the galactic center, Gonzalez said more comets and asteroids would probably develop.

"Comet showers should be more common," Gonzalez said at a meeting titled "Astrophysics of Life" earlier this month at the Space Telescope Science Institute (STScI).

Conversely, the outer reaches of the galaxy are relatively lean in the heavier elements, making planet formation difficult according to present theories. Other researchers have doubted this assertion, suggesting that regardless of the abundance of heavy elements in a star's environment, the quantities can vary greatly within a solar system, as has been observed in our own.

There are other hazards, however. The pronounced spiral arms of the Milky Way are regions where star formation is more frequent and intense. As with the center of the galaxy, gravitational chaos and heavy radiation in the arms is not conducive to long-running biological evolution.

Between the spiral arms is the only safe place, Gonzalez has been saying in recent years.

However, while stars orbit the galactic center, they are all on different courses in relation to the spiral arms and the main, fairly flat disk of the galaxy. Some stars, during their lifetimes, cross the spiral arms, and others do not. Other stars travel above, below and perilously through the main plane.

The Galactic Habitable Zone, which it has come to be called is, then, actually several shifty places whose bounds are as-yet unclear.

"We don't know where it is exactly," Gonzalez said. "We think it's in the thin disk of the galaxy. It excludes the center of the galaxy, and it excludes the outer edge of the galaxy."

And where are we?

"We're between spiral arms. We're going to stay between spiral arms for a long time."

Though Gonzalez figures the Sun "undoubtedly" experienced more dangerous regions of space in the past, he says it isn't possible to figure out where we were prior to a few hundred million years ago. What he does know is that our solar system moves around the galaxy at a pace and direction that is similar to the nearest spiral arms, so we will not soon crash through one or be overtaken.

Like Gonzalez, John Bally would love to know where the Sun was born. Bally, of the University of Colorado, examines the birthplaces of stars and has learned that the vast majority are generated from giant clouds of hydrogen that spawn not one, but many stars in a huge and dense nursery of nearly simultaneous birth.

It's anything but a pleasant womb.

The clusters are violent, chaotic places whose largest stars -- superhot, massive, short-lived objects -- bathe the smaller ones in heavy doses of ultraviolet radiation that can destroy the seeds of planets before they ever form.

Here's what happens: