May 28, 2002

Amid the Universe's Chaos, a Few Habitable Places

By Robert Roy Britt

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.

We may, it turns out, be very lucky to be here. However, we may also turn out to be very alone.

Location, location, location

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 too 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.

Galactic Habitable Zone

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.

Bully stars

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:

When stars form, some or perhaps most leave a circle of debris -- gas and dust -- that develops into rotating mass called a protoplanetary disk. From this material, planets, asteroids and comets are thought to form. At least that's how it probably happened in our solar system. Scientists are only beginning to spot and study these dusty disks around other, relatively nearby stars, and they've seen clumps that hint at planets in the making.

But nearby stars share the relative calm pocket of space -- one of Gonzalez's habitable zones -- that our Sun benefits from. Most star formation occurs in clusters, and it the bulk of it takes place in the spiral arms.

In these clusters, a few massive stars shine thousands or even millions of times more brightly than our Sun. Their UV radiation eats away at the dust disks of other stars, and can strip the planetary seeds from all nearby stars over the course of a million years or less.

Limited time

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."

Given the roughly 300 billion stars in our galaxy, Bally's limits would still allow for plenty of planets out there, but it could also mean there are far fewer than some researchers have expected. "Either planetary systems form very fast," Bally said, "or we will find planet development to be rare. Something like 5 percent of stars will have planets."

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.

Other perils

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.

Eventually, Earth will be overwhelmed by the change. Within the next 5 billion years or so, the aging Sun will have swollen so much that it envelops and vaporizes Earth. In just a billion years, the Sun could be 11 percent brighter than now, turning the planet into an inhospitable greenhouse. Had it taken another billion years for humans to evolve, only some real lowlifes would have been around to deal with the problem.

Only the smart survive

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.

File Date: 06.10.02