Is Earth a "lucky" planet, dwelling in a rare gem of a solar system that somehow escaped the gravitational wrecking balls that have knocked other planetary systems cockeyed? Or is the sun's realm fairly typical in its symmetry and orderliness?
Such questions arise from the extraordinary burst of discovery that began in 1995, in which astronomers say they have found a total of 17 worlds orbiting sun-like stars outside the solar system -- and counting.
The tally is getting high enough to show statistical patterns that, scientists say, may be telling them something new about how nature forms and destroys planets, and just how fragile the prospects for life-harboring worlds like Earth may be. The growing population of known extrasolar planets so far offers shocking contrasts with our own home solar system, defying theories based on that familiar model and challenging scientists to cobble together new ones.
All 17 extrasolar bodies appear to be roughly as massive as the largest planet in our solar system -- the giant gas ball Jupiter. It is not surprising that the hunters would find the largest, most obvious planets first. But eight of them are bound in tight, circular orbits that skim astoundingly close to their stars. The closest of these "hot Jupiters" whips around its star at such scorching proximity that its "year" -- one complete circuit -- lasts just 3.3 days.
Physical laws seem to dictate that they could not have formed that close and so must have migrated inward early in the system's history (wreaking havoc on any other fledgling worlds in their path). And yet something has halted their gravitational death spiral before it carried them "down the drain" to destruction.
All of the remaining nine extrasolar planets -- those maintaining average distances of at least 19 million miles from their stars (about one-fifth Earth's distance from the sun) -- follow "eccentric" (egg-shaped) orbits -- killer orbits.
"A trend is now being stamped on these discoveries that we thought, frankly, would go away," said Geoffrey Marcy of San Francisco State University at a recent meeting of the American Astronomical Society in Austin. "We are realizing that most of the Jupiter-like planets . . . tool around in elliptical orbits, not circular orbits, which are the rule in our solar system."
This seemingly minor distinction could spell the difference between life and its absence. Jupiter-size bodies lunging close and then veering far away from their stars are likely to sweep away smaller worlds. "None of us would be here if we had such a solar system," he said. "These Jupiters constitute a death knell for any possible habitable planet as we know it."
That's "bad news" for those in search of extraterrestrial intelligence, said Marcy, whose team leads the world in planet detection. But he noted that, of hundreds of stars surveyed so far, only about 5 percent appear to have world-wrecking planets. "The good news is that 95 percent of the sun-like stars don't have these nemeses."
Like the first photographs of Earth taken from the moon, the revelations of the extrasolar worlds may inspire a new appreciation for the fragility of human existence. Pointing to the bull's-eye symmetry of the inner solar system, Marcy said, "Look at how perfect this thing is. It's like a jewel. You've got circular orbits. They're all in the same plane. They're all going around in the same direction. . . . It's perfect, you know. It's gorgeous. It's almost uncanny."
Passing "Jupiters" flung out of other systems, black holes and neutron stars that are known to wander the galaxy are among the marauders that might have come barreling through in the 4.6-billion-year history of the solar system, he said. "Obviously our solar system represents kind of a dartboard. And no darts have hit it."
While most astronomers have been convinced that the extrasolar objects are indeed planets, broadly defined, a few skeptics have pointed to the non-circular orbits as evidence to the contrary. "It was the black-sheep property of what we've been discovering. It didn't smell like the planets in our solar system," Marcy said. But the claim that all planetary orbits must be like ours, he quipped, "may well be a circular argument."
The discoveries have pushed the very definition of the term "planet" into a state of chaos that dwarfs the periodic flaps over Pluto. The conventional definition, reflecting our own solar system, holds that planets form from a flat disk of dust and gas rotating around a newly forming star. Within this framework, planets might vary enormously, from small and rocky (like Earth and its near neighbors) to unsurfaced gas giants -- and tiny, icy eccentrics such as Pluto.
Theorists have found ways to explain some, but not all, of the properties of the newfound planets, said astronomer William Cochran of the University of Texas. In any case, even if the objects don't quite fit the mold, he added, "right now, we are still going to call them 'planets.' "
Theorists have calculated how planets born in the expected circular orbits might get kicked onto different paths in gravitational encounters with other objects (rival planets, passing stars, etc.). For example, the nine extrasolar eccentrics "may be in systems that started more or less like our own, with one Jupiter," Marcy said, "but maybe the Saturn grew too big or too close. And those two gravitationally slingshot off of each other," one being catapulted into interstellar space, the other spiraling in toward its star.
To discover how rare the solar system really is will require more time and data. No technology has yet proven capable of detecting Earth-size planets, and even the big ones are hidden from view in the overpowering glare of starlight. The leading detection technique is a complicated, indirect one that measures the very slight wobble induced in a star by the gravity of its unseen planet.
This and other techniques should gradually reveal lower-mass planets and planets of all sizes orbiting at greater distances from their stars. So far, no more than one planet has been confirmed around any sun-like star, but some of the data hint at what could be multi-planet families.
"We'll probably find many more surprises, strange systems that will make us rethink what we're doing. But we should also find lots of systems that look something like our solar system," said theorist Alan P. Boss of the Carnegie Institution of Washington.
Dozens of teams are working on improved methods and tools. Studies of dust disks around nearby stars -- the potential breeding grounds of planets -- have begun to reveal details about the processes within. And two international groups have reported the first results from a technique called microlensing, which might be used to detect planets as small as Earth.
In little more than a decade, astronomers expect to complete the first census of planets orbiting nearby stars. And NASA plans to deploy a series of space-borne telescopes that could lead to a mighty Planet Finder capable of producing actual images of distant Earth-like planets. Assuming a killer Jupiter didn't find them first.