The civil audience pelted Lin with figurative tomatoes. But a lot has changed in two years, and no one's hurling fruit now.
Indeed, Lin and other theorists are straining to keep up with the whirlwind of planetary discoveries that has blown through astronomy since late 1995. They have stretched their models of how such systems evolve to accommodate a startling variety of planets, from ones with eccentric looping orbits to "hot Jupiters" that practically skim the outer atmospheres of their stars. Even the history of our own solar system -- heretofore a peaceful scenario -- is undergoing new scrutiny.
"Planets appear ubiquitous, and planetary systems are extremely diverse," says Lin, professor of astronomy and astrophysics at the University of California, Santa Cruz. "But to form a system that looks like ours, or one that can support the existence of life, may be a rare event." Lin will give a status report on the newly energized field of planetary-system modeling on Thursday, February 13, during "Old Worlds and New Worlds," a special two-day seminar at the AAAS meeting in Seattle.
Lin launched his recent modeling efforts in October 1995 when Swiss astronomers announced the first of the new batch of planets, called 51 Pegasi B. A team at UCSC's Lick Observatory, led by UCSC alumnus Geoffrey Marcy, rapidly confirmed the planet. From the outset its very existence seemed impossible. The Jupiter-sized object raced around its star once every four days, at a distance just one-twentieth that of Earth from our Sun. How could the planet, presumably a giant ball of gas, withstand this blast-furnace orbit? More puzzling still, how did it get there at all?
Within a day of the announcement Lin, Peter Bodenheimer of UCSC, and Derek Richardson of the University of Toronto had their solution. The planet coalesced in a colder region of its star's nebula, perhaps 100 times further away than it is today. Then, in a million-year gravitational tug-of-war among the star, the planet, and gas and dust in the rest of the disk, 51 Pegasi B spiraled slowly but relentlessly toward the star. Finally, in the model's biggest surprise, inward and outward forces on the planet's orbit canceled each other out just before the star would have devoured the planet. The team published its paper in Nature on April 18, 1996.
Lin's idea that infant planets can migrate either toward or away from their stars dates to the 1970s, but he doesn't hesitate to call those initial concepts "wild speculations." If his new model is correct, the truth is even weirder. "I never thought the migration could stop, especially so close to the star," he says. "That was a real shocker."
Two other curious systems (70 Virginis and HD 114762) have large planets that swoop close to their stars and then out again, almost like huge comets. In a paper to appear in the Astrophysical Journal, Lin and Shigeru Ida of the Tokyo Institute of Technology suggest that each star may have possessed a massive disk of gas and dust, spawning several large planets. Within a few million years, the pernicious effects of gravity could have perturbed the planets sufficiently to make their orbits cross. Then, inevitable collisions created a single enormous object with a bizarre orbital path.
If this notion sounds vaguely familiar, it should: Immanuel Velikovsky proposed that similar events in our own solar system could explain certain oddities in the rotations and positions of planets. Velikovsky's 1950 book Worlds in Collision went way off the deep end, but Lin acknowledges that the basic premise has merit -- if not here, than elsewhere.
"Dynamics among planets and within a planetary disk is a very rich game," he says. For instance, our outer solar system is "marginally stable." If Saturn, Uranus, and Neptune each had the same mass as Jupiter, their orbits might degenerate within a billion years -- less than the Sun's lifetime -- and wreak havoc throughout the entire system. "We are safe, but we are just safe," Lin says. "It takes only subtle differences in initial conditions to cause very diverse evolutionary paths."
Edging further along this limb, Lin suggests that life most likely would arise in systems with single massive planets tucked close to their stars, like 51 Pegasi B. He envisions such planets as the last in a succession of gas giants that migrate to their fiery dooms, sweeping up any rocky terrestrial planets in their paths. Then, according to his models, the gas and dust that remains in the wake of the final planet spreads out, seeding a second generation of earthlike bodies. With no gaseous titans further out to disrupt them, these planets could settle into stable orbits for billions of years. It's not out of the question, Lin says, that such a chain of events marked our solar system's childhood.
This conjecture flatly counters a tenet in astronomy that earthlike planets form in the toasty conditions close to their stars while gas giants dominate the frigid outer reaches, and never the twain shall meet. "I'm not a religious person," Lin says, smiling. "I'm willing to challenge any paradigm."