James Hathaway, jim.hathaway@asu.edu
(480) 965-6375
June 22, 2004

Theory proposes new view of sun, Earth's creation

Like most creation stories, this one is dramatic: we began, not as a mere glimmer buried in an obscure cloud, but instead amidst the glare and turmoil of restless giants.

Or so says a new theory, supported by stunning astronomical images and hard chemical analysis. For years, most astronomers have imagined that the sun and solar system formed in relative isolation, buried in a quiet, dark corner of a less-than-imposing interstellar cloud. The new theory challenges this conventional wisdom, arguing instead that the sun formed in a violent nebular environment - a byproduct of the chaos wrought by intense ultraviolet radiation and powerful explosions that accompany the short but spectacular lives of massive, luminous stars.

The new theory, described in a "Perspectives" article appearing in the May 21 issue of Science, was written by a group of ASU astronomers and meteorite researchers who cite recently discovered isotopic evidence and accumulated astronomical observations to argue for a history of development of the sun, Earth and our solar system that is significantly different from the traditionally accepted scenario.

"There are two different sorts of environment where low-mass stars like the sun form," explained ASU astronomer Jeff Hester, the essay's lead author. "In one kind of star-forming environment, you have a fairly quiescent process in which an undisturbed molecular cloud slowly collapses, forming a star here. a star there. The other type of environment in which sun-like stars form is radically different. These are more massive regions that form not only low-mass stars, but luminous high-mass stars, as well."

Critical to the team's argument is the recent discovery in meteorites of patterns of isotopes that can only have been caused by the radioactive decay of iron-60, an unstable isotope that has a halflife of only a million and a half years. Iron-60 can only be formed in the heart of a massive star and thus the presence of live iron-60 in the young solar system provides strong evidence that when the sun formed 4.5 billion years ago a massive star was nearby.

Hester's coauthors on the Science essay include Steve Desch, Kevin Healy and Laurie Leshin, a cosmochemist and director of ASU's Center for Meteorite Studies. "One of the exciting things about the research is that it is truly transdisciplinary, drawing from both astrophysics and the study of meteorites - rocks that you can pick up and hold in your hand - to arrive at a new understanding of our origins," noted Leshin.

When a massive star is born, its intense ultraviolet radiation forms an "HII region" - a region of hot, ionized gas that pushes outward through interstellar space. The Eagle Nebula, the Orion Nebula, and the Trifid Nebula are all well-known examples of HII regions. A shock wave is driven in advance of the expanding HII region, compressing surrounding gas and triggering the formation of new low-mass stars.

The process leaves a sun-like star and its surrounding disk sitting in the interior of a low density cavity with a massive star close at hand. Massive stars die young, exploding in vio lent events called "supernovas." When a supernova explodes it peppers surrounding infant planetary systems with newly synthesized chemical elements - including short-lived radioactive isotopes.

"There are many aspects of our solar system that seem to make sense in light of the new scenario," noted Leshin. "For example, this might be why the outer part of the solar system - the Kuiper Belt - seems to end abruptly. Ultraviolet radiation would also have played a role in the organic chemistry of the young solar system, and could explain other peculiar effects such as anomalies in the abundances of isotopes of oxygen in meteorites."

"It is kind of exciting to think that life on Earth may owe its existence to exactly what sort of massive star triggered the formation of the sun in the first place, and exactly how close we happened to be to that star when it went supernova," mused Hester. "This new scenario has a lot of implications, and makes a lot of new predictions that we can test."

If it is accepted, the new theory may also be of use in looking for life in the universe beyond. "We want to know how common Earth-like planets are. The problem with answering that question is that if you don't know how Earth-like planets are formed - if you don't understand their connection with astrophysical environments - then all you can do is speculate," Hester said.

Hathaway, with College of Liberal Arts & Sciences, can be reached at (480) 965-6375 or ( Hathaway@asu.edu ).