http://sfgate.com/cgi-bin/article.cgi?f=/c/a/2002/06/17/MN119116.DTL

Heated debate on formation of solar system
Theorist believes 9 planets created out of chaos, not calm

Keay Davidson, Chronicle Science Writer

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A radical new theory has astronomers arguing over the origins of our solar system.

If the theory is correct, then the sun, Earth and its fellow planets were born in a place that is far different from what astronomers have assumed -- as different, in terms of cosmic glamour, violence and beauty, as New York is from Dubuque.

The theorist, Alan P. Boss of the Carnegie Institution of Washington, still recalls the "classic eureka moment" last year when he hit on his idea, which he unveiled at a NASA conference in Mountain View during the spring. He believes our solar system was born inside a cosmic maelstrom similar to the faraway, spectacular Orion nebula.

Boss' idea -- a "paradigm shift," he calls it -- contradicts the traditional belief that our nine-planet system emerged inside the comparatively calm twilight of a nebula similar to a relatively nearby one within the constellation Taurus.

The Orion nebula is one of the most gorgeous sights in the winter sky -- an immense "nursery" where baby stars are being born. Recent Hubble Space Telescope photos of Orion reveal a scene so awesome that it would have floored Dante or John Milton: Orion's fledgling star systems resemble brilliant arc lamps scattered through a boiling cloud.

Astronomers have tried to explain the origin of our solar system since the 18th century, when the philosopher Immanuel Kant proposed the first quasi- scientific hypothesis of planetary origin. Kant upset the faithful when he described how the sun and planets might have gravitationally condensed from a primordial cloud, with no divine intervention required.

2 VIEWS OF SOLAR SYSTEM'S ORIGIN

Since the 1950s, astronomers have fought over two main hypotheses of solar system origin: "core accretion" and "gaseous accretion."

These two hypotheses offer starkly different answers to a key question: How big were the "building blocks" from which the planets formed billions of years ago? As big as mountains, cities and continents? Or as minute as a fine mist?

The former, according to the core accretion hypothesis. It envisions the planets forming via the aggregation of giant blocks of rock and ice, some as hefty as small planets. The core accretion hypothesis is a variation on a theory championed in the 1950s by the Nobel Prize-winning chemist Harold Urey.

Traditionally, most astrophysicists have preferred the core accretion hypothesis to its rival, gaseous accretion. The latter hypothesis also emerged in the 1950s, the brainchild of astronomer Gerard Kuiper. He believed that each planet condensed directly from a mammoth, misty cloud of gas and tiny dust grains.

Kuiper and Urey fought bitterly over planetary origins and other issues. After their deaths, their mutual student, the late space scientist Carl Sagan, recalled: "There was a time when I believe I was the only person on speaking terms with both Urey and Kuiper, which was a great strain."

 

JUPITER AND SATURN'S FORMATION

In recent years, Boss has fought for an updated, highly computerized version of the gas accretion hypothesis that he calls "disk instability." His computer models showed that a primordial cloud of gas and dust particles could randomly develop pockets of "instability" where the gas density increased enough to begin collapsing into planets gravitationally.

He believes his model offers a convincing answer to an old question of how Jupiter and Saturn formed quickly enough to explain the great thickness of their atmospheres, which are thousands of miles deep.

Astronomers long have assumed that billions of years ago, when the solar system was young, Jupiter and Saturn acquired their atmospheres by gravitationally attracting gas from the primordial nebula, the source of the atomic and molecular "building blocks" for our solar system.

The nebular gas, however, presumably hung around only for a brief time, after which the sun switched on its "solar wind" and blew away the gas. Could Jupiter and Saturn form their hard, gravitationally attractive cores fast enough to "catch" the gas before it flew out of the solar system?

In Boss' view, the answer is yes. His computer model shows that planets' cores could have formed fast enough to catch the gas. Hence their atmospheres no longer seemed so mysterious.

Even so, there was one catch. Boss' disk instability thesis couldn't account for Uranus and Neptune's far thinner atmospheres. If disk instability gave Jupiter and Saturn thick atmospheres, then why didn't it give thick atmospheres to Uranus and Neptune?

Boss hit on the explanation after an East Coast reporter e-mailed him, inquiring about a different matter: She wanted to know "whether or not Orion would be a good place to make planets in general." The nebula is 1,500 light years away, three times more distant than the Taurus nebula.

Reflecting on her question, Boss pondered how the intense radiation of stars within the Orion nebula would have affected young planets and their atmospheres. Suddenly he found himself wondering: What if our planetary system had itself emerged inside the intense radiation bath of an Orion-type nebula?

Imagine, Boss says, that you were present when our solar system was young, about 5 billion years ago.

In front of you, you see a giant, rotating disk of gas and dust particles bathed in the light of the infant sun. Thanks to disk instability, nascent versions of Jupiter, Saturn, Uranus and Neptune are beginning to condense into little globs of matter, not unlike the "dust bunnies" that accumulate in an ill-tended home.

 

STARS EMIT INTENSE RADIATION

Meanwhile, not far from the solar system, other star systems are forming within the same titanic, Orion-style nebular cloud. As the stars ignite, they emit extreme ultraviolet (EUV) light.

Although the alien stars are many trillions of miles away, their EUV radiation is intense enough to heat nebular gas throughout the solar system's protoplanetary disk of gas and dust. The proto-Uranus and proto-Neptune planets are far from the sun, at a point where gravity is comparatively weak. Heated gas in their part of the system escapes more readily into deep space. Result: Uranus and Neptune have comparatively thin atmospheres today.

Boss explained his idea in a paper co-written with his Carnegie colleagues George W. Wetherill and Nader Haghighipour. The paper was published in the March issue of the journal Icarus, a space science journal.

Astronomers' reaction to Boss' theory is mixed. Jack Lissauer, an astrophysicist at NASA's Ames Research Center in Mountain View, said Boss' "idea is hardly advanced enough to call it a model."

One objection to the idea, Lissauer said, is that for the EUV-emitting star to have been so close to our sun for a million years, the star and our sun would have had to be "a binary pair," gravitationally bound together. Lissauer also objects to Boss' disk instability thesis of gaseous accretion.

"Although Alan may believe that a paradigm shift has occurred, few others in the field agree with him," Lissauer said.

A friendlier assessment comes from astrophysicist Steven Kortenkamp of the University of Arizona: Boss "is on to something interesting that is causing a much-needed shakeup in the field."

Kortenkamp noted that the field was rocked by the discovery of the first extrasolar planets beginning in the mid-1990s. Those planets orbiting other stars had characteristics markedly different from the planets in our solar system. For example, they tend to be gas giants that orbit amazingly close to their stars. Astronomers quickly realized the extrasolar planets' characteristics couldn't be explained by existing models of planetary formation.

"These discoveries are making it very difficult to stick to the party line endorsing the so-called standard model" of planetary formation, Kortenkamp said. "Alan Boss is one of the very few mavericks going out on a limb with a totally new theory. . . . Hopefully, we will know if Alan is right in a matter of a few years, maybe a decade."

E-mail Keay Davidson at kdavidson@sfchronicle.com.