For Levison, the main problem with Boss' theory is the present day abundance of comets that orbit the Sun in a region of the outer solar system known as the Kuiper Belt.

If Uranus and Neptune started out much more massive, as Boss maintains, their gravity would have sent more of these comets on erratic orbits. Some would have crashed into the planets, others would have been ejected from the solar system, Levison says. Further, nearby stars in the crowded Orion region would have had a similar effect.

"If the solar system formed as Boss suggests, I think it is unlikely that you would have the comets and other Kuiper Belt Objects that you see today," Levison said.

He adds that in Boss' scenario the Kuiper belt and Oort cloud (a region that extends nearly halfway to the next star) would most likely look very different from what we see. Also, comets in these regions would have been irradiated -- just like Uranus and Neptune -- and might not have as much ice near their surfaces as is thought to exist.

"These details need to be worked out before these ideas can be accepted," Levison said.

Boss also points out that there is much about the Kuiper Belt that isn't yet understood. Perhaps, he suggests, disk instability will end up explaining some of these things. Irradiation, for one thing, has been shown by other researchers to produce a tar-like layer on cometary surfaces -- a perfect sunblock, Boss said.

(Ironically, Boss' ideas build on work done by Gerard Kuiper around the middle of the last century. Kuiper first proposed the existence of objects beyond Neptune, the region now named for him. Kuiper was also among the first to suggest that planets formed in the collapse of a cloud of gas and dust. Boss uses a computer -- a tool Kuiper didn't have -- to model how bits of material could collect, forming rotating pockets of instability needed to trigger clump formation.)

Levison is also puzzled by how Uranus and Neptune could have lost most of their hydrogen and helium but hung onto their water ice, as disk instability posits. Boss replies that the icy core forms before the irradiation process begins.

Bally, the expert on star formation in Orion, is more optimistic about Boss' prospects.

"I think he has a very interesting idea that deserves serious consideration by the community," Bally said, adding that disk instability "could explain the formation of the solar system in environments like Orion."

But there are problems, Bally said. Having learned of Boss' idea, Bally did some calculations based on a suggestion by Jack Lissaur, a NASA astrophysicist who had previously criticized the disk instability model. Bally's numbers show that the Sun would have had to be extremely close to a hot star -- within 10,000 times the distance from Earth to the Sun -- for the necessary irradiation of the outer protoplanets to take place.

"This is too close," Bally says. The star would have had to be a gravitational companion of the Sun, creating a binary star system. Yet Bally says stars in the chaotic Orion region don't remain in close proximity for long.

Boss said his model could also work if the solar system was irradiated by several stars that were close but not that close.

Either way, Bally continues to maintain that Boss is on the right track. Disk instability may be necessary, he says, to explain the growing known population of giant planets being discovered around other stars.

"There are something like 100 extrasolar giant planets now known," points out Stephen Kortenkamp, of the Planetary Science Institute and the Lunar & Planetary Lab in Arizona. "Perhaps there is room for two correct theories for how they form."

Kortenkamp acknowledges that Boss is out on a limb. But out there with him is George Wetherill, also of the Carnegie Institution and a co-author of Boss' Icarus paper. Wetherill put together some of the first studies of the core-accretion process and has long been recognized as a leading expert on planet formation.

"If Wetherill is taking him seriously, then you know Boss must be onto something interesting," Kortenkamp said.