One clue to the question of a giant planet's formation would be whether the planet has a core.

Terrestrial planets like Earth grew from the accretion of planetesimals, which slammed into each other and amassed enough bulk over time to develop gravity. All this bombardment activity resulted in a rise in temperature, liquefying the material and causing the heavier elements to sink toward the center.

For the early Earth, the heaviest element was iron, so our planet has an iron core.

Mars and Venus also have metallic cores. Scientists believe Saturn, Uranus and Neptune each must have some sort of core as well. Jupiter's core, however, is still an open question.

"In the mid 1980s, the belief was that the cores of Jupiter and Saturn were both large -- about 10 to 30 Earth masses," said Boss. "However, the new data from the Galileo spacecraft, along with refined theoretical models, now indicate that Jupiter's core mass is more likely to be about six Earth masses, and could even be zero."

If Jupiter doesn't have a core, it must have formed through disk instability. Even if a solid center is present, Boss says, the planet still could have formed that core through disk instability. Enough dust could have collected and cemented together in the dense gas to form a core many times larger than the size of Earth.

The size of that core -- if any -- would provide an important clue to unraveling the process by which the planet formed.

"A six-Earth-mass core in Jupiter could have formed from the sedimentation of the dust grains expected in a Jupiter-mass gas-giant protoplanet," said Boss.

For Jupiter to have formed through accretion, some scientists say, its core would have to be at least 10 Earth masses. A smaller core would not have had enough gravity to collect the amount of gas we see on the planet today.

Alan Boss believes NASA's Space Interferometry Mission (SIM), slated for launch in April 2009, will help settle the controversy.

"SIM will be able to detect the wobbles caused by Jupiter-mass companions to nearby young stars, so SIM may well be able to settle the issue," Boss said.

This would help scientists determine the time frame in which such planets form around young stars.

Levison thinks the application of new "hydrocodes" -- large computer programs used by Boss and other scientists to simulate astrophysical processes -- could also help answer the question.

Using mathematics, these models can help scientists compute highly dynamic events as a function of time and position. New hydrocodes can adjust their resolution, allowing scientists to zoom in on an area of interest and look at it in better detail.

"These new hydrocodes could be used to model the solar nebula on a grid system," said Levison. "If an area of the gas disk starts to collapse -- if the density is going up -- then you could look at that area more closely and see what is going on. However, some of the necessary physics is missing. We still don't understand how some things work, like radiative transfer -- how light moves through these things. But at least these new hydrocodes could give us a better understanding of whether or not disk instabilities really could lead to Jupiter formation."

In the meantime, Boss hopes other scientists will continue to study and test the possibilities of both the core-accretion model and his disk-instability model.

"Core accretion is still the popular theory for explaining the formation of Jupiter and other planets in our solar system," he said.

"People have been thinking in terms of core accretion for the last two decades, and it takes a while for people to get used to any new idea, including scientists. But I think that with time, and with continued work on both core-accretion and disk-instability mechanisms, most scientists will be able to agree about the likely means for making gas-giant planets."