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How did the largest planet in our solar system form?

The traditional view is that Jupiter first formed a rocky core, several times the size of Earth, which then attracted a still larger outer envelope of gas. This process is known as “accretion.”

But there are problems with this model. The major problem is that if the large, gaseous planet did form by the gradual accretion of material, it would have taken a very long time to develop. Current estimates range between 10 million and 1 billion years. However, recent observations of distant stars suggest that planets have at most a few million years or less to gather up as much dust and gas as they can before the protoplanetary disk that feeds them disappears.

Alan Boss, a planetary scientist at the Carnegie Institution of Washington and a member of the NASA Astrobiology Institute, has developed a different theory, based on computer models, about how planets like Jupiter may have formed. He believes gas giants could form as a result of instability in a star’s protoplanetary disk.

“In the disk instability mechanism, the action occurs in a disk of gas and dust which is orbiting around a star,” says Boss. “Clumps form, contracting and increasing in density to become gas giant protoplanets.”

These clumps of denser gas would form quickly, within a few thousand – perhaps as few as several hundred – years. Such quick formation would enable the planets to develop before the protoplanetary disk disappeared.

“I think this model of disk instability is an intriguing idea,” says Hal Levison, principal scientist at the Southwest Research Institute. “This model could solve a lot of problems we have regarding Jupiter’s formation, but we’re quite far away as to knowing whether or not it is true. For instance, we don’t know whether the clump stays there, or if it eventually destroys itself. It seems to me that the technology is not quite there yet to answer whether disk instability would lead to the formation of planets like Jupiter.”

But the discoveries of planets in other solar systems, Boss argues, have illustrated hidden flaws in the core-accretion model.

“It is only very recently that the severe problems with core accretion have become obvious,” says Boss, “and only recently have we found out about extrasolar planets, many of which are much more massive than Jupiter and hence even harder to form by core accretion.”

The Evolution of Stars and Planets

Scientists now generally believe that protoplanetary disks of gas and dust last only a few million years because that is what they have observed from studies of distant newborn stars.

“We can measure the age of a star very well, so determining the age of a disk is a firm fact,” says Levison. “Most people say, ‘Within 10 million years, the gases go away.’ We know of no longer-lived disks.”

But scientists cannot say for certain that all protoplanetary disks are short-lived. It could be that our own Sun’s disk lasted much longer than average, and therefore the planets in our solar system had a much longer period of time during which to form.

“Models suggest that core accretion seems to need at least several million years to form Jupiter,” says Boss, “yet most protoplanetary disks do not seem to exist that long. Maybe the solar nebula was particularly long-lived, though – in which case, solar systems like our own may be rare.”

Until recently, scientists believed that our own solar system was typical. “Astronomers have been raised since babies on the cosmological principle that our solar system is not a special place in the Universe,” says Levison. “Now we’re finding that that may not be true, and we may have to abandon that principle. It may be that only one in a thousand stars may be capable of sustaining solar systems like ours, where Earth-like worlds capable of sustaining complex life are possible.”

Because current technology is not sufficiently sensitive to detect solar systems like ours around other stars, however, there is no way to determine at present whether our solar system is common or rare.

If planets form only through core accretion, then our solar system’s protoplanetary disk must have been long-lived. And if long-lived disks are rare, then planets in general must also be rare. If, however, planets can form by another, quicker means like disk instability, then planets could be more common.

So far, over 60 extrasolar planets have been discovered. These planets are all large, gaseous planets, most of which (because of the limitations of current observing techniques) are several times larger than Jupiter. Boss views the discovery of these super-Juptiters as strong evidence that disk instability is at work.

“The rate of discovery of extrasolar gas-giant planets seems to show that gas-giant planets are common,” says Boss. “This implies that there must an efficient mechanism for forming gas-giant planets. That seems to point to disk instability. If long-lived disks exist, they are a rarity, whereas disk instability may occur very frequently. A high frequency of extrasolar planets orbiting nearby stars would imply that disk instability must occur.”

“But,” cautions Levison, “we have only seen planets around five percent of the stars studied. We still don’t know what is typical.”

Combining the Theories

Could planets form through a combination of accretion and disk instability? Boss thinks so. He says that planetesimals (tiny bits of rocky or metallic material) would likely have been forming at the same time or even before a disk instability occurred in a protoplanetary disk. Therefore, some sort of coupled planetary evolution would have been inevitable.

“In some cases, the planetesimals would be tossed around or out of the solar system,” he explains. But in others, “they would be swallowed by a proto-Jupiter” that had been formed through disk instability.

Boss says that planetesimals could account for the abundance of heavy metals detected in Jupiter’s thick outer envelope of molecular hydrogen and helium. In order for a planet composed of only dense gases and a solid core to acquire these metals in its envelope, Boss thinks some sort of planetesimal accretion must have occurred along with disk instability. Just as asteroids and comets occasionally bombard Jupiter today, planetesimals would have been gravitationally attracted to the proto-Jupiter clump of gas in the past.

“In that sense,” says Boss, “a compromise theory combining disk instability and planetesimal accretion is not just attractive, it may be absolutely necessary.”

But Levison doesn’t see how a planet could form by the two mechanisms at the same time. “The time scales for disk instability are very, very short,” says Levison. “So short, in fact, that plantesimals wouldn’t even have a chance to form. So I don’t think a planet could form through both accretion and disk instability.”

But Boss disagrees, saying that disk instabilities might not occur until after some planetesimals had already formed.

Both scientists, however, agree that you might have both processes going on at the same time within the same protoplanetary disk, with some planets being formed by disk instability and others by accretion. In fact, if disk instability were responsible for forming Jupiter, our own solar system would be such a hybrid.

The Core of the Problem

One way of solving the question of a giant planet’s formation would be to determine whether or not the planet has a core. Terrestrial planets like the 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, making the protoplanets molten 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,” says 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 6 Earth masses, and could even be zero.”

On this point Levison agrees: “We always thought that Jupiter had to have a core, but with new information from the Galileo probes we think it’s possible now that Jupiter doesn’t have a core.”

And if Jupiter doesn’t have a core, then it must have formed through disk instability. Moreover, Boss says that even if Jupiter is proven to have a core, 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 the Earth.

Knowing the size of Jupiter’s core would provide an important clue to unravelling 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,” says Boss.

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

What Next?

Alan Boss believes NASA’s Space Interferometry Mission (SIM), slated for launch in April 2009, will help settle the controversy of how planets like Jupiter form. SIM will include a test to try to determine the time frame in which gaseous planets like Jupiter form around young stars.

“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,” says Boss.

Levison thinks the application of new “hydrocodes” could also help answer the question. Hydrocodes are large computer programs that have been used for many years by Boss and other scientists to simulate astrophysical processes. Using mathematics, these models can help scientists compute highly dynamic events as a function of time and position. The 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,” says 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,” says Boss. “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.”

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