How Jupiter Got BigBefore the discovery of planets in other solar systems, scientists thought Jupiter formed slowly, like a mammoth gaseous pearl around a solid center, but one researcher now believes the birth of planetary giants takes only a short time -- maybe even centuries.
In the
traditional view, 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."If this is the case, the large gaseous planet would have taken a very long time -- current estimates range between 10 million and a billion years -- to develop by the gradual build-up of material.
However, recent observations of distant stars suggest that planets have at most a few million years to gather up as much dust and gas as they can before the protoplanetary disk that feeds them disappears. There simply isn't enough time for massive planets like Jupiter to form.
New model "solves a lot of problems"
To solve the problem of how gas giants form, 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, he believes planets like Jupiter 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," said 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 disappears.
The theory has garnered some interest in the scientific community, but some questions remain.
"I think this model of disk instability is an intriguing idea," said 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, theory creator 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," said 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 ticking clock of stellar evolution
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," said 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 to form.
"Models suggest that core accretion seems to need at least several million years to form Jupiter," said 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."
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