Examining four-year-old data, researchers have found significantly elevated levels of certain elements in Jupiter's atmosphere that may force a rethinking of theories about how the planet, and possibly the entire solar system, formed. The work may even help explain why giant planets have been found curiously close to other stars.
The elements -- argon, krypton and xenon -- are called noble gases. They are independent characters that don't like to be trapped and strongly resist freezing except at the lowest temperatures (scientists say they are inert). Therefore, they are either rare or nonexistent in the sun, on Earth and in asteroids and comets inside the orbit of Neptune, where temperatures are relatively warm compared with the more frozen reaches of space.
So the discovery in Jupiter's atmosphere of relatively large amounts of these gases -- up to three times what exists in the sun -- has scientists puzzling over how, and possibly where, Jupiter trapped the noble gases in the first place. The puzzle will be described, though not solved, in Thursday's issue of the journal Nature.
"The implications are enormous," said Sushil Atreya, director of the Planetary Science Laboratory at the University of Michigan and part of the international team of researchers that made the discovery.
How planets formed maybe
Prevailing theories of planetary formation hold that the sun gathered itself together in the center of a pancake-shaped disk of gas and dust, then the planets begin to take shape by cleaning up the leftovers. A developing planet trapped nearby gas and dust, and its gravitational tug reigned in comets and other icy bodies, called planetesimals.
In Jupiter's current orbit, 5 astronomical units from the sun, temperatures are too warm for the planetesimals to have trapped the noble gases, researchers say (one astronomical unit, or AU, is the distance from the sun to Earth). Only in the Kuiper belt -- a frigid region of the solar system more than 40 AU from the sun -- could planetesimals have trapped argon, krypton and xenon.
"How did they become so abundant on Jupiter?" asks lead researcher Tobias Owen of the University of Hawaii's Institute for Astronomy.
Owen and his colleagues speculate that either the developing solar nebula was far colder than current models estimate, or else Jupiter wandered into its present orbit sometime after having formed. A third possibility, and the one Owen considers the most likely, is that planetesimals began forming earlier and more rapidly, before the presolar disk had warmed up. Either answer throws current theories into disarray.
And solving the puzzle, Owen says, has implications even beyond our solar system.
"If the planetesimals really formed so early and so fast, then they could build giant planets much closer to their stars than people have thought," Owen explained in an e-mail interview. "This would help to explain why the new planetary systems that are being discovered have giant planets so close to their stars. The planets would not have to migrate inward as far as people have thought."
How the finding was made
In 1995, NASA's Galileo spacecraft dropped a probe into Jupiter's atmosphere. An onboard "mass spectrometer" measured the quantities of various gases. Researchers have been analyzing the data in recent years, but they worked on the most abundant elements first. While that research was valuable, it was the more recent work that proved most surprising.
"The excitement is all about argon, krypton and xenon," Owen said. "You are breathing tiny traces of them right now as you read this."
Owen said the three noble gases are as abundant in the jovian atmosphere as are carbon and sulfur, a "surprising" result. Jupiter's primary ingredients, like that of the sun and the stars, are hydrogen and helium.
Is Jupiter a wanderer?
While Owen does not put much stock in the idea that Jupiter might have migrated inward to its present position, other scientists on the team say the idea merits consideration. As evidence of how little we know about the possibilities, they cited recent announcements of a possible tenth planet orbiting at an incredibly far-out 25,000 AU or more, as well as the fact that planets much larger than Jupiter have been found extremely close to other stars.
But the idea of Jupiter as a wanderer still leaves significant questions about the source of the noble gases. "If Jupiter had migrated inward, it would have had to come from way out there, 40 or 50 astronomical units," said Atreya, the Planetary Science Laboratory director.
Owen said that experts on the physics controlling this kind of migration think such a scenario is "highly unlikely." Researchers add that this distant region of the solar system -- the Kuiper belt at 40 to 50 AU -- does not currently have enough mass to account for something Jupiter-sized, nor are the concentrations of heavy elements comparable to what is found in Jupiter.
"You have to characterize our understanding of how the solar system got started as sort of in a state of flux," said Thomas Donahue, also of the Planetary Science Laboratory. "There may be more to the solar system than we know about."
Where do we go from here?
Since there now seems to be much more learning to do, Owen and his colleagues are calling for more spacecraft to deploy probes into the other gaseous planets. Owen expects the probes will find similarly high levels of noble gases in Saturn, Uranus and Neptune. Hints of these gases have even been found in the thick atmosphere of Venus, another planet now begging more study.
And Owen said answers to the origin of all this argon, krypton and xenon may still be lurking out there, awaiting discovery: "Comets are probably more diverse than we think, and there may still be some of these very primitive objects left in the comet pool."
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