http://www.jpl.nasa.gov/releases/99/glprobeargon.html
The discovery of these gases in such high quantities at Jupiter raises
questions about how they got there. "In order to catch these gases, Jupiter
had
to trap them physically by condensation or freezing," Owen said. This
process, he
said, requires extremely cold temperatures of about -240 degrees Celsius (-400
degrees Fahrenheit), colder than the surface of Pluto, the planet farthest
from the
Sun. Planetesimals (small objects orbiting the Sun) in the Kuiper Belt beyond
Pluto would be this cold, but Jupiter is more than six times closer to the
Sun and
thus is much warmer. For this reason, Jupiter could not have been the site
where
the three noble gases were originally trapped.
"This raises some intriguing possibilities," Owen said. "One
explanation
suggests that Jupiter was formed out in the area around the Kuiper Belt and
was
dragged inward to its present location. Another possibility is that the solar
nebula, a huge cloud of gas and dust from which our solar system formed, was
much colder than scientists believe. A third hypothesis proposes that the
solid
materials that brought these noble gases to Jupiter began forming in the
original
huge, interstellar cloud of gas and dust even before it collapsed to form
the solar
nebula. That would make these icy materials older and more primitive than we
had expected."
"If either of the last two hypotheses turns out to be correct, it
would suggest
that giant planets can form closer to their stars than current theories
predict,"
Owen said. "This could help explain the new observations of planetary systems
around other stars, in which such close-in giant planets are relatively
common."