A huge scar formed on Jupiter in July of 2009. It’s shown here near the “bottom” of the planet:
It could have been formed by a comet like body, or by an asteroid (a rocky body).
Data from three infrared telescopes enabled scientists to observe the warm atmospheric temperatures and unique chemical conditions associated with the impact debris. By piecing together signatures of the gases and dark debris produced by the impact shockwaves, an international team of scientists was able to deduce that the object was more likely a rocky asteroid than an icy comet. Among the teams were those led by Glenn Orton, an astronomer at NASA’s Jet Propulsion Laboratory, Pasadena, Calif., and Leigh Fletcher, researcher at Oxford University, U.K., who started the work while he was a postdoctoral fellow at JPL.
“Both the fact that the impact itself happened at all and the implication that it may well have been an asteroid rather than a comet shows us that the outer solar system is a complex, violent and dynamic place, and that many surprises may be out there waiting for us,” said Orton. “There is still a lot to sort out in the outer solar system.”
The new conclusion is also consistent with evidence from results from NASA’s Hubble Space Telescope indicating the impact debris in 2009 was heavier or denser than debris from comet Shoemaker-Levy 9, the last known object to hurl itself into Jupiter’s atmosphere in 1994.
This is interesting because a giant planet like Jupiter should have “cleared” its orbit of rocky bodies, which tend to be in a more or less normal solar orbit. Comets, on the other hand, are much more eccentric in their orbits, and can actually be objects from elsewhere than the solar system, and thus, could collide with virtually anything within the solar system itself.
There is a lot more information here.