An asteroid 'the size of the Titanic' caused the luminous scar on Jupiter's surface spotted back in July 2009.
In 1994 astronomers observed the planet being struck by the Shoemaker-Levy 9 comet, and most people assumed that only comets, with their erratic orbits, were likely to get close enough to Jupiter to be dragged to their doom.
Now a team, including Leigh Fletcher of Oxford University's Department of Physics, has analysed the debris and gases given off by the 2009 impact and concluded that it was caused by an object more like a rocky asteroid than an icy comet.
The researchers report their findings in the astronomy journal Icarus.
'Comparisons between the 2009 images and the Shoemaker-Levy 9 results are beginning to show intriguing differences between the kinds of objects that hit Jupiter,' Leigh told us.
'The dark debris, the heated atmosphere and upwelling of ammonia were similar for this impact and Shoemaker-Levy, but the debris plume in this case didn't reach such high altitudes, didn't heat the high stratosphere, and contained signatures for hydrocarbons, silicates and silicas that weren't seen before.'
The presence of hydrocarbons and the absence of carbon monoxide, he explained, are strong evidence for a dry object striking the planet's atmosphere instead of the expected giant hailstone.
The new finding hints that, rather ominously, Jupiter has not hoovered up all the asteroids near its orbit so that there may be other big rocks out there just waiting for their turn to make an impact.