“The atmospheric disruptions could be seen clearly and persisted longer than any impact heating”
Back in July 1994, the astronomy community was taut with anticipation. Comet Shoemaker-Levy 9 had been discovered the previous spring, as was the fact that it was on a collision course with Jupiter. The original comet would have been about 5km across, but by this time had been broken up by tidal forces from Jupiter’s powerful gravity into a long string of fragments, which slammed into the gas giant over the course of a week like machine gun fire. Of course Jupiter rotates, so the impacts were spread around the globe along a line of latitude of about 45°S, roughly corresponding to the southern tip of New Zealand on Earth. If such an object had hit our planet, the effect would have been apocalyptic. As it was, Shoemaker-Levy 9 offered us an invaluable opportunity to watch a planetary collision in realtime and study the effects on Jupiter’s atmosphere.
A natural follow-on question, which Laura Flagg at Northern Arizona University and her colleagues ponder in this month’s paper, is would a similar cosmic collision be detectable on an gas giant exoplanet? Infrared observations indicate that roughly one in five mature Sun-like stars possesses a dusty debris
is an astrobiologist at the University of Leicester and the author of The Knowledge: How to Rebuild our World from Scratch (www. the-knowledge.org) disc, produced by ongoing collisions between remaining planetesimals. Today, comets larger than 1km in diameter strike Jupiter at the rate of about one per century, with Shoemaker-Levy 9 believed to have originated in the Kuiper Belt.
Using the Shoemaker-Levy 9 impacts as a case study, Flagg worked out what signs might be visible on an exo-Jupiter. The impacts caused very bright fireballs for a few a seconds, and the hotspots from these remained in the atmosphere for a few hours. But these effects are so brief that we’d be astonishingly lucky to catch one at any given time we happened to be observing an exoplanet. But another major effect of the Shoemaker-Levy 9 impacts, and one that was much longer lasting, was the plume of small particles deposited at high altitudes in Jupiter’s atmosphere.
Jupiter contains trace amounts of methane, which absorbs strongly at the 2.3µm and 3.3µm wavelengths. The high-altitude particles deposited by ShoemakerLevy 9, however, reflect this near-infrared light back out into space before it can pass down deeper in the atmosphere and be absorbed by methane. So the disruptions to Jupiter’s atmosphere could be seen very clearly as bright reflective spots at 2.3µm and 3.3µm, and these features persisted far longer than any impact heating effects.
These same spectroscopic features could, in principle, also be detected in our observations of gas giants beyond out Solar System, says Flagg. And if we were able to measure periodic variations in the spectra as these impact-related bright spots turned with the planet, we could determine the exoplanet’s rotation rate. But more generally, studying how impacts can distort the detectable atmospheric composition is important for our understanding of the make-up of exoplanets. For example, Jupiter still has water high in its stratosphere, only because of the Shoemaker-Levy 9 impact.