The Search for Rings in the Pluto System
There are "almost certainly" rings surrounding the Pluto system, says Henry Throop, who works on the ring science and hazard avoidance teams for New Horizons. The question is whether they will be bright enough that New Horizons can observe them.
Any ring system is generally formed when a planet's small moons are impacted by meteorites, kicking up ejecta in the form of dust and ice. This material is traveling fast enough upon ejection that it escapes the gravity of the small moon, but cannot escape the larger body, and so enters orbit around it. This is the probable mechanism of formation for the faint ring systems that surround planets like Jupiter, Uranus, and Neptune, and could have been at work for Saturn's famous rings.
This Hubble photograph has had its contrast greatly amplified to capture Uranus' ring system, which is normally too faint to be visible. Credit: NASA/JPL/HST
Whether or not this occurs in the Pluto system is complicated by the relationship between Pluto and it's largest moon, Charon. As detailed previously, Charon is so large compared to its parent Pluto that the two bodies, instead of the smaller orbiting the larger, actually orbit around their barycenter -- their shared center of mass, where their two individual masses balance out. This has interesting implications for the system of smaller moons, which are expected to tumble chaotically and follow irregular orbits as a result, and may also impact the way any potential ring system orbits.
Throop's team has done some dynamical modeling in an attempt to understand what effect this could have, by taking a simulation of the Pluto system and adding a great deal of dust to see what happens. But, he says, "your guess is as good as ours."
Any potential ring system of Pluto would be very similar in albedo (light reflectivity) to that of Jupiter, which is just barely detectable by optical instruments. These rings would be composed of dust particles only a micron in size, with only a few occupying every cubic centimeter of space, residing outside the orbit of Pluto's outermost moon Hydra, and so will be very difficult for New Horizons to see.
The best time to attempt detection will be a few days after New Horizons has past Pluto, when the rings are "backlit" by the Sun. At this point, the spacecraft can turn back towards the Sun and observe any ring particles occluding the sunlight, much like seeing dust suspended in incoming light in an otherwise darkened room. In addition to the days after the encounter it will take to get to a sufficient distance behind Pluto, it will take some weeks after that for the relevant imagery to come down from the spacecraft. There may be hints of whether rings or possible in the Student Dust Counter (SDC) instrument data, which is measuring the amounts of dust present in the system and returns data regularly.
The existence or non-existence of a ring system around Pluto will tell us whether they may also be smaller moons lurking further out from Pluto that we haven't detected, and may help us understand the effect that binary systems like Pluto-Charon have on objects that orbit them.