William M. Grundy

Ever since Voyager 2 raced past Uranus in 1986, the planet's icy moons have kept a mystery: where does the carbon dioxide on their surfaces come from? The spectrometers of that era only hinted at the presence of CO₂, but couldn't say anything about its origin.

Beyond Neptune's orbit lies an ocean of icy islands that has remained nearly unchanged for four billion years. These islands, known as trans-Neptunian objects, hold the recipe for planet formation. Until recently, astronomers could only see the large ones, while the small, city-sized ones stayed invisible.