NASA’s Juno, one of many planned projects described at the symposium, is being readied for launch next year. Juno will orbit perpendicular to the planet’s equator instead of following the more traveled path around it, and will get a bird’s-eye view of the giant auroras seething at the poles. These flamboyant light displays owe their colors and shapes to supercharged interactions among the sun, Jupiter, and its satellites. Io in particular, the closest of the four Galilean moons, contributes mightily to the store of charged particles that excite Jovian auroras, due to its several hundred volcanoes.
“Io is a paradise for volcanologists,” said Rosaly Lopes of the Jet Propulsion Laboratory, “but it’s hell for cartographers, because the minute you make a map, it’s obsolete.”
Lopes’s birthday fell during the symposium week, and she commented how she wished she had been born on January 7 (the day Galileo first saw the Medicean moons) instead of on the 8th. Later she learned that although Galileo noticed the moons on the 7th, he mistook them for background stars, not questioning their true identity until the following night, when he returned to find two had moved from one side of Jupiter to the other. Eager to see them again, he was frustrated on the 9th by overcast skies.
The weather in Padua proved even worse for the 400th anniversary of these discoveries, with clouds and rain almost every night.
On the 10th, in 1610, the moons appeared in yet another configuration, and on the 11th, Galileo “arrived at the conclusion, entirely beyond doubt, that in the heavens there are...stars wandering around Jupiter like Venus and Mercury around the sun.” With those words, he reported the findings in his explosive little best seller, Sidereus Nuncius (The Starry Messenger).
Scientists now think that all of Jupiter’s moons and rings formed from a disk of debris around the newborn planet, just as the planets of the solar system took shape in the disk of leftovers surrounding the infant sun. Similar disks are observed today around other sunlike stars and are presumed to be new planetary systems in the making. Most have “hot Jupiters”—gigantic planets, some of them several times the mass of our own Jupiter, lying as close to their parent stars as Mercury is to the sun, or in many cases far closer.
The absence of a hot Jupiter in our own solar system, according to symposium speaker Alessandro Morbidelli of the Observatoire de la Côte d’Azur, France, reveals some of our ancient history: At one stage Jupiter was indeed spiraling rapidly inward, but Saturn checked its progress. Saturn “caught” Jupiter in a resonant orbital pattern that kept both planets from continuing their sunward course. Morbidelli also noted how planetesimals (protoplanets, such as the preserved relics in the Kuiper belt beyond Neptune) can take a long time to come to rest. To emphasize the point, he invoked Galileo’s famous phrase, “Eppur si muove” (“But still it moves”).
Nick Schneider, an expert on Io’s atmosphere at the University of Colorado at Boulder, stayed an extra day in Padua after the symposium ended. “I will torment you by saying that on Sunday the skies cleared,” he wrote to me, “and I had the privilege of letting some Italian kids rediscover the Medicean stars with a telescope I had brought, while their mother read aloud—in a dramatic voice—Galileo’s words from Sidereus Nuncius. You would have loved it!”