Konstantin Batygin did not set out to solve one of the solar system’s most puzzling mysteries when he went for a run up a hill in Nice, France. Batygin, a researcher at the California Institute of Technology, best known for his contributions to the search for the solar system’s missing “Planet Nine,” spotted a beer bottle. At a steep, 20-degree grade, he wondered why it was not rolling down the hill.
He realized there was a breeze at his back holding the bottle in place. Then he had a thought that would only pop into the mind of a theoretical astrophysicist: “Oh! This is how Europa formed.”
Europa is one of Jupiter’s four large Galilean moons. And in a paper published May 18 in the Astrophysical Journal, Batygin and a co-author, Alessandro Morbidelli, a planetary scientist at the Côte d’Azur Observatory in France, present a theory explaining how some moons form around gas giants like Jupiter and Saturn, suggesting that millimeter-sized grains of hail produced during the solar system’s formation became trapped around these massive worlds, taking shape into the moons we know today.
Batygin and Morbidelli say earlier theories explain only a part of how the solar system’s many objects formed. They set out to present the rest of the story with equations explaining how a new planet transitions from being surrounded by its disk of matter, to creating satellite building blocks, to the formation of moons like Europa.
When Batygin and Morbidelli ran computer simulations of their proposed theory, they found that they had accidentally re-created Jupiter’s small innermost moons as well as the four Galilean satellites, much as we see them today.
The equations amount to a recipe for how to make a moon. It starts with a mix of hydrogen and helium gas raining down onto Jupiter from above. Some of the gas gets swept out and away, spreading viscously as it goes into orbit around Jupiter.
At this point in Jupiter’s formation, the only solid particles that orbited it were smaller than 1 millimeter across. Because this dust is very small — tiny grains about two parts ice to one part rock — it can couple itself to the gas washing away from Jupiter.
As this material builds up over the course of about a million years, Batygin said, it eventually reaches a mass that approximately matches Io, Europa, Ganymede and Callisto today.