I was walking out of a classroom at the Massachusetts Institute of Technology, where students and I had been talking about the way planets form, when I was stopped by my colleague Ben Weiss. He studies magnetism in space rocks, and he was very excited. Weiss pulled me down the hall to his office to show me new data on one of these rocks, a meteorite called Allende. It was information that could change almost everything planetary geologists thought about the solar system. It was 2009, and that fall Weiss’s research team had shown that Allende—which crashed into Earth in a huge fireball in Mexico in 1969 and contained some of the oldest known material in our system—harbored signs of an ancient magnetic field in its rock. The discovery was a surprise. This kind of field, astronomers thought, was made only by a magnetic dynamo of intensely hot, flowing liquid metal inside a planet, the way Earth’s magnetic field is produced by liquid iron spinning in the planet’s core.