Every planet with a magnetic field has a radiation belt, a region of space where charged particles get trapped and flung around at extraordinary speeds. Earth has two of them, and they've been puzzling scientists for decades. Now, a physicist at the University of Helsinki has built a model that defines a universal upper limit to just how energetic those belts can ever get. The answer applies not just to Earth, but to every planet in the Solar System, every gas giant, and even the strange objects sitting halfway between planets and stars.