Around the young star MWC 480, a flat disk of gas and dust whirls, like a giant carousel where, instead of horses, you have building blocks for planets. Radio telescopes peered into this disk at different wavelengths, from millimeter to centimeter, and saw exactly where the dust accumulates and how large the grains are.

The brightest ring sits at 95 astronomical units from the star — about three times farther out than Neptune is from the Sun. For the first time, it was spotted at a wavelength of 9.1 millimeters. Inside the ring, dust grains stick together into clumps up to a centimeter in size, like snowballs gradually building a snowman. The mass of this ring alone is between 43 and 100 Earth masses. That's enough to assemble the cores of giant planets like Jupiter.

Observations at different wavelengths showed that the dust in the ring comes in two varieties: dense, compact grains or very porous ones, 90% empty space, like a sponge. Both options explain the data equally well. To choose between them, we need even broader observations, and then we'll understand more precisely what planets are made of.

The ring at 95 astronomical units is not just a storage depot. Its sharp edges and constant width across all wavelengths suggest that assembly of large bodies may already be underway inside. And a bit farther out, at 160 astronomical units, lurks a second, fainter ring, like the outer asteroid belt in our system, but around another star.