A young star called V1298 Tau is giving astronomers a front-row seat to the birth of the galaxy’s most common planets. Four massive but extremely low-density worlds orbiting the star appear to be inflated precursors of super-Earths and sub-Neptunes. By watching how the planets subtly tug on one another, scientists measured their masses and confirmed they are far puffier than expected. The system reveals how these planets dramatically shrink and transform as they age.