The young star TOI-451 wobbles slightly as three planets circle it. Each wobble is like the tremor of a spiderweb when a tiny spider crawls across it. Astronomers caught this tremor to learn how heavy the planets are.
They watched the star for several months using telescopes on Earth. The planets kept passing in front of the star, briefly dimming its light. These 'eclipses' reveal the planets' sizes, while the star's wobbles reveal their masses. But the star itself also shudders: it has spots and flares, like pimples on a teenager's face. To tell the planets' wobble from the star's own, the scientists used a clever mathematical trick. They observed the star in two ways at once: one instrument caught its light, another measured its velocity. That way they could subtract the noise and hear the planets' quiet whisper.
Planet b weighs about as much as four Earths. It could be rocky like ours but with a thin hydrogen cover, or it could be almost entirely water. Planet c is a puzzle: its mass is no more than eleven Earth masses, but we can't say more precisely yet. Planet d is heavier, about ten Earths, and definitely has a dense hydrogen coat.
These three worlds are like three teenagers of different builds. One is skinny and rocky, another plump and gassy, and the third is somewhere in between. By watching them, astronomers hope to catch a glimpse of how planets grow up and shed their first garments.
Planet b is only 125 million years old — by cosmic standards, it's a baby, but it has already lost part of its hydrogen atmosphere.