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A kindergarten where some kids are busy making sandcastles while others are already assembling building blocks. That's roughly what the V1298 Tau system looks like: it's only 20 million years old — by cosmic standards, it just left the nursery. Several planets orbit it, and two of them, b and e, turned out to be surprisingly different. The James Webb Space Telescope peered into their atmospheres, and what it saw resembles not siblings but rather playground neighbors with completely different toys.

On planet b, the air contains sulfur dioxide — a pungent gas that volcanoes emit on Earth. On planet e, there's carbon disulfide, a molecule with the sharp smell of rotten radish. This is the first time such a sulfur compound has been spotted in an exoplanet's atmosphere. It all comes down to the star's light: it bathes the planets in ultraviolet, and this harsh light triggers chemical reactions. But planet e sits a bit farther from the star, so it receives less UV. That was enough to send the chemistry down a different path, like turning on a bright lamp in one room while leaving a nightlight in another.

This difference between neighbors isn't just a curiosity. It shows that planets in the same system can mature differently. Perhaps e formed differently from b, or its atmosphere has survived its own upheavals. Carbon disulfide is like a growth chart marker: it tells us the planet is still very young and its chemistry is far from equilibrium. For us, it's a chance to glimpse what Earth and its neighbors looked like in infancy, when they were just a mere 20 million years old.