Steam over a boiling pot slightly tints the light of a flashlight: it blocks some colors and lets others through. That's roughly how we study the air of distant planets. When a planet passes in front of its star, a tiny fraction of starlight pierces its atmosphere, and molecules leave dark bands in the spectrum: their "fingerprints."

The James Webb Space Telescope JWST did this for the planet TOI-1130b — a warm mini-Neptune (about 3.7 times wider than Earth and 20 times heavier). Its atmosphere turned out to be a real chemical cocktail. Clearly visible are water (H₂O), carbon dioxide (CO₂), and even sulfur dioxide (SO₂)—the latter is rarely found on such planets. There are hints of methane (CH₄) as well, but not as clear yet.

The composition reveals the planet's origin. The atmosphere is rich in heavy elements (metallicity about 60 times solar), and the carbon-to-oxygen ratio is low. Such a mixture is typical of worlds born far from the star, beyond the "snow line"—the boundary where water freezes into ice. Most likely, TOI-1130b formed in the cold outer regions of the system and then moved closer to the star. Possibly under the influence of its more massive neighbor — a hot Jupiter on an outer orbit. That one, like a giant vacuum cleaner, could have "filtered" pebbles and ice, directing some material inward.

This discovery adds an important detail to the portrait of mini-Neptunes — the most common type of planet in the galaxy, which are absent in our solar system. It seems they don't have a single recipe for formation: some grow in place, others come from afar, bringing volatile substances with them. By studying their air, we are essentially peeking into the pot where planetary diversity is simmering.