Forty light-years away lives a star that went unnoticed for a long time: TRAPPIST-1 emits two thousand times less light than the Sun. But it turned out to have seven Earth-sized planets, three of which orbit in the zone where water could remain liquid. Yet little was known about the star itself, about its chemical kitchen. Now TRAPPIST-1 has a chemical passport, with precise numbers that had previously been impossible to extract from its dim light.

A star's light is not just brightness; it is an encoded signature of atoms. Each element and each of its variants leaves its own dark lines in the spectrum, like strokes in a passport. But in cool red dwarfs these strokes blur into a mess: water and carbon monoxide molecules overlap, and telling their traces apart is no easier than reading a signature written in wet ink. Astronomers took two different slices of infrared light, from 2 to 2.4 micrometers and from 4.6 to 5 micrometers, and combined them. In one slice, the lines of carbon monoxide with different isotopes behave differently; in the other, they also differ, but in a different way. Overlapping these two images allowed them to separate the traces of carbon-13 and oxygen-18 from their lighter siblings.

So the first entries appeared in the passport. The carbon-to-oxygen ratio for TRAPPIST-1 turned out to be 0.60, almost like the Sun's. But carbon-13 is rarer than in solar material: there is one atom of carbon-13 for every 154 atoms of carbon-12, whereas in the Sun the ratio is 89. This depletion of heavy carbon is a sign of age: the star is older than the Sun, about 7.6 billion years old, and light carbon has accumulated while heavy carbon has burned out. Oxygen-18, on the other hand, is present in the same proportions as in the Sun and other red dwarfs.

For the seven planets of TRAPPIST-1, this passport is like a birth certificate. The planets formed from the same cloud of gas and dust as the star. If the star has equal amounts of carbon and oxygen (in terms of atom numbers), then the planets likely have a similar balance. This determines which minerals formed there: an excess of carbon would produce graphite and diamonds, while an excess of oxygen would yield quartz and silicates. Now that telescopes will study the atmospheres of these planets, we have a reference for comparison.

But the passport is not only for neighboring worlds. It helps read the history of the entire Galaxy. Heavy isotopes are born in the interiors of massive stars and spread out in supernova explosions. The older the star, the less carbon-13 it has, because light carbon accumulates from the products of helium burning, while heavy carbon is gradually destroyed. TRAPPIST-1 is a rare case: for old stars with solar metallicity, isotopic composition has hardly been measured. Now we have a point on the map, and we need dozens of such points to understand how the Milky Way has changed over billions of years. The dim light of one star turned out to be a key to a much larger story.