Forty light-years away, there is a star that shines two thousand times fainter than the Sun. But it has seven Earth-sized planets. Until now, no one knew what the star itself was made of. Now astronomers have obtained its chemical passport, complete with precise numbers.

A star's light is not just brightness. Each atom leaves its own mark in it: dark lines in the rainbow. But for cool red dwarfs, these marks blur together like a signature in wet ink. To separate them, scientists took two slices of infrared light, one with shorter wavelengths and one with longer, and superimposed them. This allowed them to extract the signatures of rare carbon and oxygen isotopes.

It turned out that the star has nearly equal amounts of carbon and oxygen, just like the Sun. But it has less heavy carbon-13 than the Sun: for every 154 light carbon-12 atoms, there is one heavy one. This means the star is older than the Sun, about 7.6 billion years old. Oxygen-18 is present in the same amounts as in the Sun and other red dwarfs. For the seven planets, this passport is like a birth certificate: they formed from the same cloud as the star. Now, when telescopes peer into their atmospheres, we will know what to compare against.