A star is not just a distant sun. It's a cradle for planets, and its light carries the birth story of the entire system. The chemical composition of a star is its DNA. Astronomers peer into this DNA to understand why some stars host rocky worlds like Earth, while others host gas giants. In a new study, scientists took a huge sample — more than 28,000 stars observed by the Kepler, LAMOST, and Gaia telescopes. Of these, 629 stars host planets—865 worlds in total. But instead of simply comparing 'stars with planets' and 'stars without planets,' the authors pulled a clever trick.
They matched each host star with a 'twin'—a star of the same age and mass but with no detected planets. This approach removes confusion: what if the differences arise not because of the planets, but simply because planetary systems are more common around older or more massive stars? It turned out that in the overall sample, host stars are almost indistinguishable from their childless counterparts. But when the planets were split by size, a clear pattern emerged. Stars with small rocky planets (Earth-like) showed an excess of oxygen and a deficit of magnesium and silicon compared to their twins. And stars with sub-Neptunes, planets slightly larger than Earth but smaller than Neptune, showed the opposite: less oxygen, more magnesium and silicon.
Then it gets more interesting. Sub-Neptunes are more common around stars with lower chromospheric activity (think of it as a calm temperament — fewer flares and spots) and with a smaller birth radius. The birth radius is the distance from the galactic center where the star first ignited. So sub-Neptunes prefer calmer, inner regions, while Earth-like planets prefer more turbulent outskirts. Even hot Jupiters showed their temperament: their stars turned out to be slightly richer in metals, more active, and also born closer to the center than hosts of more distant Jupiters.
These findings paint a picture where planet size is not a coincidence but a result of the cosmic cradle in which it grew. Like children from different neighborhoods, planets bear the imprint of their environment. And now, looking at a distant star, we can read in its light what worlds it is most likely to harbor.
If our Sun were slightly poorer in magnesium and silicon, Earth might have been larger or might not have formed at all — stellar chemistry dictates the menu for planets.
In Stanisław Lem's novel 'Solaris', the planet's ocean holds memories of the past — much like starlight holds chemical clues about the birth of worlds.