Imagine you can tell what city someone grew up in by their accent. A similar principle works for distant planets: the composition of their atmosphere holds a memory of their birthplace. Especially intriguing are the so-called hot Jupiters — these are exoplanets-giants like our own Jupiter, but located incredibly close to their stars. The first such planet was discovered in 1995 by Michel Mayor and Didier Queloz. Since then, astronomers have been wondering where they formed. After all, near the star it's too hot and there's little building material. They could only have been born farther out, in the cold regions of the protoplanetary disk — a vast cloud of gas and dust around a young star, where material gradually clumps together into planets.

Water plays a decisive role. In the protoplanetary disk, there is a clear boundary — the snow line of water, where the temperature drops so much that water freezes and turns to ice. Planets born inside this line receive less water ice and are therefore enriched in oxygen; those born outside — the opposite. Scientists used computer models to trace the growth of planets in the accretion disk — the flow of material falling onto the star and planets. Comparing the results with real data on carbon and oxygen content in the atmospheres of nine hot Jupiters, obtained by the transit method (when a planet passes across the star's disk) and spectroscopy (splitting light into colors), they concluded: at least six out of nine planets formed beyond the water snow line, and some even farther, beyond the line of carbon dioxide. This suggests that future hot Jupiters began their journey in the frosty outskirts of the system, where there's plenty of ice.

But how did they end up almost hugging their stars? Scientists believe that in addition to gradual drift in the disk, a huge role is played by planetary migration due to gravitational jostling with other planets. This explains why the orbits of many hot Jupiters are stretched and tilted, even though they are now on very tight orbits. It turns out that chemical analysis of atmospheres — specifically the carbon-to-oxygen ratio, which depends on the overall metallicity of the environment — becomes the key to unraveling the turbulent history of these systems.