Astronomers using the JWST (James Webb Space Telescope) have for the first time studied in detail the atmosphere of a young exoplanet, V1298 Tau c. This super-Earth, a planet more massive than Earth but less so than gas giants, is only 23 million years old. By cosmic standards, that's infancy: the planet has just finished forming and still radiates residual accretion heat, like a freshly baked pie that hasn't had time to cool.

The key observation method was transmission spectroscopy. As V1298 Tau c passed in front of its star (an event called a transit), part of the starlight pierced the upper layers of its atmosphere. Gas molecules absorbed radiation at specific wavelengths, leaving characteristic absorption lines in the spectrum — colorful fingerprints of the chemical composition. In these spectral data, scientists found a clear signal of water (H₂O). Notably, no traces of methane (CH₄) or carbon dioxide (CO₂) were detected — at least at the current sensitivity level.

In astrophysics, all elements heavier than hydrogen and helium are called 'metals'—a professional jargon where stellar metals are like spices in cooking: they are few, but they drastically change the properties of the medium. Measurements showed that in the atmosphere of V1298 Tau c, the abundance of such 'spices'—metallicity — is 15 times the solar value. However, this figure is noticeably lower than that of similar but fully formed super-Earths of mature age. This evokes an analogy with slow cooking: planets, like a complex stew, gradually accumulate heavy elements over time — possibly through bombardment by planetesimals or long-term geochemical processes.

Of particular note is the stability of water at the colossal atmospheric temperature, where it would be expected to dissociate under high-energy radiation. The world itself is still inflated from the heat of birth: its radius is anomalously large for its mass due to residual thermal energy. These observations hint that the evolution of planets is a slow 'maturing' process stretched over billions of years, and JWST for the first time allows us to glimpse the earliest 'recipes' of planetary system formation, when matter differentiation processes are far from complete.