2025Natur.643..649M

Terrestrial planets and small bodies in our Solar System are theorized to have assembled from interstellar solids mixed with rocky solids that precipitated from a hot, cooling gas^1,2. The first high-temperature minerals to recondense from this gaseous reservoir start the clock on planet formation^3,4. However, the production mechanism of this initial hot gas and its importance to planet formation in other systems are unclear. Here we report the astronomical detection of this t = 0 moment, capturing the building blocks of a new planetary system beginning its assembly. The young protostar HOPS-315 is observed at infrared and millimetre wavelengths with the James Webb Space Telescope (JWST) and the Atacama Large Millimeter Array (ALMA), revealing a reservoir of warm silicon monoxide gas and crystalline silicate minerals low in the atmosphere of a disk within 2.2 au of the star, physically isolated from the millimetre SiO jet. Comparison with condensation models with rapid grain growth and disk structure models suggests the formation of refractory solids analogous to those in our Solar System. Our results indicate that the environment in the inner disk region is influenced by sublimation of interstellar solids and subsequent refractory solid recondensation from this gas reservoir on timescales comparable with refractory condensation in our own Solar System.