It doesn’t happen often that astronomers manage to find the universe in the process of creating planets. Yet, in a moment as rare as it is revelatory, a global team has directly observed the first condensation of refractory solids in a protoplanetary disk the very seeds from which planets are born around the young star HOPS-315, located about 1,300 light-years away in Orion B. This observation is more than a technical milestone; it’s a glimpse into the deep past of our own Solar System, offering a living laboratory for theories long built on the study of ancient meteorites and indirect evidence.
This achievement was the result of merging the powerful capabilities of the James Webb Space Telescope’s (JWST) infrared spectroscopy and the Atacama Large Millimeter/submillimeter Array (ALMA) located in Chile. JWST’s infrared sensors detected the spectral signatures of silicon monoxide (SiO) both in its gaseous and crystalline mineral states, while ALMA’s radio interferometry located where the materials were within the protoplanetary disk. The joint data showed that the initial planetesimal-forming solids are condensing in an area similar to our Solar System’s asteroid belt a vital piece of information in reconstructing the timeline of planet formation.
“For the first time, we have identified the earliest moment when planet formation is initiated around a star other than our Sun,” wrote Leiden University professor and lead author Melissa McClure in the Nature study. Her co-author, Purdue University’s Merel van ‘t Hoff, said it simply: “We’re seeing a system that looks like what our Solar System looked like when it was just beginning to form.”
This discovery was not made in a vacuum. For years, planetary scientists have used meteorite analysis to date the creation of our Solar System. The oldest solids, which are called calcium-aluminum-rich inclusions (CAIs), occur in chondritic meteorites and are more than 4.5 billion years old. The inclusions are created at temperatures in excess of 1,300 Kelvin, conditions that prevail only in the inner parts of a protoplanetary disk. The thickness of crystalline SiO in the meteorites and HOPS-315’s disk indicates a process common to all: high-temperature condensation of refractory minerals is first in planetary construction.
The technology that makes this discovery possible is no less impressive than the science itself. JWST’s MRS allows molecular and mineral signatures to be detected at mid-infrared wavelengths, essential for detecting even the earliest solids in crowded, dusty environments. Concurrently, ALMA’s submillimeter interferometry resolves spatial structures in disks on scales as small as a few astronomical units, enabling astronomers to map the precise region in which solids are condensing. This combination of infrared and radio observations gives a unique insight into the chemistry and physics of planetesimal formation.
The process itself is a subtle interplay of physics. As predicted by theoretical models for a long time, dust grains in the protoplanetary disk stick and collide when they come together, growing from below micron size to macroscopic pebbles. When the aggregates grow to a critical size, they also decouple from the gas and start to accumulate in regions of enhanced pressure, like the disk’s midplane or local pressure humps. With time, gravitational instabilities and more collisions give rise to kilometer-scale planetesimals the precursors to planets. HOPS-315 observations show the first direct indication that this process starts with the condensation of refractory minerals, as the models and meteorite analyses have predicted.
The implications run deeper than a single star system. As Edwin Bergin of the University of Michigan said in the press release, “This process has never been seen before in a protoplanetary disc—or anywhere outside our Solar System.” The area where SiO is condensing in HOPS-315 is where the asteroid belt is located in our own system, providing an intriguing analogue for how the inner planets and gas giant cores could have originated.
For scientists and members of the public alike, the possibility to see firsthand the very beginning of planet building rather than speculate about it constitutes a turning point. The union of JWST and ALMA is not only revealing the window into the past; it is equipping the means to interpret the physical and chemical processes that built every rocky planet, including Earth.
