Imagine being able to replay the formation of the Solar System, not in theory or simulation, but in real time, around a distant star. Just that is what scientists have achieved, capturing the first direct evidence of planet formation in its nascent stages around a young star named HOPS-315 a process once deemed all but impossible.
HOPS-315, a protostar about 1,300 light-years away from us in Orion, is more than a dot in the evening sky. It’s a living fossil, the virtual twin of our Sun in its youth, and now, a cosmic laboratory for the discovery of the secrets of planetary birth. “For the first time, we have identified the earliest moment when planet formation is initiated around a star other than our Sun,” Leiden University’s Melissa McClure said, as quoted by the ESO press release.
The secret to the breakthrough was combining the record-breaking sensitivity of the James Webb Space Telescope (JWST) with the millimeter-wavelength ability of the Atacama Large Millimeter/submillimeter Array (ALMA). These telescopes pierced through the dense, turbulent envelope of gas and dust that is powering HOPS-315 to reveal a planetic disk in which the first seeds of planets planetesimals are only beginning to develop.
What’s distinctive about this discovery is that the researchers have observed warm silicon monoxide gas and silicate grains the telltale signs of summary from gas-to-solid. University of Michigan’s Edwin Bergin reported in Phys.org, “This process has never been seen before in a protoplanetary disk or anywhere outside our Solar System.” The minerals were found at a distance of approximately 2.2 astronomical units from HOPS-315, a region strikingly close to the orbit of the asteroid belt in our own Solar System. Logan Francis of Leiden University went on further to say, “We’re really seeing these minerals at the same location in this extrasolar system as where we see them in asteroids in the Solar System.”
This is a region of importance: in our Solar System, old meteorites possess calcium-aluminum-rich inclusions (CAIs) the oldest solids, which set the “time zero” of planetary development. Detection of crystalline silicates and silicon monoxide in HOPS-315’s disk would be expected from theoretical models that predict the conditions of CAI formation, says Phil Armitage of Stony Brook University in Scientific American.
How then are these tiny grains built into planets? In this case, the physics of coagulation and planetesimal formation takes center stage. New simulations, such as those described in a July 2024 arXiv study, indicate that micron-sized dust grains can increase by coagulation and streaming instability processes that form dense clumps. When these clumps gain a high enough density, they become gravitationally unstable and collapse to kilometer-sized planetesimals. The grain shattering speed of the dust grains larger for ice grains, smaller for silicates is instrumental in determining the rate and efficiency with which this growth is occurring.
JWST’s infrared spectroscopy was instrumental in this study. Its medium-resolution spectrometer can distinguish between gas-phase and solid-phase molecules, allowing researchers to determine the specific phase of summary in the disk. ALMA radio observations then mapped the spatial distribution of the minerals, confirming their location within the disk, not in outflowing jets or winds a resolution earlier telescopes could not achieve.
The implications are profound. HOPS-315 offers a “baby picture” of a solar system that is very similar to the early Solar System, providing a rare analog on which to observe the processes that created Earth, Jupiter, and everything in between. According to Merel van ‘t Hoff of Purdue University, as quoted in the University of Michigan release, “This system is one of the best that we know to actually probe some of the processes that happened in our Solar System.”
As the same signs are hunted for by astronomers in other young stars, the combined power of JWST and ALMA will be able to transform our understanding of the formation of planets spanning from the chemistry of initial dust grains to the architecture of planetary systems as a whole.
