Peering through a cosmic keyhole at distant baby star, astronomers may have opened a new window on the deep past of our own solar system.
Using combined observations from the James Webb Space Telescope (JWST) and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, an international research team has glimpsed the earliest moments of planetary creation around the protostar HOPS-315, which lies in a giant star-forming region that is located about 1,400 light-years away in the constellation of Orion. Their findings appear in a study published on Wednesday in Nature.
Weighing in at 0.6 solar mass, HOPS-315 should someday grow to become a star much like our own sun; this makes it a promising stand-in for studying the first stages of our solar system’s history. For now, however, it’s shrouded by a vast and obscuring envelope of inflowing material—baby food for a hungry stellar newborn.
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But JWST’s infrared and ALMA’s radio observations have pierced this veil, peering through a gap in the envelope to probe other structures around HOPS-315 in unprecedented detail—most notably a whirling halo of hot gas and dust called a protoplanetary disk. Such disks are wombs for embryonic worlds; in them, clumps of rock called planetesimals coalesce and eventually build up into full-fledged planets.
Yet no planetesimals can form without smaller grains of crystalline minerals first condensing within the disk, which occurs as the disk’s gas cools. For generations, astronomers have been literally in the dark about this process, as the enveloping clouds that nourish a protostar typically obscure its intimate details. Planetary scientists studying our own solar system haven’t fared much better because more than four and a half billion years lie between them and the birth of our own star and its retinue of worlds.
What little evidence we have from that distant era mostly comes in the form of calcium-aluminum-rich inclusions (CAIs) preserved in ancient meteorites. Precise radiometric dating has shown these to be the oldest solid objects to arise around the sun, suggesting CAIs may be the primordial seeds from which future planets would grow. Scientists set the clock for everything around the sun using CAIs, marking their emergence as “time zero” in our solar system’s history.
Presumably the CAIs were formed by mineral grains showering from the slowly cooling disk of hot gas that must have once surrounded our infant sun. But exactly how, where and when they came into being, no one really knows. Short of having a time machine to go back and look, the only way to solve this mystery is to study what we can see of this process around other infant stars—which, until these observations of HOPS-315, hasn’t been very much.
“Most of what we’ve seen is colder, older protoplanetary disks,” says the new study’s lead author Melissa McClure, an astronomer at Leiden University in the Netherlands. “The period [for the formation of mineral grains and CAIs] is really short, like 100,000 years. Blink, and you’ll miss it. And these young protostars are still enveloped in dense molecular clouds, which are hard to see through.”
HOPS-315, however, is not only very young but also tilted at a certain angle with respect to our solar system—a position that lets astronomers see deeper and closer to the protostar. “This system is a unicorn,” says Fred Ciesla, a planetary scientist at the University of Chicago, who peer-reviewed the Nature paper and penned an accompanying commentary. “It has a hot inner disk that’s still going through this early phase, and it’s oriented so we can actually see it. That makes it very special, and I expect we still have a lot to learn from it.”
Another critical contributor was JWST; earlier observations by other facilities, such as NASA’s Spitzer Space Telescope, had flagged the system as a promising target yet lacked the capability for thorough follow-up. “It was Webb’s massive improvements in sensitivity and spectral resolution that allowed this to happen,” McClure says.
With the stars literally and figuratively aligned, McClure and several colleagues observed HOPS-315 with JWST in March and September 2023. A painstaking analysis of the data revealed the molecular fingerprints of gaseous silicon monoxide, as well as a mix of crystalline silicates—all telltale signs of solid mineral grains condensing out as the hot gas in the protoplanetary disk cools. HOPS-315 is also burping up an outflowing jet of material as it feeds, however, which the researchers worried might be the source of those signals. Subsequent observations with ALMA in November 2023 helped to confirm the mineral grains were present not in the jet but rather in a region of the protostar’s disk that spans twice the distance between the Earth and the sun—and that is located at the equivalent orbit around our star of our solar system’s main asteroid belt. The churning of the disk or intense stellar winds from the growing protostar may help the grains accumulate there.
Although the JWST and ALMA observations did not directly detect CAIs, the ratios of the detected minerals and their location around HOPS-315 are consistent with many models’ predictions of the conditions for the emergence of CAIs at “time zero” in the very early solar system.
“This new work strongly suggests that, for [HOPS-315], conditions suitable for CAI formation occur within about [one Earth-sun distance] at an early time—a fraction of a million years” after a protostar’s formation, says Phil Armitage, a planet-formation theorist at Stony Brook University and the Flatiron Institute in New York City, who was not involved in the new work. This isn’t necessarily surprising, he adds, although “you could certainly imagine other possibilities” in which CAIs would form significantly earlier or later in a protostar’s evolution. Consequently, “it will be interesting to see if similar signatures can be detected in systems of different ages.”
Ilaria Pascucci, an astronomer at the University of Arizona, who was also not part of the new study, emphasizes that the result is so fundamentally profound that it demands very careful investigation and follow-up. “It would be extremely important to detect CAIs in protoplanetary disks because it would allow us to connect the evolution of these disks with that of the solar system,” she says. “But in this paper, the authors clearly state they haven’t detected CAIs; they’ve [instead] detected crystalline grains that could have formed in an environment where CAIs could form, too. It’s a very interesting link.”
Observations of protostars such as HOPS-315, she adds, can be very difficult to interpret. “There is the star, the disk, the wind, the jet, the envelope—these are very complex objects,” she says. “The authors have done a really nice job of teasing out all the information they can from their observations [of HOPS-315], but this is a challenging object, so we definitely need to find and look at more.” One protostar in particular, Pascucci notes, is HOPS-68. Other astronomers observed it with Spitzer in 2011 and found similar features in the lower-resolution data that was available then. At the time, they interpreted those features as part of the protostar’s obscuring envelope rather than its inner protoplanetary disk, she says, yet this new result suggests it may be time to revisit that object with JWST for another, deeper look.
As for HOPS-315, McClure speculates that the system may still hold surprises. Her team’s JWST data, she says, show that the outflow jet that complicated their analysis is conspicuously depleted in silicon—which happens to be the most important element for making the silicates that serve as planetary building blocks. Perhaps, then, instead of feeding the jet, the silicon has been locked away elsewhere—such as in reservoirs of dust or even larger rocky objects that are deeper in the disk.
“Our estimates suggest that something like 98 percent of the silicon we’d expect relative to the carbon we see [in the jet] is missing,” she says. “That may be a hint that planetesimals are already forming there in a similar way that they must have in our solar system.”