For the first time, astronomers have captured a young star just beginning to assemble its planets. An international team has observed mineral particles solidifying around the infant star HOPS-315, located 1,300 light-years away, using the Atacama Large Millimetre/submillimetre Array (ALMA) radio telescope and the James Webb Space Telescope (JWST). These mineral specks will collide and grow into rocky Earth-like worlds, revealing how terrestrial planets (planets composed primarily of silicate rocks) and the rocky cores of the gas giants are born.
“For the first time, we have identified the earliest moment when planet formation is initiated around a star other than our Sun,” says Melissa McClure, a professor at Leiden University in the Netherlands and lead author of the study, published recently in Nature.
A massive cloud of gas and dust collapses under its own gravity, forming a protostar (an infant stage of a star’s formation) like HOPS-315. These protostars have leftover gas, dust, and ice that flatten into a spinning protoplanetary disc (a rotating disk of matter around a young star), where new planets form. Over time, the disc cools and particles begin to stick together, eventually forming rocky planetesimals (precursor building-blocks of planets) that become planets or remain as smaller bodies.
When our solar system formed, the first grains to cool and solidify near Earth’s orbit were minerals containing silicon monoxide (SiO). These solid grains clumped together to form rocky planetesimals, the building blocks of terrestrial planets, such as Earth. Some SiO-rich grains were trapped in ancient meteorites during the solar system’s formation, preserving a record of early planet formation. Scientists now study these meteorites to uncover when the first solid grains appeared.
Astronomers have now observed the birth of solid stardust around HOPS-315, using JWST and ALMA. They detected silicon monoxide (SiO) as both gas and freshly forming crystals, explaining how building blocks of planets appear in the protoplanetary disc.
The team used JWST’s infrared telescope to study the chemical composition of material around the star. Different molecules absorb specific wavelengths of infrared radiation based on their chemical properties, creating distinct dips (absorption lines) in the spectrum, which is a graph showing how much light is absorbed at each colour. These lines, like unique fingerprints, allow astronomers to identify the molecules present and determine their temperature, providing insights into the composition and conditions of molecules in the protoplanetary disc (the disk-shaped region of matter around a young star where planets can form).
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In HOPS-315’s disk, the team detected warm silicon monoxide (SiO) alongside SiO-rich silicate minerals for the first time. The star’s midsection, nearly 1200K, was hot enough to vaporise rocks. In this region, silicates turned into gas, which later cooled and condensed into solid minerals, forming the material that would eventually become planets. “This hot mineral is the first feedstock that you have to start growing things in the disk,” says McClure.
JWST identified key chemical ingredients, while ALMA pinpointed their location. Using ALMA, the team traced the sources of these signals. JWST detected SiO gas moving at 10 km/s, but ALMA found SiO jets moving ten times faster, showing that slower SiO is concentrated in a region near the star, similar to our Solar System’s asteroid belt.
While SiO jets shoot from the disc, the SiO gas concentration in the jets is less than in the disc itself. This suggests that some SiO gas is cooling and turning into solid dust, similar to steam condensing into water.
The team identified these chemical signatures in a small section of HOPS-315’s disc, which is similar in size to our asteroid belt. They recognised the same minerals seen in ancient meteorites from our solar system, confirming signals from a region where Earth-like planets may form. The jets contained less silicon and iron gas than expected, hinting that these elements are being absorbed by growing planetary seeds.
“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,” says co-author Logan Francis, a postdoctoral researcher at Leiden University.
HOPS-315 is among 410 young stellar objects identified by the Herschel Orion Protostar Survey using the Spitzer Space Telescope. The survey identified different types of protostars, ranging from very young to more developed ones, and demonstrated how their surroundings evolve over time, ultimately leading to the formation of planetary systems.
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HOPS-315, in the Orion molecular cloud complex about 1,300 light years away, drew astronomers’ attention for its crystalline silicate minerals, a sign of early planet formation. McClure and her team focused on this protostar using JWST and ALMA.
An added benefit came from HOPS-315’s accidental alignment. While jets from newborn stars often block the disc, HOPS-315’s tilt allowed astronomers a direct view of its gas and dust disk. The protoplanetary disc of HOPS-315 extends 35 Astronomical Units (AU), or the distance between Earth and the Sun, resembling a solar system in formation. “The star is on track to grow to be as large as the Sun,” McClure says, “and the disc is about the same mass and radius… as the Sun’s disc at a similar age.”
Until now, astronomers debated whether rocky planets formed farther out (where water freezes) or closer to the star (where SiO — silicon monoxide — is abundant). HOPS-315 confirms the latter, hot minerals condense into rock near the star.
“Our results… provide physical constraints on what the region around the sun within 1 AU might have looked like for our solar system,” McClure says, “which will allow people to test these theories.”
“What we’ve been trying to do is find a baby version of our Solar System somewhere else,” says Merel van’t Hoff, an astronomer at Purdue University in West Lafayette, Indiana, and a co-author of the study. HOPS-315 provides a wonderful analogue for studying our own cosmic history. As van’t Hoff says, “This system is one of the best that we know to actually probe some of the processes that happened in our solar system.”
