Located in the Atacama desert in northern Chile, the Atacama Large Millimeter-submillimeter Array (ALMA) and its many radio antennas are dedicated to studying the unseen Universe. Meanwhile, the James Webb Space Telescope (JWST) has been using its advanced infrared instruments to visualize another unseen part of the Universe. Together, the two observatories have seen, for the first time, hot minerals in a distant solar system beginning to solidify to create a small planetesimal. In short, these observations have allowed astronomers to pinpoint the moment when planets begin to form.
Located about 1300 light-years from Earth, this system (HOPS-315) is one of many studied by scientists with ALMA and Webb because of its age and stellar type. Using their advanced radio and infrared optics, these observatories have observed many planet-forming disks around young stars. However, this is the first time astronomers have witnessed a planetary system in the earliest stages of formation. What's more, HOPS-315 is a nascent version of our Sun, a G-type yellow dwarf, which means these observations are offering the first clues about what our Solar System may have looked like billions of years ago.
The international team responsible for this discovery was led by Melissa K. McClure, an Assistant Professor and Veni Laureate at the Leiden Observatory in the Netherlands. She was joined by researchers from the University of Michigan, Purdue University, the National Tsing Hua University, the Onsala Space Observatory, the Centre National de la Recherche Scientifique (CNRS), and the Southwest Research Institute (SwRI). Their paper, "Refractory solid condensation detected in an embedded protoplanetary disk," appeared yesterday (July 16th, 2025) in Nature.
In the Solar System, the first material to condense and create bodies is found today in the form of ancient meteorites. Astronomers not only use these rocks to determine when the Solar System began to form, but also to provide insight into the conditions that led to it. For example, these meteorites are largely composed of crystalline minerals containing silicon monoxide (SiO) that can only condense at extremely high temperatures, like those found in protoplanetary disks. Over time, these solids bind together, growing in size and mass to create planetimals that measure in the kilometer range.
As more material accumulated, scientists theorize that heavier elements (like iron and nickel) settled into the interior while silicate minerals formed the outer layers. These larger objects grew to become the terrestrial (rocky) planets of the inner Solar System and the cores of the gas/ice giants in the outer Solar System. As McClure said in an ESO press release: “[W]e've always known that the first solid parts of planets, or ‘planetesimals’, must form further back in time, at earlier stages. For the first time, we have identified the earliest moment when planet formation is initiated around a star other than our Sun,” said McClure.
Their results show that SiO is present in these crystalline minerals and around the young star in its gaseous state, suggesting they are just beginning to solidify. The minerals were first identified from spectra obtained by the JWST, while ALMA radio data was used to determine their location in the system. With this combined data, the team confirmed that the SiO signal was coming from a small region in the disk equivalent to the distance between the Main Asteroid Belt and the Sun. This essentially means that this system is an analog for studying early planet formation and how solar systems like our own evolved.
"[W]e're seeing a system that looks like what our Solar System looked like when it was just beginning to form," said Merel van ‘t Hoff, an Assistant Professor at Purdue University. "[T]his system is one of the best that we know to actually probe some of the processes that happened in our Solar System."
Further Reading: ESO
Understanding Planet Formation
Planet formation is a complex process that occurs over millions of years and involves the interaction of various physical and chemical processes. This section explores the stages and components that contribute to the formation of planets in a solar system.
Physical Stages of Planet Formation
- Collapse of the Molecular Cloud: The initial phase involves the gravitational collapse of a molecular cloud, which creates a protostar.
- Formation of the Protoplanetary Disk: As material falls into the forming star, it creates a rotating disk of gas and dust around it.
- Condensation of Solids: High temperatures cause some materials to condense into solid grains, marking the beginning of planetesimal formation.
- Agglomeration: These grains stick together, forming larger bodies called planetesimals.
- Formation of Protoplanets: Planetesimals collide and merge to form protoplanets, leading to the growth of planets.
Chemical Processes in Planet Formation
The transition from gas to solid state is critical in forming the basic building blocks of planets. This involves several key chemical processes:
- Thermal Condensation: In high-temperature environments, atoms and molecules condense to form solid phases.
- Refractory Materials: Elements like silicon and metals condense into robust solid structures, even at high temperatures.
- Volatile Compounds: Elements with low boiling points remain gaseous until temperatures drop, allowing for the condensation of water, ammonia, and methane.
The Role of Meteorites
Meteorites provide essential clues about the early Solar System and the processes of planet formation. By studying these ancient rocks, scientists can identify:
- The age of the solar system.
- The composition and structure of the original protoplanetary disk.
- Insights into the thermal history of early materials.
Table of Meteorite Types
| Type of Meteorite | Characteristics | Significance |
|---|---|---|
| Chondrites | Primitive, composed of small grains. | Most common type, provides clues about the solar nebula. |
| Achondrites | Broke away from larger bodies, lacking chondrules. | Helps understand differentiation processes of planetary bodies. |
| Iron Meteorites | Made mostly of iron and nickel. | Suggest insights into the core formation of differentiated bodies. |
Further Research and Observations
With advancements in observational technology, astronomers can now observe these processes in real-time across various distant star systems. Future research will focus on:
- Identifying more examples of nascent planetary systems.
- Understanding the impact of stellar environments on planet formation.
- Improving models of planet formation based on observational data.
Conclusion
The recent findings in the study of HOPS-315 offer a significant leap forward in our understanding of how planets form around stars. With the combination of ALMA and JWST, scientists are uncovering the building blocks of planets much like our own Solar System. As these observations continue to evolve, they will provide deeper insights into the origins of planets and the conditions necessary for life to develop.
