Planet-forming disks are places of chaotic activity. Not only do planetesimals slam together to form larger worlds, but it now appears that the process involves the destructive recycling of water within a disk. That’s the conclusion from scientists studying JWST data from a planetary birth crèche called d203-506 in the Orion Nebula.
The data they studied suggest that an amount of water equivalent to all of Earth’s oceans is created and replenished in a relatively short period—about a month. According to study co-lead Els Peeters at Western University in Canada, it was relatively easy to discover this process in the protoplanetary disk. “This discovery was based on a tiny fraction of our spectroscopic data,” she said. “It is exciting that we have so much more data to mine and I can’t wait to see what else we can find.”
The Orion Nebula is a vast active star- and planet-forming region and the d203-506 protoplanetary disk lies within it at a distance of about 1,350 light-years away from Earth. Astronomers study the nebula to understand all aspects of star birth since there are so many newborn stars there. In addition, many are surrounded by disks of gas and dust, called protoplanetary disks (proplyds, for short). Those regions are excellent places to observe planet-formation processes, and particularly the interplay between the young stars and their disks.
The Orion Nebula is one of the most studied objects in the sky. Many of its protostars and their planetary disks likely contain water in some form. Image: NASA
We all know that water is an important ingredient for life. It certainly played a role in creating and sustaining life on our planet. As it turns out, water is a significant fraction of the materials in a proplyd. In the infant Solar System, water existed throughout our proplyd long before any of the planets formed, largely in their icy form, either as icy bodies or locked into asteroids and planetesimals. It also exists in interstellar space.
This view of Earth’s horizon by an Expedition 7 crewmember onboard the International Space Station. A new study suggests that Earth’s water didn’t all come from comets, but likely also came from water-rich planetesimals. Credit: NASA
Most of Earth’s water got delivered to the forming planet over millions of years. It melted or outgassed to form the oceans, rivers, and lakes we see today. But, some fraction of the water in our system’s birth disk probably went through a “freeze-thaw” cycle within the disk. That happened when the Solar System was still just a disk of gas and dust. The water was essentially destroyed and then re-formed at higher temperatures.
We can’t see that effect anymore in our system. But, astronomers can point telescopes at other proplyds to see if the same process happens there. That’s what Peeters and her team did. They used JWST to look at d203-506. There, bright young stars flood the nearby regions in the proplyd with intense ultraviolet radiation. The UV breaks up water molecules to form hydroxyl molecules and that process also releases infrared light. JWST can search out that light and report back on how much hydroxyl is in the birth cloud. The team estimates that the process in d203-506 regularly destroys and replenishes about an Earth oceans-full of water each month.
The d203-506 system is currently forming new worlds, but it began as a cloud of gas and dust without a star. That’s exactly how our own Solar System began—as a cloud of gas and dust more than 4.5 billion years ago. The cloud it formed from was a cold, dark nebula containing some amount of water ice, or water-rich material. Something nudged the cloud to coalesce into a region of higher density, and that continued to shrink in on itself under the force of gravity. Temperatures rose, and eventually, a protostar began to form. Ultraviolet from the Sun irradiated the birth cloud, and that led to a similar water-destroying and replenishing activity. Heat and radiation from the Sun also forced lighter elements to migrate out to cooler regions in the system.
So, d203-506 makes a great analog to study the water cycle in the infant Solar System. Based on this JWST data, it’s very likely that water in Earth’s oceans went through this same process. Eventually, that water made its way to the planetesimals and icy bodies that helped form the worlds of the Solar System.
The icy bodies of the outer solar system probably didn’t experience the same extremes of heating, destruction, and replenishment. That’s because they migrated out to (or already existed at) great enough distances that the irradiation from the Sun didn’t have the same effect. That’s one reason planetary scientists are also interested in sampling those distant bodies. Their “primordial” water ices are a good sample of what conditions were like in the original nebula before it coalesced to form the Sun and planets.
Researchers Find Destruction of Oceans’ Worth of Water per Month in Orion Nebula
OH as a Probe of Warm-water Cycle in Planet-forming Disks (journal link)
OH as a Probe of Warm-water Cycle in Planet-forming Disks (arXiv link)
