The origins of Earth’s water is a complicated mystery that scientists have been untangling for decades. Life is impossible without water, so the origin of Earth’s life-giving water is a foundational question. As the power of our telescopes grows, researchers have made meaningful headway on the question.
Previous research uncovered links between Earth’s water and the Solar System’s comets and icy planetesimals. But newer research follows the chain back even further in time to when the Sun itself had yet to form.
“We can now trace the origins of water in our Solar System to before the formation of the Sun.”
John J. Tobin, NRAO
A new study examined the star V883 Orionis, which is about 1300 light-years away. V883 Orionis is a young protostar, only about 500,000 years old, a mere infant in stellar terms. At such a young age, the star has not cleared away the cloud of gas and dust it was born in. And inside that whirling cloud, called a protoplanetary disk, the distant solar system’s planets are still forming.
Astronomers struggle to see the detail in young solar systems like the V883 Orionis system. The activity is hidden inside the veil of gas and dust. But new observations with the Atacama Large Millimetre/submillimetre Array (ALMA) detected water vapour in the system, and the scientists behind a new study also measured the water’s ‘chemical fingerprint.’
The study is “Deuterium-enriched water ties planet-forming disks to comets and protostars” and it’s published in the journal Nature. The lead author is John J. Tobin from the USA’s National Radio Astronomy Observatory. “We can now trace the origins of water in our Solar System to before the formation of the Sun,” Tobin said in a press release announcing the results.
Detecting water in young solar systems is tricky because it’s mostly frozen into ice. “Most of the water in planet-forming discs is frozen out as ice, so it’s usually hidden from our view,” says co-author Margot Leemker, a PhD student at Leiden Observatory in the Netherlands. That restricts the water’s molecular motion. But when water is in its gaseous form, the molecules spin and vibrate. They’re constantly moving in relation to each other, and the hydrogen bonds continually break and reform, and all that activity emits radiation that ALMA can see.
Typically, water far from the star is frozen, while water nearer the star is gaseous. However, the region nearest the star is hidden from view by the cloud of gas surrounding the star, and so is the radiation from the gaseous water.
What sets this study apart from others is that the researchers detected water vapour a great distance from V883 Orionis. Not only did they detect the water, they measured its chemical fingerprint.
This image shows the difference between ‘normal’ hydrogen called Protium, and hydrogen with a neutron, called deuterium. Water can contain both types of hydrogen, and the ratio between the two is water’s ‘chemical fingerprint.’ Image Credit: By Dirk Hünniger;Derivative work in english – Balajijagadesh – This file was derived from: Hydrogen Deuterium Tritium Nuclei Schmatic-de.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=46295940
Normally, water is made up of two regular hydrogen atoms and one oxygen atom (H2O.) But sometimes one of the hydrogen atoms in the water molecule is heavy hydrogen, also called deuterium. (Deuterium contains a neutron, while regular hydrogen doesn’t.) H2O with one deuterium atom is called semi-heavy water, because the additional neutron increases its density. A body of water, whether in one of Earth’s oceans or somewhere in a distant solar system, can contain both types of water, and the ratio between the two is the water’s chemical fingerprint.
V883 Orionis’ disk is unusually hot, a fact revealed by previous research. As a young protostar, it’s still accreting material. But accretion is an uneven process, and as V883 Orionis accretes material, it also experiences outbursts of energy. That energy can push the snow line further away from the star, sublimating what was water ice into water vapour. That’s what happened a few years ago, when an outburst of energy from the star raised its luminosity, warmed the disk up, and sublimated the ice into vapour, making it visible.
“The warm nature of V883 Ori’s disk enables us to characterize its water reservoir with spatially-resolved observations, unlike most proto-planetary disks,” the authors explain in their research.
This image from the new paper shows how V883 Orionis heated its disk during an outburst. The outburst pushed the snow line further from the star and melted water ice into water vapour, making it visible to ALMA. (COMs = complex organic molecules.) Image Credit: Tobin et al. 2023.
That visibility has helped illustrate another link in water’s long journey from gaseous, star-forming cloud to planetary oceans like Earth’s. Until now, that link has been missing, according to lead author Tobin. “V883 Orionis is the missing link in this case,” says Tobin. “The composition of the water in the disc is very similar to that of comets in our own Solar System. This is confirmation of the idea that the water in planetary systems formed billions of years ago, before the Sun, in interstellar space, and has been inherited by both comets and Earth, relatively unchanged.”
The team measured the HDO:H2O ratio of the disk and found that it’s comparable to both comets and protostellar envelopes, the authors explain, but exceeds that of Earth’s oceans.
“We conclude that disks directly inherit water from the star-forming cloud and this water becomes incorporated into large icy bodies, such as comets, without substantial chemical alteration.”
from “Deuterium-enriched water ties planet-forming disks to comets and protostars.”
This image of the planet-forming disc around the young star V883 Orionis was obtained by ALMA in 2016. The star was experiencing an outburst, which pushed the water snow line further from the star and allowed it to be detected for the first time. The dark ring midway through the disc is the water snowline, the point from the star where the temperature and pressure dip low enough for water ice to form. The orbits of the planet Neptune and dwarf planet Pluto in our Solar System are shown for scale. Image Credit: ALMA (ESO/NAOJ/NRAO)/L. Cieza
It takes a lot of sleuthing to untangle what’s happening in a solar system around a young protostar like V883 Orionis. ALMA has the power to observe the molecular emission lines of not only different water molecules, but also molecules like methanol. The location of each type of molecule reveals the nature of the system.
This image from the research shows the intensity of water and other molecules at V883 Orionis. HDO is semi-heavy water with one deuterium atom. H218O is water enriched with O18, a natural stable isotope of oxygen. CH3OH is methanol, and C17O is carbon monoxide contain Carbon 17, an unstable carbon isotope. Each molecule has different sublimation temperatures, so their locations and concentrations tell astronomers a lot about the disk surrounding the young protostar. Image Credit: Tobin et al. 2023.
Overall, the results uncover a link between the water that existed in the cloud before V883 Orionis even formed, and the water in the solar system. The composition of the water in the disk around the young star is the same as the composition of water in objects in our Solar System like comets.
“We conclude that disks directly inherit water from the star-forming cloud and this water becomes incorporated into large icy bodies, such as comets, without substantial chemical alteration,” the authors write. The team also found that the disk contains 1200 times more water than all of Earth’s oceans.
Previous research shows that at least some of Earth’s water was delivered by comets early in Earth’s history. This study shows that the water’s history stretched back to before the Sun formed, illustrating its epic journey that lasts millions of years before ending up as part of Earth’s biosphere.
Not only did Earth’s water complete an epic journey to become part of the planet, but the effort to understand water’s origins is likewise epic. ALMA’s sensitivity enabled this research, and future observations with even more powerful telescopes can strengthen these findings.
This artist’s impression shows the European Extremely Large Telescope (E-ELT) in its enclosure. The E-ELT will be a 39-metre aperture optical and infrared telescope. It’s powerful METIS instrument will allow even more detailed study of disks around young stars like V883 Orionis. ESO/L. Calçada
The ESO’s Extremely Large Telescope should see first light in 2028. The ELT will have include a powerful instrument named METIS (Mid-infrared ELT Imager and Spectrograph.) METIS, when combined with the telescope’s enormous 39-meter diameter mirror, will enable more detailed study of circumstellar disks like the one surrounding V883 Orionis. It’s power will allow astronomers to resove the gas phase of water in these disks, and will shed more light on the links in water’s long journey from gas cloud to planets.
“This will give us a much more complete view of the ice and gas in planet-forming discs,” concludes Leemker.
