Bridging the water gap in solar-system formation
The protoplanetary disk around V883 Orionis as seen by the Atacama Large Millimeter/Submillimeter Array. The water snow line is the dark ring (indicated in white) midway through the disk. Beyond that, the temperature and pressure are low enough for water ice to form. For scale, the orbits of Neptune and Pluto are shown.
ALMA (ESO/NAOJ/NRAO)/L. Cieza
The journey of water from molecular clouds to Earth and other planets has a gap. Water was likely delivered to Earth by comets, but its deuterium enrichment in evolved solar-system bodies is different from its enrichment in molecular clouds. What is the water like in between, when the star is young and comets haven’t yet formed? A chance discovery has provided astronomers with a new opportunity to understand the presence of water in an infant solar system.
V883 Orionis (V883 Ori) is a Sunlike protostar—a still-forming star that cannot yet sustain nuclear fusion—in the constellation of Orion. A protostar develops out of a collapsing molecular cloud, which contains water ice. As the protostar matures, a disk of gas and dust, known as a protoplanetary disk, forms around the young star and later coalesces into comets and planets. Observations at the Atacama Large Millimeter/Submillimeter Array (ALMA) caught the system shortly after a large amount of material from the disk fell into the protostar and triggered an outburst.
The outburst has proved fortuitous for astronomers. Because water sublimates inside a protoplanetary disk’s water snow-line radius, gas emission lines can be used to determine the abundance of water in the disk. But the radius is typically less than 10 AU, and thus observations of the emission lines so close to a distant star are usually overwhelmed by the star’s brightness. But observations farther away from the radius are impossible since most water is frozen out onto dust grains.
The burst of heat in V883 Ori, however, caused the water snow-line radius in the system to be pushed out to 40 AU, about the radius of Pluto’s orbit. John Tobin (National Radio Astronomy Observatory) and collaborators took advantage of that to obtain integrated spectra at millimeter wavelengths. Using ALMA, they determined the total flux of gas emission lines in the protoplanetary disk.
Tobin and colleagues measured the column densities of water, both as HDO and H2O, to characterize the disk’s water reservoir. Tracing the deuterium enrichment, or water ratio HDO:H2O, indicates how much water processing has occurred since the interstellar medium (ISM) phase of solar-system formation (see the article by Kathrin Altwegg, Physics Today, January 2022, page 34 ). The water ratio is high, at roughly 1.0 × 10−3, in the cold ISM because of an abundance of free deuterium-bearing molecules. A similar ratio is found in young protostars, but the next evolutionary stage, that of young protoplanetary disks, had not been measured until now. The researchers found that unlike some comets and planets that have a lower water ratio—comet water ratios vary from 3.0 × 10−4 to 10−3—V883 Ori has a high water ratio of 2.26 × 10−3, demonstrating that the water at that stage in solar-system formation has not yet undergone significant evolution. The water in V883 Ori’s disk comes directly from the infalling envelope of molecular gas in the cold ISM.
Tobin and colleagues’ finding fills the gap in our understanding of where water comes from. The researchers speculate that their study is evidence that the water in our solar system originated in the cold ISM before the Sun was formed. Observations of other young protoplanetary disks are key to figuring out how water arrives at Earthlike planets. (J. J. Tobin et al., Nature 615, 227, 2023 .)