Where do stars and planets get their water from? There has long been evidence that young stars and their protoplanetary disks "inherit" their water from the interstellar medium - the clouds of gas and dust from which new stars form. Now astronomers have discovered the missing evidence for this. They have discovered the signature of interstellar water in the planet-forming disk of young star V883 Orionis, some 1300 light-years away. The deuterium content in the gaseous water around this star is similar to that of the water in solar system comets on the one hand and that in the interstellar medium on the other, the team reports. This suggests that our water also once came from interstellar space.
Our earth is a water planet: three quarters of its surface is now covered by oceans and the atmosphere also contains plenty of water vapour. So far, however, it has only been partially clarified where all this water comes from. According to current theory, much of this water comes from the young Sun's protoplanetary disk. The Earth got its water from planet formation, but also from later impacts from asteroids and comets that formed in different areas of the protoplanetary disk. Similarities in the isotopes of hydrogen and oxygen in water suggest this, but there are also some discrepancies.
It was also unclear until now where the sun's protoplanetary disk got its water from. Planetary researchers have long assumed that young stars and the clouds of matter surrounding them "inherit" their water from the interstellar medium. "We can think of the path of water through the cosmos as a chain or path: we already know what the end links of it look like - the water on planets and in comets," explains first author John Tobin of the National Radio Astronomy Observatory (NRAO) in the US -American Charlottesville. “So far we have been able to link the earth to comets and protostars to the interstellar medium. But the link from protostars to comets was missing.”
Hot pane allows water views
Whether the protoplanetary discs of stars actually contain water from interstellar space has not yet been clearly demonstrated. In our solar system, the reason is clear: we would have to travel back in time to do this. There is a different problem with protoplanar disks around alien young stars: "Most of the water in planet-forming disks is frozen as ice, so it is usually hidden from us," explains co-author Margot Leemker of Leiden Observatory in the Netherlands. When water has frozen to ice, astronomers cannot determine its isotopic composition, for example the proportion of the heavy hydrogen isotope deuterium, using spectral analysis - this is only possible with gaseous substances. In protoplanetary disks there is water vapor in addition to frozen water. However, this zone with sufficient heat is usually too close to the star and is cloaked by the surrounding dust clouds.
But Tobin and his team have now discovered a protostar whose snow line - the limit at which water freezes - is unusually far out. The protostar V883 Orionis lies around 1305 light-years away in the constellation of Orion and has a gas and dust disk that extends up to 320 astronomical units into space. A good 130 years ago, a strong outburst occurred in this growing system, which astronomers report heated the protoplanetary disk of the protostar. As a result, the snow line shifted far outward and much of the protoplanetary water became water vapor. Using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, Tobin and his team were able to capture and analyze the spectral signatures of this water. "The heated disk of V883 Orionis allows us to characterize its water reservoir in a spatially resolved manner, which is not possible for most protoplanetary disks," explain the astronomers.
Link between interstellar and solar water
On the one hand, the analyzes revealed that the protoplanetary disk contains enough water in the form of water vapor to fill the Earth's oceans 1,200 times over. "That's the lower limit because it doesn't include the water closer than 40 astronomical units to the star, nor the water ice on the outer edge of the disk," the team said. On the other hand, they were able to use the spectral analyzes to determine how high the proportion of deuterium is in this protoplanetary water. The proportion of this hydrogen with an extra neutron provides important clues as to where the water originally came from. In V883 Orionis, the researchers found correspondences with the water of comets in the solar system as well as with the interstellar medium. "The water molecules in this system and in our solar system have similar proportions of deuterium and hydrogen," says Tobin. "This supports the idea that the water in planetary systems originated in interstellar space and was taken over relatively unchanged by both comets and Earth," says Tobin.
With this, V883 Orionis now provides the missing link in the chain of water: on the one hand, the star connects the interstellar water with protostars and their protoplanetary disk and, on the other hand, shows a connection from its disk to the solar comets and thus also to our solar system. This confirms the assumption that our water also comes at least in part from interstellar space and is probably far older than our sun or the earth. "By looking at the water in the disk of V883 Orionis, we're essentially looking back in time and seeing what our own solar system looked like when it was much younger," says Leemker. Tobin adds: "V883 Orionis is the missing link: we now have an unbroken chain of comets and protostars to the interstellar medium."
Source: John Tobin (National Radio Astronomy Observatory, Charlottesville) et al., Nature, doi: 10.1038/s41586-022-05676-z