Where do the stars and planets get their water? There has long been evidence that young stars and their protoplanetary disks “inherit” water from the interstellar medium – the clouds of gas and dust where new stars form. Now astronomers have found missing evidence for it. They detected traces of interstellar water in the planet-forming disk of the young star V883 Orionis, about 1,300 light years away. The team showed that the deuterium content in the gaseous water around this star is similar to the water content in the solar system’s comets on the one hand, and the water content in the interstellar medium on the other. This suggests that our water once also came from interstellar space.
Our Earth is a water planet: three-quarters of its surface is now covered by oceans and its atmosphere also contains a lot of water vapor. However, so far it has only been partially clarified where all this water comes from. According to current theory, most of this water came from the young Sun’s protoplanetary disk. Earth gets its water from the formation of planets, but also from the after-effects of asteroids and comets that form in different regions of the protoplanetary disk. The similarity of the isotopes of hydrogen and oxygen in water suggests this, but there are also some differences.
It is not clear where the sun’s protoplanetary disk gets its water from. Planetary scientists have long assumed that young stars and clouds of matter around them “inherit” their water from the interstellar medium. “We can think of the passage of water through the universe as a chain or path: we already know what the final link is like – water on planets and comets,” explained first author John Tobin of the National Radio Astronomy Observatory (NRAO) in Charlottesville, US. “So far we have been able to connect Earth to comets and protostars to the interstellar medium. But the link from protostar to comet has been lost.”
The heated section allows views of the water
It is not clear whether the stellar protoplanetary disk actually contains water from interstellar space. In our solar system, the reason is clear: we have to go back in time to do this. There’s another problem with a flat proto-disk around a strange young star: “Most of the water in the planet-forming disk is frozen like ice, so it’s usually hidden from us,” explains co-author Margot Lemker of the Leiden Observatory in the Netherlands. . When water freezes into ice, astronomers cannot determine its isotopic composition, such as the proportion of the heavy hydrogen isotope deuterium, using spectroscopy – this is only possible with gaseous materials. In the protoplanetary disk, there is water vapor in addition to frozen water. However, these regions with sufficient heat are usually very close to stars and shrouded in clouds of dust around them.
But Tobin and his team have now found a protostar whose snow line – the freezing point of water – is very far away. Protostar V883 Orionis is located approximately 1,305 light years in the constellation Orion and contains a disk of gas and dust that stretches 320 astronomical units into space. 130 years ago, a powerful explosion occurred in this growing system, which astronomers say heated the protoplanetary disk of the protostar. As a result, the snow line shifted far outward and most of the protoplanetary water has become water vapour. Using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, Tobin and his team were able to capture and analyze these spectral signatures of water. “V883 Orionis’ hot disk allows us to characterize its water reservoir in a spatially resolved manner, which is impossible for most protoplanetary disks,” the astronomers explained.
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The link between the stars and the sun’s water
On the one hand, the analysis revealed that the protoplanetary disk contained enough water in the form of water vapor to fill Earth’s 1,200 times larger oceans. “This is the lower limit because it excludes water closer than 40 AU to the star, or water ice at the outer edge of the disk,” the team said. On the other hand, they could use spectral analysis to determine how high levels of deuterium were in the protoplanetary waters. The ratio of this hydrogen to the extra neutrons provides an important clue as to where the water came from. In V883 Orionis, the researchers found a match with the cometary water of the solar system as well as with the interstellar medium. “The water molecules in this system and in our solar system contain similar proportions of deuterium and hydrogen,” Tobin said. “This supports the idea that water in planetary systems comes from interstellar space and comets and Earth are taken over relatively unchanged,” said Tobin.
With this, V883 Orionis now provides the missing link in the water chain: on the one hand, the star connects interstellar water to the protostar and its protoplanetary disk, and on the other hand, it demonstrates the connection from its disc to the solar comet and thus to our solar system. This confirms the assumption that our water also originates at least partially from interstellar space and may be much older than our Sun or Earth. “Looking at the water in V883 Orionis’ disc, we are basically looking back in time and seeing what our solar system was like when it was much younger,” Limaker said. Tobin added, “V883 Orionis is the missing link: we now have an unbroken link from comets and protostars to the interstellar medium.”
Source: John Tobin (National Radio Astronomy Observatory, Charlottesville) et al., Nature, Available Here. doi: 10.1038/s41586-022-05676-z
