Deuterium-Enriched Water Ties Protoplanetary Disks to Comets and Protostars

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have detected gaseous water in the planet-forming disk around V883 of Oriona protostar located 1,305 light-years away in the constellation of Orion. This water carries a chemical signature that explains the journey of water from star-forming gas clouds to planets, and supports the idea that water on Earth is even older than our Sun.

Water usually consists of one oxygen atom and two hydrogen atoms.

Dr. John Tobin, an astronomer at NSF’s National Radio Astronomy Observatory, and his colleagues studied a slightly heavier version of water where one of the hydrogen atoms is replaced with deuterium — a heavy isotope of hydrogen.

Because simple and heavy water form under different conditions, their ratio can be used to trace when and where the water was formed.

For instance, this ratio in some Solar System comets has been shown to be similar to that in water on Earth, suggesting that comets might have delivered water to Earth.

“We can think of the path of water through the Universe as a trail,” Dr. Tobin said.

“We know what the endpoints look like, which are water on planets and in comets, but we wanted to trace that trail back to the origins of water.”

“Before now, we could link the Earth to comets, and protostars to the interstellar medium, but we couldn’t link protostars to comets.”

“V883 Ori has changed that, and proven the water molecules in that system and in our Solar System have a similar ratio of deuterium and hydrogen.”

Observing water in the circumstellar disks around protostars is difficult because in most systems water is present in the form of ice.

When astronomers observe protostars they’re looking for the water snow line or ice line, which is the place where water transitions from predominantly ice to gas, which radio astronomy can observe in detail.

“If the snow line is located too close to the star, there isn’t enough gaseous water to be easily detectable and the dusty disk may block out a lot of the water emission,” Dr. Tobin said.

“But if the snow line is located further from the star, there is sufficient gaseous water to be detectableand that’s the case with V883 Ori.”

V883 Ori’s disk is quite massive and is just hot enough that the water in it has turned from ice to gas.

That makes this protostar an ideal target for studying the growth and evolution of planetary systems at radio wavelengths.

“This observation highlights the superb capabilities of ALMA in helping astronomers study something vitally important for life on Earth: water,” said Dr. Joe Pesce, NSF Program Officer for ALMA.

“An understanding of the underlying processes important for us on Earth, seen in more distant regions of the galaxy, also benefits our knowledge of how nature works in general, and the processes that had to occur for our Solar System to develop into what we know today.”

To connect the water in V883 Ori’s protoplanetary disk to that in our own Solar System, the astronomers measured its composition using ALMA and found that it remains relatively unchanged between each stage of solar system formation: protostar, protoplanetary disk, and comets.

“This means that the water in our Solar System was formed long before the Sun, planets, and comets formed,” said Merel van ‘t ’Hoff, an astronomer at the University of Michigan.

“We already knew that there is plenty of water ice in the interstellar medium.”

“Our results show that this water got directly incorporated into the Solar System during its formation.”

“This is exciting as it suggests that other planetary systems should have received large amounts of water too.”

Although the Sun is believed to have formed in a dense cluster of stars and V883 Ori is relatively isolated with no nearby stars, the two share one critical thing in common: they were both formed in giant molecular clouds.

“It is known that the bulk of the water in the interstellar medium forms as ice on the surfaces of tiny dust grains in the clouds,” said Dr. Margot Leemker, an astronomer at Leiden University.

“When these clouds collapse under their own gravity and form young stars, the water ends up in the disks around them.”

“Eventually, the disks evolve and the icy dust grains coagulate to form a new solar system with planets and comets.”

The findings were published in the journal Nature.

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J.J. Tobin et al. 2023. Deuterium-enriched water ties planet-forming disks to comets and protostars. Nature 615, 227-230; doi: 10.1038/s41586-022-05676-z