ALMA Detects Water Vapor in Protoplanetary Disk around Young Star

The water molecule is a key ingredient in the formation of planetary systems. Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have detected water vapor in the disk around the young star HL Tauri exactly where planets may be forming. Their analysis implies a stringent lower limit of 3.7 Earth oceans of water vapor available within the inner 17 astronomical units of the HL Tauri system.

The water molecule is undoubtedly one of the most important molecular species in the whole Universe.

Being an extremely efficient solvent, water had a key role in the emergence of life as we know it on our planet.

For this reason, the chemical characterization of exoplanetary atmospheres is often focused on detecting this particular molecule.

Formed by the common hydrogen and oxygen atoms, water plays a fundamental role in the physics of the formation of planetary systems, due to its very high abundance in both gaseous and icy forms.

“I had never imagined that we could capture an image of oceans of water vapor in the same region where a planet is likely forming,” said Dr. Stefano Facchini, an astronomer at the University of Milan.

The HL Tauri system is thought to be less than 100,000 years old and about 17.9 billion km in radius. It lies in the direction of the constellation of Taurus at a distance of 450 light-years.

HL Tauri’s protoplanetary disk is unusually massive and bright, which makes it an excellent place to search for signs of forming planets.

The new ALMA observations revealed at least three times as much water as in all of Earth’s oceans in the inner part of the disk.

“It is truly remarkable that we can not only detect but also capture detailed images and spatially resolve water vapor at a distance of 450 light-years from us,” said Dr. Leonardo Testi, an astronomer at the University of Bologna.

The spatially-resolved observations with ALMA allow astronomers to determine the distribution of water in different regions of the disk.

“Taking part in such an important discovery in the iconic HL Tauri disk was beyond what I had ever expected for my first research experience in astronomy,” said Dr. Mathieu Vander Donckt, an astronomer at the University of Liège.

“Our recent images reveal a substantial quantity of water vapor at a range of distances from the star that include a gap where a planet could potentially be forming at the present time,” Dr. Facchini said.

“This suggests that this water vapor could affect the chemical composition of planets forming in those regions.”

“To date, ALMA is the only facility able to spatially resolve water in a cool planet-forming disk,” said Professor Wouter Vlemmings, an astronomer at the Chalmers University of Technology.

“It is truly exciting to directly witness, in a picture, water molecules being released from icy dust particles,” said Dr. Elizabeth Humphreys, an astronomer at ESO.

“The dust grains that make up a disk are the seeds of planet formation, colliding and clumping into ever larger bodies orbiting the star.”

“Our results show how the presence of water may influence the development of a planetary system, just like it did some 4.5 billion years ago in our own Solar System,” Dr. Facchini said.

The findings were published in the journal Nature Astronomy.

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S. Facchini et al. Resolved ALMA observations of water in the inner astronomical units of the HL Tau disk. Nat Astron, published online February 29, 2024; doi: 10.1038/s41550-024-02207-w