Astronomers discover oceans of water vapor in planet-forming disc around young star

Astronomers discover oceans of water vapor in planet-forming disc around young star

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered vast reservoirs of water vapor in the disc around HL Tauri, a young star 450 light-years away from Earth, unveiling processes that may lead to the birth of planets rich in water and redefining our search for life in the universe.

An international group of researchers unveiled findings that have profound implications for our understanding of planet formation and the potential for life elsewhere in the universe.

Utilizing the powerful Atacama Large Millimeter/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) plays a key role, scientists have detected vast quantities of water vapor within a disc surrounding a young star, specifically in regions where planets are likely taking shape.

water vapour in a disc around a young star exactly where planets may be forming
Astronomers have found water vapor in a disc around a young star exactly where planets may be forming. In this image, the new observations from the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner, show the water vapor in shades of blue. Near the center of the disc, where the young star lives, the environment is hotter and the gas brighter. The red-hued rings are previous ALMA observations showing the distribution of dust around the star. Credit: ALMA (ESO/NAOJ/NRAO)/S. Facchini et al.
protoplanetary disc surrounding the young star HL Tauri
This is the sharpest image ever taken by ALMA — sharper than is routinely achieved in visible light with the NASA/ESA Hubble Space Telescope. It shows the protoplanetary disc surrounding the young star HL Tauri. These new ALMA observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system.Credit: ALMA (ESO/NAOJ/NRAO)

This discovery, led by Stefano Facchini, an astronomer at the University of Milan, Italy, and recently published in Nature Astronomy, marks the first time water distribution has been mapped in a stable, cool disc.

Such discs provide the most conducive environments for planet formation around stars. Facchini expressed his astonishment at the findings, stating, “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.”

The young Sun-like star, HL Tauri, located about 450 light-years away in the constellation Taurus, is at the center of the new study.

The observations revealed that the inner disc of HL Tauri harbors at least three times the amount of water found in all of Earth’s oceans. Leonardo Testi, an astronomer at the University of Bologna, Italy, and co-author of the study, highlighted the exceptional nature of these “spatially resolved” observations, which allow astronomers to pinpoint water distribution across different regions of the disc.

A significant concentration of water vapor was discovered within a known gap in the HL Tauri disc. These ring-shaped gaps, often carved by orbiting young planetary bodies as they accumulate material and grow, are critical areas for planet formation. The presence of water vapor in such a gap suggests that it could play a crucial role in the chemical composition of forming planets.

The challenge of observing water with ground-based telescopes, given the interference caused by Earth’s own water vapor, makes this discovery all the more remarkable. ALMA’s location in the high, dry Atacama Desert at an elevation of approximately 5 km (3.1 miles) is instrumental in minimizing atmospheric degradation and ensuring exceptional observing conditions. According to co-author Wouter Vlemmings, a professor at the Chalmers University of Technology in Sweden, ALMA is currently the only facility capable of resolving water in a cool, planet-forming disc.

The study provided insights into the conditions that may facilitate the development of planetary systems elsewhere in the universe. As Elizabeth Humphreys, an astronomer at ESO and participant in the study, points out, the findings illustrate how water molecules are released from icy dust particles within the disc.

This process is believed to enhance the efficiency of planet formation, echoing the conditions that likely existed in our own Solar System approximately 4.5 billion years ago.

With ongoing upgrades to ALMA and the forthcoming operation of ESO’s Extremely Large Telescope (ELT), our understanding of planet formation and the critical role of water is set to expand significantly. The Mid-infrared ELT Imager and Spectrograph (METIS), for example, will offer astronomers unparalleled views of the inner regions of planet-forming discs, where terrestrial planets like Earth emerge.


1 Resolved ALMA observations of water in the inner astronomical units of the HL Tau disk – Facchini, S., Testi, L., Humphreys, E. et al. – Nature Astronomy – February 29, 2024 – doi:10.1038/s41550-024-02207-w

2 Astronomers reveal a new link between water and planet formation – ESO – February 29, 2024


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