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Oceans of water vapor may be involved in planet formation

Astronomers have detected water vapor within a disc surrounding a young star, a region believed to be ripe for planet formation. This finding underscores water’s crucial role in both the genesis of life on Earth and the process of planet formation. 

This significant advancement in our understanding of planetary formation environments was enabled by the Atacama Large Millimeter/submillimeter Array (ALMA), a project in which the European Southern Observatory (ESO) participates.

Astonishing discovery 

Stefano Facchini, an astronomer at the University of Milan, Italy, who led the research expressed his astonishment: “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 study uncovered a volume of water at least triple that of Earth’s oceans within the inner disc of HL Tauri, a young star resembling our Sun, situated 450 light-years away in the constellation Taurus.

Brand new insights

“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,” added co-author Leonardo Testi, an astronomer at the University of Bologna, Italy. 

The ALMA’s “spatially resolved” observations have provided insights into how water is distributed across different areas of the disc. 

“Taking part in such an important discovery in the iconic HL Tauri disc was beyond what I had ever expected for my first research experience in astronomy,” said co-author Mathieu Vander Donckt, a master student at the University of Liège.

Water vapor and planet formation 

The experts found a considerable concentration of water in a section of the HL Tauri disc where a gap, potentially carved by a forming planet, exists. 

“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,” Facchini said. This finding indicates the potential impact of water vapor on the chemical makeup of emerging planets within these gaps.

Revolutionary observations 

Capturing water observations from a ground-based telescope poses a challenge due to the interference caused by Earth’s atmospheric water vapor

ALMA, situated in Chile’s high-altitude Atacama Desert, was designed in such a dry environment to overcome this obstacle, as noted by Wouter Vlemmings, a professor at Chalmers University of Technology in Sweden.

Elizabeth Humphreys, an astronomer at ESO and study participant, shared her excitement: “It is truly exciting to directly witness, in a picture, water molecules being released from icy dust particles.” 

Study implications 

The presence of water, which aids in the agglomeration of dust grains into larger bodies, is believed to play a pivotal role in planet formation. 

“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,” Facchini explained.

With ongoing upgrades to ALMA and the upcoming operation of ESO’s Extremely Large Telescope (ELT), our understanding of planet formation and water’s role therein is set to deepen. The Mid-infrared ELT Imager and Spectrograph (METIS) will particularly enhance our view of the inner regions of planet-forming discs, shedding light on the birthplaces of Earth-like planets.

More about planet formation 

Planet formation is a complex and intriguing process that unfolds over millions to billions of years within the swirling disks of dust and gas surrounding young stars. This process is often referred to as the protoplanetary disk phase and is fundamental to understanding how planets, including those in our own solar system, come to exist.

Setting the stage

The journey begins when a cloud of interstellar gas and dust collapses under its own gravity, often triggered by external forces such as the shock waves from nearby supernovae. 

As the cloud collapses, it begins to spin, flattening into a spinning disk with the new star at its center. This disk is rich in materials, including water ice, silicates, and organic compounds, setting the stage for planet formation.


Within this disk, dust particles collide and stick together, forming larger clumps in a process known as accretion. Over time, these clumps grow into kilometer-sized planetesimals, the building blocks of planets. 

Through a combination of further accretion and gravitational interactions, these planetesimals can grow into protoplanets.

Fate of the protoplanets 

The fate of these protoplanets depends on their location within the disk. Closer to the star, where it is hotter, only metal and rock can condense, leading to the formation of terrestrial planets like Earth and Mars. 

Farther from the star, where it is cooler, ices can also condense, allowing for the formation of gas giants and ice giants. These larger planets can then attract surrounding gas to form thick atmospheres.

Gravitational interactions 

Throughout this process, gravitational interactions among protoplanets and with the disk itself can lead to migration, where planets change their orbits. This can result in a dynamic and sometimes chaotic evolution of the planetary system.

Mature system

Eventually, the gas in the disk is either accreted onto the star, blown away by the star’s radiation, or locked up in planets, leaving behind a mature planetary system. 

The leftover planetesimals that did not become part of planets may remain as asteroids, comets, and other small solar system bodies, acting as remnants of the system’s formation process.

The study is published in the journal Nature Astronomy.

Image Credit: ALMA (ESO/NAOJ/NRAO)/S. Facchini et al.

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