Investigating the role of water in planet formation

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Astronomers have detected water vapour in the disc surrounding a young, Sun-like star, HL Tauri. Water is a molecule that is a key ingredient to life as we know it here on Earth, but it’s also thought to be important in the process of planet formation. However, astronomers have never been able to map the distribution of water in a stable, cool disc around such a star – the perfect environment for planet formation – until now.

HL Tauri is a star in the constellation Taurus, around 450 light years away. These observations astonishingly reveal the equivalent of at least three times the volume of water in all of Earth’s oceans, within the inner 17 astronomical units of this system. To put this into context, this is about the same distance to Uranus, one of the outer planets of our Solar System.

“I had never imagined that we could capture an image of oceans of water vapour in the same region where a planet is likely forming,” says Stefano Facchini, lead author of the study recently published in the journal Nature, and an Associate Professor at the University of Milan.

I had never imagined that we could capture an image of oceans of water vapour in the same region where a planet is likely forming

Prof. Leonardo Testi, a co-author of the study from the University of Bologna, also expresses their amazement at the findings: “It is truly remarkable that we can not only detect but also capture detailed images and spatially resolve water vapour at a distance of 450 light-years from us.”

This astronomical feat has only proven possible thanks to the increased spatial resolution and sensitivity of our astronomical observatories, namely the Atacama Large Millimeter/Submillimeter Array (ALMA). For example, ALMA can show astronomers details as small as a human hair a kilometre away.

The Earth’s atmosphere is full of water vapour, causing interference and making ground-based observations much more challenging. So how does ALMA overcome this challenge? This array of over 60 radio dishes is situated in both a high and dry environment, about 5000 metres above sea-level in the Chilean Atacama Desert. “To date, ALMA is the only facility able to spatially resolve water in a cool planet-forming disc,” says coauthor Prof. Wouter Vlemmings, from the Chalmers University of Technology in Sweden.

This water was found within a region in the HL Tauri protoplanetary disk where a gap is known to exist. This gap is produced as young planetary bodies orbit the star; these cosmic sculptors carve out paths in the disk as they accumulate gas and dust. “Our recent images reveal a substantial quantity of water vapour at a range of distances from the star that include a gap where a planet could potentially be forming at the present time,” says Facchini.

An astronomer at the European Southern Observatory (ESO), Dr. Elizabeth Humphreys, adds: “It is truly exciting to directly witness, in a picture, water molecules being released from icy dust particles.” So, how does the significance of water vapour fit into the picture? Well, even the most colossal planet-like body begins as the smallest grains of dust. Astronomers believe that as these seeds of planet formation collide with one another and form larger clumps, if water can freeze onto these then they can stick together much more efficiently.

If water can freeze onto clumps of dust, then they can stick together much more efficiently

“Directly measuring the amount of water vapour where planets are forming takes us a step closer to understanding how easy it could be to make worlds with oceans – how much water is attached to the agglomerating rocks, or is it mostly added later to an almostfully-formed planet?” says Dr. Anita Richards from the University of Manchester.

According to Facchini, “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.” The pivotal role water plays in this process is only going to become clearer, with further technological advancements currently in progress: ALMA is being upgraded and ESO’s Extremely Large Telescope (ELT) is scheduled to have its first light in 2028.

An instrument called METIS, the Mid-infrared ELT Imager and Spectrograph, will enable astronomers to probe further into the inner regions of disks like those surrounding HL Tauri. This research paves the way to revealing more crucial information about the birthplaces of planets like our own.

Image: New observations from ALMA show water vapour in blue, with hotter gas closer to HL Tauri in brighter blue. The red rings show dust around the star from previous observations. Credit: ALMA (ESO/NAOJ/NRAO) / S. Facchini et al.

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