Water

This Planet-Forming Disk has More Water Than Earth’s Oceans

Astronomers have detected a large amount of water vapour in the protoplanetary disk around a young star. There’s at least three times as much water among the dust as there is in all of Earth’s oceans combined. And it’s not spread throughout the disk; it’s concentrated in the inner disk region.

No water means no life, so finding this much water in the part of a protoplanetary disk where rocky planets form is an intriguing discovery. And this isn’t just any disk. It’s a cold, stable disk, the type most likely to form planets.

The findings are presented in a new paper published in Nature Astronomy. It’s titled “Resolved ALMA observations of water in the inner astronomical units of the HL Tau disk.” The lead author is Stefano Facchini, an astronomer at the Dipartimento di Fisica, Università degli Studi di Milano, Milano, Italy.

“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,” said Facchini.

The star, HL Tau (HL Tauri), is a young star about 450 light-years away. It’s likely less than 100,000 years old, making it a prime observing target in the quest to understand planet formation. When it comes to seeing inside the gas and dust surrounding young stars like this, ALMA is our best tool. One of ALMA’s first high-resolution images is of HL Tau and its disk. The image shows rings in the disk that indicate where young planets are probably forming.

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. Image Credit: ALMA

HL Tau has always intrigued scientists, and now that they’ve detected such a large amount of water vapour in its planet-forming disk, the young star is an even more compelling target for observations.

“These observations pave the way to the characterization of the water content of the inner regions of protoplanetary disks,” the researchers write in their paper. “The tremendous angular resolution and sensitivity of the ALMA telescope, even in spectral ranges of low atmospheric transmission, are providing spatially and spectrally resolved images of the vapour of the main water isotopologue in a planet-forming disk.”

Not only did ALMA detect the water, but it also determined where it is in the disk and how much of it there is. “Our analysis implies a stringent lower limit of 3.7 Earth oceans of water vapour available within the inner 17 astronomical units of the system,” the researchers write in their paper.

When planets take shape in a protoplanetary disk like the one around HL Tauri, they clear out lanes in the dust. Nothing else is likely to create the tell-tale gaps that signal the presence of young, still-forming planets. We have the powerful ALMA to thank for this understanding.

“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,” said study co-author Leonardo Testi, an astronomer at the University of Bologna, Italy. The spatial resolution Testi is referring to is thanks to ALMA. The radio interferometer allowed astronomers to see how the water vapour is distributed throughout the disk. “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,” added Mathieu Vander Donckt from the University of Lie?ge, Belgium, a master’s student when he participated in the research.

ALMA is a radio interferometer, meaning it observes wavelengths from 0.3 mm to 3.6 mm, which correspond to the range from 84 GHz to 950 GHz. In this study, the researchers observed different “flavours” of water molecules at different temperatures. “We observed HL Tau in two different ALMA bands (band 5, originally developed with the goal of studying water in the local Universe, and band 7) to target three transitions of water,” the researchers explain.

This figure from the research illustrates some of the findings. The blue line is water detected by ALMA at 321 GHz, a high-excitation state for water vapour. The yellow line is water detected at 183 MHz, an important diagnostic line used in remote sensing of water vapour. Both lines indicate more water vapour in the inner regions of the disk. Image Credit: Facchini et al. 2024.

The observations didn’t just find water in the inner region where rocky planets form. It found water in one of the gaps that indicate a planet is sweeping up disk material and adding it to its mass. “Our recent images reveal a substantial quantity of water vapour at a range of distances from the star that includes a gap where a planet could potentially be forming at the present time,” said Facchini. The natural conclusion is that the water is becoming part of the planet.

These results are all thanks to ALMA’s power. It’s the only facility we have that can detect water in a disk like this. “To date, ALMA is the only facility able to spatially resolve water in a cool planet-forming disc,” said study co-author Wouter Vlemmings, a professor at the Chalmers University of Technology in Sweden.

ALMA’s different observational frequencies capture water as it transitions, and part of this research looks at water as it’s liberated from dust particles. The relationship between water and dust in a planet-forming disk is important. Where it’s cold enough for water to freeze onto dust particles, the particles stick together more readily, aiding the planet formation process.

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

Some of what astronomers see in the disk around HL Tauri is like a window into the past. Our planet formed in a similar way, and the same processes and mechanisms must be similar from disk to disk.

“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 said.

ALMA really flexed its muscles in this work, and the facility has played a primary role in our study of protoplanetary disks around young stars. But upcoming telescopes will surpass ALMA and give us even deeper, more detailed looks inside the dusty, obscured disks. The Extremely Large Telescope is due to see first light in 2028. Its powerful METIS (Mid-infrared ELT Imager and Spectrograph) will give us unprecedented insight into the process of planet formation.

Evan Gough

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