JWST observations reveal for the first time the presence of water in the inner disc of a young star that harbours giant planets

The international MINDS (MIRI Mid-Infrared Disk Survey) research team - which includes researchers from ULiège - has just detected the presence of water in the planet-forming disk of the young star PDS 70, located around 370 light years away. This is an important discovery, as it allows us to probe the region where rocky planets similar to Earth are generally formed. The results of this study have been published in the scientific journal Nature.

W

ater is essential for life on Earth. However, scientists do not agree on how it came to be on our planet and on the fraction of rocky planets that could harbour water around other stars. The results of a study carried out by the MINDS international research group, which includes researchers from ULiège, provide new clues to answer these questions. The study used the James Webb Space Telescope (JWST) to observe the young star PDS 70 (370 light years away), still surrounded by its circumstellar disc,  Using JWST's MIRI (Mid-InfraRed Instrument), the scientists discovered traces of water in the inner region of this disc of gas and dust, close to the host star PDS 70.

It is in this zone that astronomers expect terrestrial planets to form. The researchers' discovery suggests that any rocky planet forming therein would therefore benefit from a large local reservoir of water at its disposal, which would improve its subsequent chances to develop and sustain life. “Until now, the mechanism favoured by scientists for bringing water to rocky planets has been via the bombardment of water-bearing asteroids," explains Giulia Perotti, an astronomer at the Max Planck Institute for Astronomy (MPIA) in Heidelberg and first author of the article. “We have now found evidence that water could also be one of the initial ingredients of certain rocky planets, and be available from their birth, in addition to potentially being brought later by asteroids."

These observations were obtained as part of the MINDS (MIRI Mid-Infrared Disk Survey) programme, in which research institutes from eleven European countries are participating, including the University of Liège PsiLab (STAR Institute/ Faculty of Science). This programme aims to identify the properties of the gas and dust disks around young stars so that we can learn about the conditions that determine the composition of the planets that form there.

"What is exceptional about this discovery is that, on the one hand, it is the first detection of water in a relatively evolved disc, but on the other hand, it is also the first detection of water in a disc containing forming planets that we have been able to observe! Enthuses Valentin Christiaens, FNRS research fellow at ULiège, second author of the article and responsible for processing the data leading to the high-quality infrared spectrum that enabled the identification of water in the inner disc of the system. "These two protoplanets, we have been able to image them in this system since 2018 - this is actually the first system in which we had been able to image planets literally in their birth cradle".

The two giant planets forming in this system carve out a relatively wide and deep annular gap within the disc, with potential rocky planets forming in the inner part of the disc, closer to the star with respect to the gap. Studies prior to the advent of the JWST had failed to detect water in these inner regions for discs with similar morphology, so astronomers suspected that water could not survive the intense stellar radiation prevailing there, resulting in dry, rocky environments for planet formation. "In fact, we did not expect to discover significant amounts of water vapour in the inner disc of PDS 70. This discovery somewhat challenges the view of evolved protoplanetary disks as being 'dry', and therefore calls into question the hypothesis that water is supplied to rocky planets solely through a subsequent bombardment of water-bearing asteroids", adds Valentin Christiaens.

Where does the water in the PDS 70 disc come from?

The MINDS team is studying several scenarios to explain its somewhat unexpected discovery. One possibility is that the water is a remnant of an initially water-rich nebula that preceded the disc stage. Water is fairly common in this type of nebula, particularly in the state of ice, covering tiny particles of dust. When subjected to heat near a star in formation, the water evaporates and mixes with the other gases. Unfortunately, water molecules are quite fragile and break up into smaller components, hydrogen and oxygen, when hit by the harmful UV radiation from the nearby star. However, surrounding materials such as dust and the water molecules themselves, if sufficiently abundant, can act as a protective shield. Given the evolved stage of the disk and the low amount of residual dust in the inner part of the disk, the detection of water, therefore, suggests an efficient water formation or replenishment mechanism to compensate for this destruction.

Under certain circumstances, oxygen and hydrogen can combine to form water vapour. If the temperature and gas density conditions are right, water vapour can be formed on-site, in the inner disc, and compensate for the star's UV destruction. Another source of water vapour could come from ice-rich dust particles initially located at the outer edges of the disc. The friction of ice-rich dust with the rotating gas in the disc can cause these particles to spiral in from the outer edges of the disc towards the interior.  When the dust grains enter the inner disc close to the star, the ice then transforms into water vapour.

“The truth probably lies in a combination of all these options," concludes Giulia Perotti. “Nevertheless, it is likely that one mechanism plays a more decisive role than the others in maintaining the water reservoir of the PDS 70 disc. The task ahead will be to find out which mechanism it is.”

The MINDS (MIRI Mid-Infrared Disk Survey) collaboration

Max Planck Institute for Astronomy, Heidelberg, Germany; Max Planck Institute for Extraterrestrial Physics, Garching, Germany; Université Paris-Saclay, Orsay and Gif-sur-Yvette, France; Leiden Observatory, Netherlands; KU Leuven, Belgium; Rijksuniversiteit Groningen, Netherlands; University of Vienna, Austria; ETH Zürich, Switzerland; University of Liège, Belgium; Centro de Astrobilogía, CSIC-INTA, Villaneuva de la Cañada and Torrejón de Ardoz, Spain; LESIA, Observatoire de Paris, Meudon, France; INAF, Naples, Italy; Dublin Institute for Advanced Studies, Ireland; UK Astronomy Technology Centre, Edinburgh, United Kingdom; Radboud University, Nijmegen, Netherlands; Space Research Institute, Graz, Austria; SRON, Groningen and Leiden, Netherlands; University of Stockholm, Sweden; DTU Space, Lyngby, Denmark; Onsala Space Observatory, Sweden; University of Amsterdam, Netherlands.

Scientific reference

Perotti and al., "Water in the terrestrial planet-forming zone of the PDS 70 disk", Nature (2023).

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