Webb telescope detects water vapor for first time in faraway Earth-like planet-forming disk

Presence of water in the inner ring

The Webb's MIRI (Mid-Infrared Instrument) closely observed the captivating alien star system called PDS 70. This system features a K-type star, which is 5.4 million years old and notably cooler than our Sun.

This star's surroundings have two protoplanetary rings: inner and outer disks of gas and dust. According to the study, these two huge disks are separated by a massive five billion-mile (eight billion-kilometer) gap that contains two giant exoplanets. 

Water vapor molecules have been identified in the inner disk of the star system — about less than 100 million miles (160 million kilometers) from the star. 

“We’ve seen water in other disks, but not so close in and in a system where planets are currently assembling. We couldn’t make this type of measurement before Webb,” said Giulia Perotti, lead author from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany.

Within our solar system, the inner disk is the birthplace of rocky planets like Earth, Mercury, Venus, and Mars. Webb's observations indicate that any rocky worlds taking shape in this inner region have the potential to acquire substantial amounts of water molecules from their very inception, significantly improving their prospects of becoming habitable in the future.

Up to the present time, no rocky planets have been observed within the inner disk of PDS 70. Nevertheless, there is a ray of hope, as Webb identified hints of raw materials, such as silicate molecules, which suggest the potential formation of rocky worlds.

Where did the water came from?

The discoveries give rise to two primary inquiries: firstly, the source of the water, and secondly, how the molecules managed to persist in such proximity to the star within the inner ring. The authors put forth two potential explanations for the origin of water.

The first hypothesis proposes that water molecules formed in the water-rich nebula, which led to the formation of PDS 70. This implies that water molecules have been present in this inner location for a long period.

Alternatively, it is possible that the water originated in the outer ring and subsequently migrated to the inner disk. The ice-coated dust particles could have traveled from the relatively cooler outer region to the significantly hotter inner region. Once within the inner area, the water ice undergoes sublimation, transitioning into water vapor.

“Such a transport system would be surprising since the dust would have to cross the large gap carved out by the two giant planets,” noted NASA. 

In response to the second inquiry, vast clouds of dust act as a protective shield for the molecules even at such a close proximity to the star. The encompassing dust acts as a shield, safeguarding the water molecules from being disintegrated by the harmful UV radiation and intense stellar winds.

The scientists intend to undertake more observations with Webb's advanced instruments to gain a deeper understanding of the water vapor mechanisms at work in this system. 

The results have been published in the journal Nature.

Study Abstract:

Terrestrial and sub-Neptune planets are expected to form in the inner (less than 10 AU) regions of protoplanetary disks. Water plays a key role in their formation, although it is yet unclear whether water molecules are formed in situ or transported from the outer disk. So far Spitzer Space Telescope observations have only provided water luminosity upper limits for dust-depleted inner disks, similar to PDS 70, the first system with direct confirmation of protoplanet presence. Here we report JWST observations of PDS 70, a benchmark target to search for water in a disk hosting a large (approximately 54 AU) planet-carved gap separating an inner and outer disk. Our findings show water in the inner disk of PDS 70. This implies that potential terrestrial planets forming therein have access to a water reservoir. The column densities of water vapour suggest in-situ formation via a reaction sequence involving O, H2 and/or OH, and survival through water self-shielding. This is also supported by the presence of CO2 emission, another molecule sensitive to ultraviolet photodissociation. Dust shielding, and replenishment of both gas and small dust from the outer disk, may also play a role in sustaining the water reservoir. Our observations also reveal a strong variability of the mid-infrared spectral energy distribution, pointing to a change of inner disk geometry.