Webb detects water vapor in rocky planet-forming zones


July 24, 2023

(Nanowerk News) Water is essential to life as we know it. However, scientists debate how it reached Earth and whether the same process could produce rocky exoplanets orbiting distant stars. New insights may come from the planetary system PDS 70, located 370 light years away. The star hosts an inner and outer disk of gas and dust, separated by a gap 5 billion miles (8 billion kilometers) wide, and within that gap are the two known gas giant planets.

New measurements by MIRI (Mid-Infrared Instrument) of NASA’s James Webb Space Telescope have detected water vapor in the system’s inner disk, at a distance of less than 100 million miles (160 million kilometers) from the star – a region where rocky terrestrial planets might form. (Earth orbits 93 million miles from our Sun.) This is the first detection of water in a terrestrial region of the disk that is already known to have hosted two or more protoplanets. This artist's concept depicts the star PDS 70 and its inner protoplanetary disk This artist’s concept depicts the star PDS 70 and its inner protoplanetary disk. New measurements by NASA’s James Webb Space Telescope have detected water vapor less than 100 million miles from the star – a region where rocky terrestrial planets might form. This is the first detection of water in the terrestrial region of a disk that is already known to harbor two or more protoplanets, one of which is shown above right. (Image: NASA, ESA, CSA, J. Olmsted (STScI))

These findings have been published in the journal Natural (“Water in the terrestrial planet-forming zone of the PDS 70 disk”).

“We’ve seen water on other disks, but not as close and within systems where planets are currently clustered. We couldn’t make measurements like this before Webb,” said lead author Giulia Perotti of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany.

“This discovery is very exciting, as it investigates the region where rocky planets similar to Earth usually form,” added MPIA director Thomas Henning, a co-author of the paper. Henning was principal investigator with MIRI (Mid-Infrared Instrument) Webb, which performed the detections, and principal investigator with the MINDS (MIRI Mid-Infrared Disk Survey) program, which retrieved the data.

Steamy Environments to Form Planets

PDS 70 is a K-type star, cooler than our Sun, and has an estimated age of 5.4 million years. It’s relatively old in terms of stars with disk-forming planets, which makes the discovery of water vapor surprising.

Over time, the gas and dust content of the planet-forming disc decreased. Either the central star’s radiation and winds blew the material away, or the dust grew into larger objects that eventually formed planets. Because previous studies failed to detect water in a similarly aged central disk region, astronomers suspect it may not have survived the harsh radiation of stars, leading to a dry environment for the formation of any rocky planets.

Astronomers have yet to detect any planets forming within the inner disk of PDS 70. However, they do see the raw material for building rocky worlds in the form of silicates. The water vapor detection implies that if rocky planets formed there, they would have had water available from the start.

“We found a relatively high number of small dust grains. Combined with our water vapor detection, the inner disc is a really interesting place to be,” said co-author Rens Waters of Radboud University in the Netherlands.

What is the Origin of Water?

The discovery raises the question of where the water comes from. The MINDS team considered two different scenarios to explain their findings.

One possibility is that water molecules form where we detect them when hydrogen and oxygen atoms combine. The second possibility is that ice-coated dust particles are transported from the cold outer disk to the hot inner disk, where water ice sublimates and turns into steam. Such a transport system would be surprising, as the dust has to pass through the huge gap carved by the two giant planets.

Another question raised by this discovery is how water can survive so close to a star, when the star’s ultraviolet light has to break apart water molecules. Most likely, the surrounding material such as dust and other water molecules served as a protective shield. As a result, the water detected in the inner disc of the PDS 70 can survive destruction.

Ultimately, the team will use two more of Webb’s instruments, NIRCam (Near-Infrared Camera) and NIRSpec (Near-Infrared Spectrograph) to study the PDS 70 system in an effort to gain a greater understanding.


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