Scientists, using Webb, discover water vapor within a planet-forming disk

by Haygen Warren

Using the joint NASA/European Space Agency/Canadian Space Agency James Webb Space Telescope, a team of scientists has discovered, for the first time, water vapor within a planet-forming disk. The water vapor, which was found within the inner disk of two circumstellar disks around star PDS 70, is allowing scientists to research the ways by which water makes its way into rocky, terrestrial planets.

Star system PDS 70, located 370 light-years away from Earth, has been the subject of recent discoveries in exoplanet and star system research. In 2018, PDS 70b, which is one of two confirmed exoplanets around PDS 70, became the first protoplanet to ever be directly imaged by a telescope. What’s more, in 2021, a team detected the first-ever circumplanetary disk around PDS 70c — PDS 70b’s sibling planet.

PDS 70 has now returned to the spotlight, as a team of scientists using Webb’s Mid-Infrared Instrument (MIRI) has discovered water vapor within a disk of the star system, which is home to two circumstellar disks (an inner and outer disk). The inner disk is made up of gas, dust, and other cosmic material that may be fueling the formation of new exoplanets. The water vapor detected by Perotti et al. was found less than 160 million kilometers from the star — directly within the region of the inner disk where planets are possibly forming, and at a similar distance to Earth’s separation from the Sun (149.6 million kilometers).

This image, taken with the Atacama Large Millimeter/submillimeter Array (ALMA), shows the PDS 70 star system. PDS 70 is the faint dot in the middle of the circumstellar disk, and the bright spot to the right of PDS 70 is exoplanet PDS 70c. (Credit: ALMA (ESO/NAOJ/NRAO)/Benisty et al.)

Perotti et al.’s detection of water vapor is the first time water has been found within the terrestrial region of a stellar disk wherein one or more protoplanets are thought to be forming. Scientists have long debated the exact process by which water arrived on Earth, and are using star systems with planet-forming regions like PDS 70 to investigate the ways water can make its way into planets, especially those like Earth.

“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 Perotti.

Interestingly, the detection of water vapor came as a surprise to Perotti et al., as they believed PDS 70 was too old to efficiently create an environment conducive to planet formation.

PDS 70 is a 5.4-million-year-old K-type star that is cooler than our Sun. Its age and type make it relatively old when compared to other stars with planet-forming disks, so the team of scientists was not expecting to see water vapor in its inner disk.

While these planet-forming disks require a significant amount of material to form planets, the amount of material within the disks can decrease over time and as the host star ages. It is currently thought that either the star’s radiation and stellar wind blow material out of the disks, or the material within the disk clumps together into larger objects that could eventually form planets. Previous studies have not found water within these aged planet-forming disks, which then led scientists to believe that the disks could not survive the stellar radiation and that they were too dry for the formation of rocky planets.

Scientists have not yet confirmed the presence of protoplanets within the inner planet-forming disk of PDS 70; though the silicates and materials needed for the formation of rocky planets have been confirmed to exist within the disk. If scientists do find protoplanets within the disk, Perotti et al.’s detection of water vapor seemingly implies that the planets would have water available to them from the moment they form.

“We find a relatively high amount of small dust grains. Combined with our detection of water vapor, the inner disk is a very exciting place,” said Rens Waters, a co-author and scientist at Radboud University in The Netherlands.

The emission spectrum of PDS 70’s inner disk, which was obtained by MIRI. (Credit: NASA/ESA/CSA/J. Olmsted (STScI))

However, where is the water within the disk coming from?

With Perotti et al.’s research contributing to the MIRI mid-INfrared Disk Survey (MINDS), the MINDS team was able to generate two possible scenarios for how water entered the disk. The first scenario is that water is naturally forming within the disk via the combination of hydrogen and oxygen atoms.

The second scenario is slightly more complex, with ice-coated dust particles from the cooler outer disk being transported to the warmer inner disk. Once the icy particles reach the warm environment of the inner disk, the ice around the dust particles melts and turns into water vapor. This second scenario is considered more unlikely, as the dust particles would have to travel across the large gap between the inner and outer disk, which is carved out by PDS 70’s two exoplanets.

In addition to the question of how water gets into the disk, Perotti et al. are investigating how water can survive so close to PDS 70, as the ultraviolet radiation from the star should break apart the water molecules within the disk. This obviously isn’t occurring within PDS 70’s inner disk, and the team believes that the gas, dust, and cosmic material within the disk may be serving as a shield from PDS 70’s immense radiation — which would allow the water molecules within the disk to survive.

To answer these questions and further investigate PDS 70’s inner disk, Perotti et al. are planning on using Webb’s Near-Infrared Camera (NIRCam) and Near-Infrared Spectrometer (NIRSpec). NIRCam and NIRSpec, along with MIRI and Webb’s Fine Guidance Sensor/Near Infrared Imager and Slitless Spectrograph, make up Webb’s incredible instrument suite, which has allowed scientists to peer further into the universe and star systems than any telescope before it.

Perotti et al.’s research was published in the journal Nature on July 24, 2023, as part of Webb’s Guaranteed Time Observation program 1282. 

(Lead image: Artist’s concept of PDS 70 and its inner disk where the water vapor was detected. Credit: NASA/ESA/CSA/J. Olmsted (STScI))

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