JWST sees newly forming planets floating in water

A big question about the formation of the earth is: where did all the water come from? New data from the James Webb Space Telescope (JWST) show that newly forming planets in a system 370 light-years away are surrounded by water vapor in their orbits. Although astronomers have previously detected water vapor in protoplanetary disks, this is the first time water vapor has been observed where planets form.

“We’ve seen water in other disks, but not as close in and in a system where planets are currently assembling. Before Webb, we couldn’t make this kind of measurement,” said Giulia Perotti of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, lead scientist on the observations.

Dubbed PDS 70, the distant planetary system contains both inner and outer disks of gas and dust separated by a gap 8 billion kilometers wide. JWST’s MIRI (Mid-Infrared Instrument) detected the water vapor in the system’s inner disc, where rocky terrestrial planets and sub-Neptune planets are expected to form.

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The water vapor has been detected at distances of less than 160 million kilometers from the star. The earth orbits our sun at a distance of 150 million kilometers.

While no true planets have yet been discovered in this inner region of PDS 70, astronomers have seen evidence of protoplanets whose raw materials for building rocky worlds come in the form of silicates. The astronomers, in their paper published in Nature, said that the detection of water vapor implied that if rocky planets formed there, they would have water from the start.

A spectrum of PDS 70’s protoplanetary disk taken with Webb’s MIRI (Mid-Infrared Instrument) shows a series of emission lines from water vapor. Scientists determined that the water resides in the system’s inner disk, less than 100 million miles from the star — the region where rocky, terrestrial planets may be forming. Download the full resolution version from the Space Telescope Science Institute. Image credits: NASA, ESA, CSA, J. Olmsted (STScI)

“This discovery is extremely exciting as it examines the region where Earth-like rocky planets typically form,” added MPIA Director Thomas Henning and co-author of the study. Henning is co-principal investigator on Webb’s MIRI (Mid-Infrared Instrument), which made the discovery, and principal investigator on the MINDS (MIRI Mid-Infrared Disk Survey) program, which compiled the data.

There are two known planets in this system, PDS 70b and PDS 70c, both the size of Jupiter. These gas giants reside in the gap between the two dust disks, and the c-planet could have a moon orbiting it. That other planets have already formed in this system bodes well for the future formation of other worlds.

But the question remains: Where did the water come from? The researchers said one possibility is that hydrogen and oxygen atoms in the region meet and form water. Another possibility is that the water migrates from the cool outer pane into the inner system.

PDS 70 is a K-type star, cooler than our Sun and estimated to be 5.4 million years old. The fact that planets are still forming in a star is relatively old, since the gas and dust content of the planet-forming disks decreases over time.

The scientists said either the radiation and winds from the central star are blowing out this material, or over time the dust coalesces and grows into larger objects that eventually form planets.

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

This late planet formation also makes the discovery of water vapor surprising. The team plans to use two more instruments from JWST, NIRCam (Near-Infrared Camera) and NIRSpec (Near-Infrared Spectrograph), to study the PDS 70 system to gain an even better understanding of what’s going on in the inner disk.


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