JWST Sees Newly Forming Planets Swimming in Water

One big question about Earth’s formation is, where did all the water come from? New data from the James Webb Space Telescope (JWST) shows newly forming planets in a system 370 light-years away are surrounded by water vapor in their orbits. Although astronomers have detected water vapor in protoplanetary disks before, this is the first time it’s been seen where the planets are forming.

“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 of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, lead scientist of the observations.

The distant planetary system, named PDS 70, contains both an inner disk and outer disk of gas and dust, separated by a 5 billion-mile-wide (8 billion kilometer) gap. JWST’s MIRI (Mid-Infrared Instrument) detected the water vapor in the system’s inner disk, where rocky terrestrial and sub-Neptune planets are expected to form.

The water vapor was detected at distances of less than 100 million miles (160 million kilometers) from the star. Earth orbits 93 million miles (150 million km) from our Sun.

While actual planets haven’t yet been detected in this inner region of PDS 70, astronomers have seen evidence of protoplanets, with the raw materials for building rocky worlds in the form of silicates. The astronomers said in their paper, published in Nature, that the detection of water vapor implies that if rocky planets are forming there, they will have water available to them from the very start.

A spectrum of the protoplanetary disk of PDS 70, obtained with Webb’s MIRI (Mid-Infrared Instrument), displays a number of emission lines from water vapor. Scientists determined that the water is in the system’s inner disk, at distances of 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. Credits: NASA, ESA, CSA, J. Olmsted (STScI)

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

There are two known planets in this system, PDS 70b and PDS 70c, both of which are the size of Jupiter. These gas giants are located within the gap between the two dusty disks, and the c planet may 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 that one possibility is that hydrogen and oxygen atoms are meeting in the region, forming water. Another possibility is that the water migrates from the cool outer disk into the inner system.

PDS 70 is a K-type star, cooler than our Sun, and is estimated to be 5.4 million years old. This is relatively old for a star to have planets still forming, since over time, the gas and dust content of planet-forming disks declines.

The scientists said that either the central star’s radiation and winds blow out such material, or the dust combines and grows into larger objects over time, which eventually form planets.

“We find a relatively high 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 disk is a very exciting place.”

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