NASA’s James Webb Space Telescope scientists have made an important discovery about how planets form. By observing water vapor in “protoplanetary disks”, the disks of dust and gas that swirl around young stars, Webb has confirmed a physical process key to planet formation.

Scientists have long thought that small icy pebbles that form in the cold outer parts of protoplanetary disks, similar to where comets come from in our solar system, are important for building planets.

The theories say these pebble seeds should slowly drift inward through the gaseous disk due to friction. This inward movement provides solid material and water to developing planets closer to the star.

Concept art: This concept art compares two types of typical planet-forming disks around newborn Sun-like stars. On the left, a compact disk and, on the right, an extended disk with gaps. Scientists using the Webb telescope recently studied four protoplanetary disks: two compact and two extended disks. | photo NASA, ESA, CSA, Joseph Olmsted (STScI)

An important prediction is that when icy pebbles cross the “snow line” – where it’s warm enough for ice to turn to vapor – they should release a lot of cold water vapor. This is exactly what Webb saw in two compact protoplanetary disks!

The researchers used Webb’s MIRI instrument, which is good at detecting water, to look at four disks around stars like our Sun.

Two disks were compact without gaps, while two had spreading rings. They expected more efficient inward pebble drifting and extra inner water for the compact disks.

Emission Spectra – Water Abundance: This plot compares spectral data for hot and cold water in the GK Tau disk, which is a compact disk with no rings, and the extended CI Tau disk, which has at least three rings in different orbits. The science team employed the unprecedented resolving power of MIRI’s MRS (Medium Resolution Spectrometer) to separate the spectra into individual lines that probe water at different temperatures. These spectra, shown in the top plot, clearly reveal an excess of cold water in the GK Tau compact disk compared to the large CI Tau disk. The lower plot shows the excess cold water data in the GK Tau compact disk minus the cold water data in the extended CI Tau disk. The actual data, in purple, are superimposed on a model cold-water spectrum. Note how they are aligned | photo NASA, ESA, CSA, Leah Hustak (STScI)

Webb’s powerful observations clearly showed excess cold water in the compact disks inside the snow line, right where planet seeds are forming. This confirms that icy pebbles bring water from the outer to inner disk regions during planet formation.

Before, scientists thought planet-forming regions didn’t interact much. Now we see icy pebbles drifting long distances, just like what probably happened to bring water to Earth.

Jupiter may have blocked drifting pebbles from reaching our inner planets, keeping them drier.

This plot is an interpretation of Webb’s MIRI (Mid-Infrared Instrument) data, sensitive to water vapor in disks. It shows the difference between pebble drift and water content in a compact disk versus an extended disk with rings and gaps. In the compact disk on the left, the ice-covered pebbles drift unimpeded toward the warmer region closer to the star. As they cross the snow line, their ice turns to steam and provides a large amount of water to enrich the forming rocky inner planets. On the right is an extended disk with rings and gaps. As the ice-covered pebbles begin their inward journey, many are stopped by the gaps and trapped in the rings. Fewer icy pebbles are able to break through the snow line to bring water to the inner region of the disk | photo NASA, ESA, CSA, Joseph Olmsted (STScI)

Webb’s amazing images give us the clearest view yet of how water and rocky planets form together in swirly disks around young stars. Its discoveries will help reveal more secrets of planetary nurseries across the galaxy.

Sources

NASA | Andrea Banzatti, Klaus M. Pontoppidan, et al., JWST Reveals Excess Cool Water near the Snow Line in Compact Disks, Consistent with Pebble Drift. The Astrophysical Journal Letters, vol.957, no.2. DOI 10.3847/2041-8213/acf5ec

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