JWST Findings Support Ice Pebble Drift Theory, Gives New Insight Into Planet Formation

Astronomers used the James Webb Space Telescope to make a breakthrough in the search for clues as to how planets are formed. By observing water vapor in distant protoplanetary discs, they confirmed a long-proposed theory of how celestial bodies came into existence.

Ice Pebble Drift Theory

It has been long proposed that icy pebbles could be the fundamental components of planetary formation. Icy pebbles are objects measuring one foot (30 cm) in diameter that formed in the protoplanetary disks' cold outer regions, the exact spot where comets originate in the Solar System.

This theory assumes that pebbles should drift inward toward the star due to friction in the gaseous disks, allowing them to deliver solids and water to planets. Its fundamental prediction is that as icy pebbles enter the warmer region within the "snowline," where ice transitions to vapor, they should release large amounts of cold water vapor.

Tracing the Roots of the Earth

The connection between water vapor in the inner disk and the drift of icy pebbles from the outer disk has finally been revealed by the James Webb Space Telescope. Principal investigator Andrea Banzatti of Texas State University believes this finding holds the key to studying the formation of rocky planets.

In the past, experts had a very static picture of planet formation, almost like there were isolated zones that planets formed out of. The researchers currently have evidence that these zones interact, and this event is proposed to have happened in our Solar System.

In this study, the research team used Webb's Mid-Infrared Instrument (MIRI) to investigate two compact and two extended disks around Sun-like stars. These stars are estimated to be between 2 and 3 billion years old, considered newborns in astronomical time.

The astronomers expected that the two compact disks would experience efficient pebble drift where pebbles would be delivered within a distance equivalent to Neptune's orbit. Meanwhile, the extended disks are assumed to have their pebbles retained in multiple rings as far out as six times the orbit of Neptune.

The observations from JWST were designed to determine whether compact disks have a higher water abundance in their inner, rocky planet region. This condition is expected if pebble drift is more efficient and delivers a lot of solid mass and water to the inner planets. The researchers decided to use MIRI's Medium-Resolution Spectrometer (MRS) due to its sensitivity to water vapor in disks.

The result of the study confirmed expectations by revealing excess cool water in the compact disks compared with the large disks. Pebbles encounter a pressure bump as they drift. This increase in pressure does not necessarily shut down pebble drift; the same happens in the large disks with rings and gaps.

The current research findings propose that large planets may cause rings of increased pressure where pebbles tend to collect. This is also assumed to be the role of Jupiter in our Solar System, where it inhibits pebbles and water delivery to small, inner, and relatively water-poor rocky planets.