Scientists using NASA’s James Webb Space Telescope just made a breakthrough discovery in revealing how planets are made. By observing water vapor in protoplanetary disks, Webb confirmed a physical process involving the drifting of ice-coated solids from the outer regions of the disk into the rocky-planet zone.
Theories have long proposed that
icy pebbles forming in the cold, outer regions of protoplanetary disks — the
same area where comets originate in our solar system — should be the
fundamental seeds of planet formation. The main requirement of these theories
is that pebbles should drift inward toward the star due to friction in the
gaseous disk, delivering both solids and water to planets.
A fundamental prediction of this
theory is that as icy pebbles enter into the warmer region within the
“snowline” — where ice transitions to vapor — they should release large amounts
of cold water vapor. This is exactly what Webb observed.
“Webb finally revealed the
connection between water vapor in the inner disk and the drift of icy pebbles
from the outer disk,” said principal investigator Andrea Banzatti of Texas
State University, San Marcos, Texas. “This finding opens up exciting prospects
for studying rocky planet formation with Webb!”
“In the past, we had this very
static picture of planet formation, almost like there were these isolated zones
that planets formed out of,” explained team member Colette Salyk of Vassar
College in Poughkeepsie, New York. “Now we actually have evidence that these
zones can interact with each other. It’s also something that is proposed to
have happened in our solar system.”
Image: Planet-forming Disks
Artist’s Concept: This artist’s concept compares two types of typical, planet-forming disks around newborn, Sun-like stars. On the left is a compact disk, and on the right is an extended disk with gaps. Scientists using Webb recently studied four protoplanetary disks—two compact and two extended. The researchers designed their observations to test whether compact planet-forming disks have more water in their inner regions than extended planet-forming disks with gaps. This would happen if ice-covered pebbles in the compact disks drift more efficiently into the close-in regions nearer to the star and deliver large amounts of solids and water to the just-forming, rocky, inner planets. Current research proposes that large planets may cause rings of increased pressure, where pebbles tend to collect. As the pebbles drift, any time they encounter an increase in pressure, they tend to collect there. These pressure traps don’t necessarily shut down pebble drift, but they do impede it. This is what appears to be happening in the large disks with rings and gaps. This also could have been a role of Jupiter in our solar system — inhibiting pebbles and water delivery to our small, inner, and relatively water-poor rocky planets. NASA, ESA, CSA, Joseph Olmsted (STScI)
Harnessing the Power of Webb
The researchers used Webb’s MIRI (the Mid-Infrared Instrument) to study four disks — two compact and
two extended — around Sun-like stars. All four of these stars are estimated to
be between 2 and 3 million years old, just newborns in cosmic time.
The two compact disks are expected
to experience efficient pebble drift, delivering pebbles to well within a
distance equivalent to Neptune’s orbit. In contrast, the extended disks are
expected to have their pebbles retained in multiple rings as far out as six
times the orbit of Neptune.
The Webb observations were designed
to determine whether compact disks have a higher water abundance in their
inner, rocky planet region, as expected if pebble drift is more efficient and
is delivering lots of solid mass and water to inner planets. The team chose to
use MIRI’s MRS (the Medium-Resolution Spectrometer) because it is sensitive to
water vapor in disks.
The results confirmed expectations
by revealing excess cool water in the compact disks, compared with the large
disks.
Image: Water Abundance
Emission Spectrum – Water Abundance: This graphic
compares the spectral data for warm and cool water in the GK Tau disk, which is
a compact disk without rings, and extended CI Tau disk, which has at least
three rings on different orbits. The science team employed the unprecedented
resolving power of MIRI’s MRS (the Medium-Resolution Spectrometer) to separate
the spectra into individual lines that probe water at different temperatures.
These spectra, seen in the top graph, clearly reveal excess cool water in the
compact GK Tau disk, compared with the large CI Tau disk. The bottom graph
shows the excess cool water data in the compact GK Tau disk minus the cool
water data in the extended CI Tau disk. The actual data, in purple, are
overlaid on a model spectrum of cool water. Note how closely they align.
NASA, ESA, CSA, Leah Hustak (STScI)
As the pebbles drift, any time they encounter a pressure bump — an increase
in pressure — they tend to collect there. These pressure traps don’t
necessarily shut down pebble drift, but they do impede it. This is what appears
to be happening in the large disks with rings and gaps.
Current research proposes that
large planets may cause rings of increased pressure, where pebbles tend to
collect. This also could have been a role of Jupiter in our solar system —
inhibiting pebbles and water delivery to our small, inner, and relatively
water-poor rocky planets.
Solving the Riddle
When the data first came in, the
results were puzzling to the research team. “For two months, we were stuck on
these preliminary results that were telling us that the compact disks had
colder water, and the large disks had hotter water overall,” remembered
Banzatti. “This made no sense, because we had selected a sample of stars with
very similar temperatures.”
Only when Banzatti overlaid the
data from the compact disks onto the data from the large disks did the answer
clearly emerge: the compact disks have extra cool water just inside the
snowline, at about ten times closer than the orbit of Neptune.
“Now we finally see unambiguously
that it is the colder water that has an excess,” said Banzatti. “This is
unprecedented and entirely due to Webb’s higher resolving power!”
Image: Icy Pebble Drift
This graphic is an interpretation of data from Webb’s
MIRI, the Mid-Infrared Instrument, which is 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, as the ice-covered pebbles drift inward toward the warmer region closer
to the star, they are unimpeded. As they cross the snow line, their ice turns
to vapor and provides a large amount of water to enrich the just-forming,
rocky, inner planets. On the right is an extended disk with rings and gaps. As
the ice-covered pebbles begin their journey inward, many become stopped by the
gaps and trapped in the rings. Fewer icy pebbles are able to make it across the
snow line to deliver water to the inner region of the disk.
NASA, ESA, CSA, Joseph Olmsted (STScI)
The team’s results appear in the Nov. 8 edition of the Astrophysical Journal
Letters.
The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.
Source: NASA’s Webb Findings Support Long-Proposed Process of Planet Formation - NASA
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