Planetary systems take millions of years to form, which introduces quite a
challenge for astronomers. How do you identify which stage they are in, or
categorize them? The best approach is to look at lots of examples and keep
adding to the data we have—and NASA's upcoming James Webb Space Telescope
will be able to provide an infrared inventory. Researchers using Webb will
observe 17 actively forming planetary systems. These particular systems were
previously surveyed by the Atacama Large Millimeter/submillimeter Array
(ALMA), the largest radio telescope in the world, for the Disk Substructures
at High Angular Resolution Project (DSHARP).
Webb will measure spectra that can reveal molecules in the inner regions of
these protoplanetary disks, complementing the details ALMA has provided
about the disks' outer regions. These inner regions are where rocky,
Earth-like planets can start to form, which is one reason why we want to
know more about which molecules exist there.
A research team led by Colette Salyk of Vassar College in Poughkeepsie, New
York, and Klaus Pontoppidan of the Space Telescope Science Institute in
Baltimore, Maryland, seek the details found in infrared light. "Once you
switch to infrared light, specifically to Webb's range in mid-infrared
light, we will be sensitive to the most abundant molecules that carry common
elements," explained Pontoppidan.
Researchers will be able to assess the quantities of water, carbon monoxide,
carbon dioxide, methane, and ammonia—among many other molecules—in each
disk. Critically, they will be able to count the molecules that contain
elements essential to life as we know it, including oxygen, carbon, and
nitrogen. How? With spectroscopy: Webb will capture all the light emitted at
the center of each protoplanetary disk as a spectrum, which produces a
detailed pattern of colors based on the wavelengths of light emitted. Since
every molecule imprints a unique pattern on the spectrum, researchers can
identify which molecules are there and build inventories of the contents
within each protoplanetary disk. The strength of these patterns also carries
information about the temperature and quantity of each molecule.
"Webb's data will also help us identify where the molecules are within the
overall system," Salyk said. "If they're hot, that implies they are closer
to the star. If they're cooler, they may be farther away." This spatial
information will help inform models that scientists build as they continue
examining this program's data.
Knowing what's in the inner regions of the disks has other benefits as well.
Has water, for example, made it to this area, where habitable planets may be
forming? "One of the things that's really amazing about planets—change the
chemistry just a little bit and you can get these dramatically different
worlds," Salyk continued. "That's why we're interested in the chemistry.
We're trying to figure out how the materials initially found in a system may
end up as different types of planets."
If this sounds like a significant undertaking, do not worry—it will be a
community effort. This is a Webb Treasury Program, which means that the data
is released as soon as it's taken to all astronomers, allowing everyone to
immediately pull the data, begin assessing what's what in each disk, and
share their findings.
"Webb's infrared data will be intensively studied," added co-investigator Ke
Zhang of the University of Wisconsin–Madison. "We want the whole research
community to be able to approach the data from different angles."
Why the up-close examination?
Let's step back, to see the forest for the trees. Imagine you are on a
research boat off the coast of a distant terrain. This is the broadest view.
If you were to land and disembark, you could begin counting how many trees
there are and how many of each tree species. You could start identifying
specific insects and birds and match up the sounds you heard offshore to the
calls you hear under the treetops. This detailed cataloging is very similar
to what Webb will empower researchers to do—but swap trees and animals for
chemical elements.
The protoplanetary disks in this program are very bright and relatively
close to Earth, making them excellent targets to study. It's why they were
surveyed by ALMA. It's also why researchers studied them with NASA's Spitzer
Space Telescope. These objects have only been studied in depth since 2003,
making this a relatively newer field of research. There's a lot Webb can add
to what we know.
The telescope's Mid-Infrared Instrument (MIRI) provides many advantages.
Webb's location in space means that it can capture the full range of
mid-infrared light (Earth's atmosphere filters it out). Plus, its data will
have high resolution, which will reveal many more lines and wiggles in the
spectra that the researchers can use to tease out specific molecules.
The researchers were also selective about the types of stars chosen for
these observations. This sample includes stars that are about half the mass
of the Sun to about twice the mass of the Sun. Why? The goal is to help
researchers learn more about systems that may be like our own as it formed.
"With this sample, we can start to determine if there are any common
features between the disks' properties and their inner chemistry," Zhang
continued. "Eventually, we want to be able to predict which types of systems
are more likely to generate habitable planets."
Beginning to answer big questions
This program may also help researchers begin to answer some classic
questions: Are the forms taken by some of the most abundant elements found
in protoplanetary disks, like carbon, nitrogen, and oxygen, "inherited" from
the interstellar clouds that formed them? Or does the precise mix of
chemicals change over time? "We think we can get to some of those answers by
making inventories with Webb," Pontoppidan explained. "It's obviously a
tremendous amount of work to do—and cannot be done only with these data—but
I think we are going to make some major progress."
Thinking even more broadly about the incredibly rich spectra Webb will
provide, Salyk added, "I'm hoping that we'll see things that surprise us and
then begin to study those serendipitous discoveries."
This research will be conducted as part of Webb General Observer (GO)
programs, which are competitively selected using a dual-anonymous review
system, the same system that is used to allocate time on the Hubble Space
Telescope.
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