Abstract
The study of protoplanetary disks allows us to better understand our origins. Protoplanetary disks provide a direct view of planet formation in progress, allowing us to refine our solar system-centric planet formation scenario. But protoplanetary disks also allow us to directly observe the more varied paths that planet formation might take. The incredible sensitivity of JWST, coupled with its instrumentation well-suited for studying planet formation chemistry, is allowing us to explore the diversity of planet formation pathways.
1. Introduction
1.1 The study of protoplanetary disks helps us understand our origins
The study of protoplanetary disks – the disks of gas and dust out of which planets form – is motivated, at its heart, by two questions fundamental to humans: Why are we here? and, Are we alone? One approach to answering these questions is to search for an Earth analog elsewhere in the galaxy. However, this type of work requires long exposures, unsuitable to large surveys, and getting sufficiently detailed information about Earth analogs will remain difficult for the foreseeable future.
As an alternative, we might try to understand several key steps in the origin of humans, and understand how likely each of those steps are to occur. We might perhaps begin with understanding the origin of the universe, then the origin of Earth, then the origin of life, and, finally, the origin of human life. If we want to understand whether the production of an Earth-like planet is common or rare in the universe, it is essential to not just hypothesize about Earth’s formation. Instead, we need to understand the varied ways in which planets can form, and figure out which of those ways produces Earth-like vs. other types of planets.
The study of protoplanetary disks therefore aims to illuminate all of the planet formation pathways that occur in our universe. If we can study the varied pathways, we can understand which environments are likely to form Earth-like planets (or not), and why.
1.2 The solar system story
The solar system has a wide array of planet types, from gas giants, to tiny Mercury, with only our planet Earth seemingly habitable. For the solar system, the key variable that influences the type of planet formed seems to be distance from the sun, which is also a proxy for temperature. The standard formation story for our solar system (e.g., Grossman 1972) envisions a well-mixed solar-composition gas, known as the solar nebula, condensing into planetary building blocks according to the local temperature. Therefore, terrestrial planets and planetary cores close to the sun are composed of relatively more refractory-rich materials, while those far from the sun are composed of relatively more volatile-rich materials. In addition, the condensation of water ice at the so-called snow line allows for the formation of more massive planetary cores, aiding in the formation of gas giants (e.g., Hayashi 1981).
1.3 Wrinkles in the solar system story
However, the solar system story has some wrinkles, notably the influences of non-equilibrium chemistry and mixing processes. Meteorite and cometary compositional analyses have shown that solid materials can be mixed across large radial distances (e.g., Nakamura et al. 2008); in addition, cometary analyses suggest that some materials, especially in the outer solar system, may have been inherited directly from the solar system’s birth cloud, rather than being vaporized and then thoroughly mixed about (Pontoppidan et al. 2014, and references therein).
One potential cause of mixing is the radial migration of solids that occurs in the presence of pressure gradients. In a smooth solar nebula disk, pressure is expected to decrease uniformly with distance from the sun, which would result in inward radial migration of solids (Weidenschilling et al. 1977). However, the relatively recent discovery that a majority of protoplanetary disks have radial structures indicative of many localized pressure bumps (Andrews et al. 2018, Long et al. 2018} suggests that migration is much more complex – grains might move in or out, or even stop altogether, depending on the local conditions. When this radial migration of solids occurs, local chemistry can vary depending on how much solid material was delivered to a region. Thus, planet composition won’t just depend on distance from the sun, but will also depend on the detailed radial structure of the early solar system, and how it affects the transport of solid materials.
1.4 Multiple dimensions of planet formation
While the solar system provides a way to explore how distance from the sun affects planet formation, there are many other factors that could affect planet formation processes throughout the galaxy, or the universe (see Figure 1). Studying protoplanetary disks, as opposed to just the solar system, allows us to see how the formation process proceeds across many different axes.
It is important to understand how stellar mass might affect planet formation, given that low-mass stars are the most common types of stars. Metallicity may play an important role in planet formation efficiency since solids are the seeds of planet formation (e.g., Johnson et al. 2010), and this would affect the formation of planets at the edges of our galaxy, and in the early universe. Radiation environment may play a role by eroding disks and shortening planet formation timescales (e.g., Mann et al. 2014) affecting planets forming in large clusters containing massive stars. Stellar multiplicity can also dynamically shape disks, likely affecting planet formation when binary separations are similar to disk size scales (e.g., Kraus et al. 2012).
If all of these axes of planet formation are taken into account, there might still be stochasticity inherent to the planet formation process. Can we ever fully predict a planetary system from initial conditions, or might very different planets arise from similar disks? Whether or not there is inherent stochasticity in the process can only be understood once the other factors are fully explored.
A final axis inherent in all studies of planet formation using protoplanetary disks is the time axis. Studies of the solar system only allow us to look at the final result of planet formation, long after it occurred. Instead, studies of protoplanetary disks can allow us to study the planet formation process as it progresses. In addition, multi-epoch studies can allow us to study rapid changes in real time.
2. The JWST Advantage
JWST offers several advantages over ground-based facilities and its space-based predecessor, Spitzer, for the study of protoplanetary disks.
2.1 Spatial resolution
The increased mirror size of JWST as compared to Spitzer, coupled with the use of integral field unit (IFU) spectrographs, enables spatially-resolved spectro-imaging of some disk structures. JWST’s diffraction limit corresponds to ~30 AU at the 150 pc distance of nearby star-forming regions. Thus, terrestrial planet-forming regions are not resolvable, but the outer parts of protoplanetary disks are. Also resolvable are jets and outflows, which regulate disk evolution, ultimately setting the timescale available for planet formation.
2.2 Sensitivity
The greatly increased sensitivity of JWST as compared to its predecessor, Spitzer, is allowing a fuller exploration of the many axes of planet formation. In particular, with greater sensitivity, we can now study the fainter disks around low mass stars, and lower-metallicity disks in the distant outer galaxy.
JWST’s sensitivity also aids in the detection of less abundant molecular species, and weaker atomic and molecular emission lines in general.
2.3 Infrared spectroscopy
JWST’s presence in space places it above the obscuring effects of Earth’s atmosphere; ground-based observations struggle particularly in the study of important chemical building block molecules like H2O that are also common in our atmosphere. Although JWST’s spectrographs do not provide resolving powers as high as high-resolution ground-based spectrographs, they still provide substantial improvement over Spitzer’s resolving power. The increased resolving power provides greater sensitivity to weak lines, as the line strength scales with resolving power for unresolved lines. In addition, the increased resolving power allows for the disentangling of emission from the multiple overlapping energy levels of water and other molecules.
3. First results
3.1 Spectral imaging
Spectral imaging is only beginning to be highlighted by the planet formation community, but it bears mentioning that the IFU data from JWST provides free spatial information on size scales of 10’s of AU. So far, emission lines from most molecules commonly studied in inner protoplanetary disks (including CO, H2O, HCN, C2H2 and OH) show no resolvable spatial extent, consistent with our understanding that they arise from the few AU region of the disk. However, some atomic transitions and H2 rovibrational transitions, both of which trace jets and outflows, are demonstrating interesting structures.
Pontoppidan et al. (2024) show an example of spectral imaging revealing an unidentified ring-like molecular hydrogen structure surrounding the protoplanetary disk FZ Tau. Its physical origin is not yet understood. Sturm et al. (2023), observing the edge-on HH 48 disk, demonstrate how a combination of tracers can map out the disk structure, plus a wide angle outflow, and a more collimated jet. In edge-on disks, it may also be possible to use line-of-sight probes to measure ice abundances as a function of disk radius; however, Sturm et al. caution that the optical paths can be complex, so simplistic interpretations should be avoided.
3.2 The distance axis, including the transport wrinkle
JWST continues to explore the solar system-based formation story, as well as the important caveats regarding transport of solids. A key take-away from JWST results so far is that protoplanetary disk chemistry in the few AU region can vary significantly from source to source. The reasons for these differences, however, are still being teased out. For some disks, various gaps may preferentially reveal different radial regions of the disk, and their specific chemistry. In others, radial transport, or its inhibition, may affect the inner disk C/O ratio. In all disks, the chemistry we see is also influenced by the local radiation field, and radiative transfer effects.
Pontoppidan et al. (2024) report strong water emission from the disk around FZ Tau, and show how JWST can allow for the decomposition of the spectrum into multiple temperature components. Gasman et al. (2023) take a similar approach, deriving temperatures for a series of water-emitting regions from the disk around Sz 98. Both studies thereby use the JWST spectra to empirically derive a disk radial temperature structure – the key input into a solar nebula-like chemical model. In addition, Pontoppidan et al. show how the full disk chemical abundance structure might be revealed by coupling JWST inner disk data with data from facilities probing the outer disk.
Several studies are revealing a variety of line strength ratios between H2O and C-bearing species, including CO2, HCN and C2H2, and discussions in each of these works highlight the continuing attempts to disentangle the multiple processes discussed above (revealing radial chemistry, vs. disk transport, vs. radiative effects). Grant et al. (2023) detect strong CO2 emission, including from the 13CO2 isotopologue, indicating high CO2 abundances in the GW Lup inner disk. While this could reflect inward transport of CO2 rich solids, Grant et al. (2023) argue that the transport should bring in H2O as well. Instead, they suggest that an inner cavity might be preferentially revealing a region between the H2O and CO2 snowlines. Schwarz et al. (2024) and Gasman et al. (2023) find, instead, a relatively low C/O ratio in the inner disk of SY Cha and Sz 98, respectively. This could be caused by inward transport of O-rich icy grains, although Sz 98’s gapped structure should prevent or at least slow such transport. Perhaps the C/O ratio is sometimes a relic of an earlier time when transport was more feasible, or perhaps other radiative transfer effects are affecting the observed ratio of C- to O-bearing molecules.
Results from Perotti et al. (2023) also remind us of the importance of large disk gaps on chemistry, in their observations of the gapped (and planet-bearing) disk PDS 70. As first noted in past work using Spitzer and ground-based facilities (e.g., Salyk et al. 2015), the presence of large disk gaps indicates depletion of the grains needed to shield molecules from photodissociation; therefore, gapped disks can show extreme reductions in molecular emission, even if gas is present in the inner disk. However, using the incredible sensitivity of JWST, Perotti et al. (2023) were able to detect water line luminosities in PDS 70 nearly two orders of magnitude below that seen in typical protoplanetary disks, and estimate water abundances in this planet-bearing disk. The impact of a disk gap on inner disk chemical signatures, however, is highly dependent on the exact properties of the gap (Salyk et al. 2015). As an example, Schwarz et al. (2024) show that the SY Cha disk, which has a mm-wave cavity but a full disk SED indicating the presence of micron-sized grains, has a more typical molecular emission spectrum.
Banzatti et al. (2023) report results from a program specifically designed to study the possible effects of radial transport. Comparing water vapor emission spectra from four disks, two large (indicating restricted radial transport) and two compact (indicating efficient radial transport), they find stronger water vapor emission in the compact disks. With the excellent spectral resolution of MIRI-MRS, however, they are further able to attribute the strength differences to the presence (or absence) of a cool water component. This component could be evidence for the inward transport and subsequent sublimation of water vapor at or near the snowline.
3.3 The (real) time axis
Although current programs are not focused on variability, JWST is already finding some variability in molecular emission. While continuum variability was detected commonly with Spitzer, especially in gapped disks (e.g., Espaillat et al. 2011), molecular line variability has typically only been detected in unusually active systems (e.g., Banzatti et al. 2015). Muñoz-Romero et al. (2024) find significantly weaker water vapor emission in the disk around AS 209, as compared to Spitzer observations, while Schwarz et al. (2024) find continuum variability and tentative line variability from the disk around SY Cha. The cause of the line variability is not yet known.
3.4 The stellar mass axis
Spitzer revealed notable differences in observed chemistry between lower mass T Tauri disks and their higher mass Herbig Ae/Be counterparts (Pontoppidan et al. 2010), and also hinted at differences between solar-mass and very low mass stars (Pascucci et al. 2009, Pascucci et al. 2013). Thanks to JWST’s sensitivity, it is now possible to more deeply explore the lower stellar mass regime, to better understand how planets form and develop around such stars. First published JWST results for the low-mass (M4.75) star 2MASS-J16053215-1933159 (Tabone et al. 2023) were in line with past results, in that this disk showed a high ratio of C-bearing to O-bearing species. However, the disk has a dramatically rich spectrum of C-bearing species never seen in Spitzer spectra, including C2H2 (and its main isotopologue), C6H6, and possibly CH4, with such molecular emission covering wide swaths of the MIRI-MRS band. On the other hand, Xie et al. (2023), studying the M5 star Sz 114, found a more T Tauri-like emission spectrum with significant amounts of water, and moderate amounts of organic molecules. Pinning down the origin of these differences likely requires expanding our sample beyond two objects (which will be tackled by S. Grant’s cycle 2 program 3886); nevertheless, there may be a complex interplay between differences in chemistry due to the lowered stellar mass (which may, for example, speed up radial migration; Pinilla et al. 2013), and differences due to the presence or absence of gaps (Xie et al. 2023).
4. Conclusions
In short, JWST is revolutionizing our ability to study chemistry in protoplanetary disks, and therefore to better understand the diverse pathways of planet formation. So far, JWST has clearly revealed that the chemical environments in which planets form are diverse. However, the origins of this diversity are still being elucidated, and may reflect different radial migration histories, different radiation environments, different stellar masses, or some combination of effects. The sensitivity, as well as spectral and spatial resolution provided by JWST spectroscopy, are allowing us to fully explore the multiple axes of planet formation for the first time. The work has just begun, but it is clear that there will be many lessons learned.
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