Abstract
The formation and properties of our inner Solar System bodies have long been proposed to have required the delivery of icy solids from the cold regions away from the Sun, regions where comets originate from today. Recent observations of protoplanetary disks around other stars, the young analogs of the “Solar Nebula” that formed our Solar System, have confirmed that icy solids can either be retained in systems of rings at large distances from the star, or be efficiently delivered into the inner region where rocky planets are forming. Together with the solid mass necessary to build up planetesimals and planet cores, the drifting solids carry water ice into the inner warm region of disks, which will sublimate and produce large amounts of water vapor. While this fundamental dynamical process is included in theories of planet formation, its confirmation has been elusive for decades due to the extreme angular resolution needed to observe planet-forming regions around other stars. With the deployment of the James Webb Space Telescope (JWST), the long-awaited test has arrived. By providing an unprecedented sharp view of water vapor spectra, JWST has revealed that disks expected to have a large flux of icy migrators do indeed have excess cold water vapor near the ice sublimation front. Observing this fundamental ice migration process has exciting implications for using JWST to estimate the mass in solids and water that is delivered to the disk region where rocky planets are forming and clarify the origins of different types of planets and their potential habitability.
1. A fundamental problem: how planets get their essential ingredients
Planets are formed in disks of dust and gas that rotate around young stars. These circumstellar or “proto-planetary” disks are feeding mass to their central stars until nuclear fusion is ignited in their core and they begin their mature life on the Main Sequence, the current phase of our Sun. By then, the natal disk is gone and a new planetary system is formed. While this general picture is now accepted and supported by multiple lines of evidence, some of its fundamental steps still challenge current planet formation theories. Two such fundamental problems are: 1) how planets build up their solid mass, or how solids grow hugely in size from interstellar dust grains up to planetary cores and rocky planets, and 2) how planets obtain their water to possibly host liquid oceans and life.
These questions have first and for long been analyzed in the context of the formation of our Solar System. A global temperature gradient must have been present in the “Solar Nebula” (the protoplanetary disk of dust and gas that formed our Solar System) with a hotter inner region and increasingly colder outer regions (Figure 1). With this structure, the inner region is populated with dust particles and gas, while at larger distances the disk is cold enough to be populated with ice. The separation between these two global regions has been called the “snowline”. The water snowline has long been proposed to have had a fundamental role in shaping the structure of our Solar System, because by being made of two of the most abundant elements (hydrogen and oxygen) water is very abundant and provides a large fraction of solid (ice) mass available to planets.
One fundamental process proposed early on in studies of the Solar System is that solid particles in disks should migrate efficiently towards the star due to gas drag forces [42]. This global process would cause an “ice drift” effect (Figure 1) bringing icy solid particles from the outer disk into the snowline region and within it. After crossing the snowline, ice sublimates in the inner hotter region producing an increase in water vapor abundance, which would in turn partly diffuse outward. This global process could solve two problems at the same time: 1) deliver enough solid mass at ∼ 5 au to explain the rapid formation of Jupiter’s core, and 2) radially mix the ice and vapor content across the snowline to explain chemical gradients measured in asteroids as a function of their distance from the Sun [28, 38, 16, 14].
Yet, the solution came with a new critical problem: the drift of icy solids, later called “pebbles”, that have the right size to be marginally decoupled from the gas (i.e. not too small or they are completely coupled, not too large or they are completely decoupled) should be so fast to deplete the inner disk in as short as 100 years [42]. The particle size that maximizes drift efficiency depends on the gas density and is about 1 meter at a distance of 1 astronomical unit (and 1 millimeter at a distance of 100 astronomical units), so that this problem came to be known as the “meter-size barrier”: no solids larger than 1 meter should have been formed in the Solar System, and protoplanetary disks should disappear before planets may form in them. However, the existence of planets demonstrates the opposite.
New observations of protoplanetary disks from modern millimeter interferometers, which observe the cold thermal emission from pebbles and reveal their distribution as a function of distance from the star, later came to help in finding a solution to overcome the barrier problem. They demonstrated that another process must be slowing down the drift, retaining pebbles in the disk for a longer time, and even accumulating them in specific regions where the growth of solids towards planets would become more efficient. This process was proposed to be the formation of local pressure enhancements, or “bumps”, that would trap pebbles in systems of rings preventing their rapid drift into the star [24, 32]. While being only a theoretical prediction at first, new sharp images from the ALMA interferometer in 2015 and onward demonstrated its reality: protoplanetary disk can indeed be carved by systems of rings where pebbles are trapped (Figure 2) [1, 18, 27, 2].
The radial distribution of pebbles observed in disks recently became a proxy for the relative efficiency of the drift process in different disks (Figure 2): some disks are observed to retain pebbles in systems of rings out to 100-200 au, while other disks are very compact and are proposed to have had a very efficient inward drift delivering a large mass in icy pebbles to the inner planet forming region [e.g. 35, 3, 44]. The radial drift and accumulation of pebbles are now considered fundamental processes that shape the mass architecture and chemical composition of planetary systems [e.g. 19, 10, 11, 15] and determine which type of planetary system would form in a given disk, whether composed of super-Earths (in disks with efficient pebble drift delivering a larger solid mass to within the snowline) or of small terrestrial planets (in disks where pebble traps retain solid mass in the outer disk) [25, 40].
Despite how fundamental the process of ice drift in disks is, both from the physical point of view as well as for its implications for planet formation, its confirmation has long eluded direct observations due to the extreme resolution needed to observe planet-forming regions around other stars. An observable consequence of ice drift, if truly happening in disks, was however already known from previous Solar System studies, as described above: the rapid drift of large amounts of icy pebbles should produce an enrichment in water vapor in the region within the snowline (Figure 2) [22, 21].
2. A long-awaited answer: JWST reveals excess water emission near the snowline
Spectroscopy of water and molecular emission from planet-forming regions has become possible only about 20 years ago, with observations from the NASA Spitzer Space Telescope [12, 36]. The first analyses of these spectra suggested that water and organic molecules are abundant in planet-forming regions, but our understanding of the distribution, temperature, and density of molecular gas has been critically limited by the low resolution of Spitzer instruments, which was not optimized for this type of observations [34, 13, 37, 31, 9]. Yet, these spectra provided first hints that the drift of icy pebbles might indeed deliver large amounts of water to the rocky planets region in disks [4, 30, 6]. Even in this case, the interpretation of results was severely limited by the low resolution of Spitzer spectra that blended emission lines together, providing a “blurred” vision of their properties (Figure 3).
With the deployment of JWST, the view of the molecular environments of planet formation has significantly sharpened. Figure 3 demonstrates how the factor ∼4 increase in resolving power (R = λ/∆λ, where ∆λ is the spectral detail that can be distinguished in the spectrum) results in the effective de-blending of emission lines from different molecules that overlap in the spectra. The possibility to separate different lines and measure their individual strength provides a “thermometer” that is most essential to researchers in all fields of astronomy: the population of different energy levels, which produce the different emission lines observed in the spectrum, is indeed dictated by the temperature of the gas in addition to other properties (including the gas density and the irradiation spectrum).
Equipped with this sensitive thermometer, JWST spectra observed from planet-forming regions have been used to test the fundamental predictions described in Section 1: that drift-dominated, compact protoplanetary disks should have more water vapor than the extended disks, which instead trap icy pebbles in systems of rings and prevent them from drifting to the snowline and feeding rocky planet formation. A mixed sample of compact and extended disks was observed in the first cycle of observations with JWST in February 2023, providing spectra of astounding quality thanks to improved data calibration techniques [33]. The comparison of their water spectra revealed a strong difference between the two types of disks: while both had similar emission in the higher-energy lines emitting from the inner, hotter region of the disk, the compact disks have excess emission in the lower-energy lines emitting from colder disk regions that extend to the water snowline ([7] and Figure 4). A fit to the excess emission provided temperatures of 400 K extending down to 170 K, matching the sublimation temperature of water ice in inner disks. The snowline region in compact disks indeed seems to be enriched in cold water vapor supplied by sublimation of icy pebbles that have drifted inward (Fig. 5).
3. Conclusions and future prospects
The discovery of the cold-water excess in drift-dominated disks [7] is a major achievement that comes directly from the increased resolving power, sensitivity, and data quality of JWST. The picture emerging from JWST spectra of molecules in planet-forming regions now supports the long-proposed, fundamental process describing the dynamics and re-distribution of solids and water in protoplanetary disks first proposed for the Solar Nebula and later applied as a standard for planet formation theories in general (Section 1). The implications from using JWST to study such a fundamental process are multiple, from aiding the analysis of other processes that impact the chemistry of planet-forming regions to providing essential input to planet formation models.
From the point of view of the global chemistry in planet-forming regions, the analysis of water and organic molecules from the past 20 years has already demonstrated that the problem is multi-dimensional by nature. Multiple factors determine which molecules will be formed and destroyed as a function of disk radius and time: irradiation from stars of different temperature and the high-energy radiation from shocks where disk gas is feeding the central star, the evolution of dust grains that are shielding molecules from dissociation from high-energy radiation, the relative abundance of molecules delivered as ice from the outer disk which can be stopped by traps at different radii [e.g. 37, 31, 41, 43, 9, 5, 8]. This multiplicity of factors is emerging from the first JWST observations too [e.g. 39, 17, 23], and larger samples of disks are now being observed to explore some of them, including stellar mass and irradiation, age, dust evolution, and environment (see papers by C. Salyk and I. Kamp at this conference).
From the point of view of planet formation models, the solid mass delivered to inner disks through icy pebble drift is proposed to be a critical factor determining which type of planetary system will form [25]. The possibility to now extract ice mass estimates from JWST spectra of the cold water vapor emission from disks is supporting the scenario that drift-dominated disks may deliver a few 100s Earth masses of solids into the rocky planet region [29], which would support the formation of systems of super-Earths; on the contrary, disks with multiple pebble traps provide estimates of only a few 10s Earth masses, which would only sustain the formation of smaller terrestrial planets [25]. Determining which disks may form which types of planetary systems has the potential to significantly advance our understanding of planet formation in general, the origins of different planet types (including their water content and potential for habitability), and the place of our Solar System in the broader context of exoplanet populations in the galaxy.
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