New Window into Planet Formation with Webb‘s MIRI

Christine Chen, cchen@stsci.edu, & Klaus Pontoppidan, pontoppi@stsci.edu

With its exquisite sensitivity, high angular resolution, and moderate spectral resolution, the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope will open new windows into many facets of the universe, including the processes by which planetary systems form and evolve. To advance this area of science, MIRI will make unique observations of two types of circumstellar disks—protoplanetary disks around young stars, where planets maybe forming, and debris disks around more mature stars, where the debris is produced by the collisions between small planetary bodies. For example, MIRI will constrain the location and abundance of water in protoplanetary disks, and will spatially resolve thermal emission from dust in debris disks—in both cases around stars with a wide range of masses, from brown dwarfs to solar-mass stars.

Volatiles in protoplanetary disks

Volatile molecules—e.g., H2O, CO2, CH4, and HCN—play a central role in the formation of planetary systems. This is reflected in the structure and composition of the Solar System, and theory predicts that volatiles are equally important for the formation of exoplanetary systems. Out beyond the snow line—the minimum distance from a star at which water condenses, typically 1–5 AU—volatiles dominate the solid mass in protoplanetary disks, and therefore drive the formation of planetesimals (e.g., Johansen et al. 2009) and planets that are formed through core accretion processes (Dodson-Robinson et al. 2009). Also, the evolution of volatiles dictates the availability of ingredients for life–on the Earth, as well as on terrestrial planets and moon systems around other stars (Raymond et al. 2004).

MIRI will permit detailed studies of bulk volatiles (species that dominate the condensable mass) near snow lines in the era when planetesimals form in gas-rich protoplanetary disks. This research will be accomplished by mid-infrared spectroscopy of the molecular “emission forest” that characterizes typical planet-forming disks around young (~1–5 Myr) stars. Recent Herschel observations find far-infrared lines are generally absent or weak, demonstrating extremely low abundances of water beyond a few AU in typical disks (e.g., Hogerheijde et al. 2011). Combining the Herschel results with mid-infrared observations of the molecular forest can measure the location of the snow line (Zhang et al. 2013).

As illustrated in Figure 1, the InfraRed Spectrograph (IRS) on Spitzer acted as a critical trailblazer for MIRI. Spitzer made the first mid-infrared detections of the molecular forest in protoplanetary disks, and developed the demographics of molecular gas in planet-forming regions (prevalence and chemistry, as well as dependencies on the stellar mass and evolutionary stage; Carr & Najita 2008; Salyk et al. 2008; Pontoppidan et al. 2010; Carr et al. 2011). These initial Spitzer results have raised a long list of questions that can only be answered by Webb. For instance, the physical parameters of disk molecules—e.g., abundance, temperature, and distribution—are uncertain due to the IRS’s relatively low spectral resolving power (Salyk et al. 2011). At R~600, molecular lines are highly blended, making it very difficult to measure individual lines. For water, this defect is critical since the blends typically include and combine transitions from widely separated energy levels. The higher resolving power of MIRI (R~3000) permits the separation of most water lines, and will enable detailed measurements of molecular abundance and spatial distribution (Pontoppidan et al. 2009).

Further, Spitzer was sensitivity-limited to disks around stars more massive than ~0.5 MSun. Conversely, MIRI can observe molecular emission from disks spanning the entire stellar and substellar mass range within several kpc. Further, while Spitzer observed essentially all of the ~100 disks that were bright enough for this type of study, MIRI will be able to select from a diverse potential sample of up to 100,000 protoplanetary disks. This permits a careful construction of robust, unbiased statistical studies, which may turn out to be critical, as the total number of targets that can be observed one-by-one with Webb in a reasonable time is limited.

Finally, Spitzer did not detect water in disks around early-type stars (spectral types F–B). One possible explanation is that the bright continuum of these disks masked the very under-resolved lines in the Spitzer spectra, making the line-to-continuum ratio too small for detection. Simulations show that the higher spectral resolution of MIRI may reveal water emission around these objects by increasing the dynamic range of line sensitivity above the bright disk continuum by at least an order of magnitude.