MIRI Mid-Infrared Disk Survey (MINDS)

• MINDS is a comprehensive survey that maps the molecular inventory and chemical diversity in the inner regions of protoplanetary and debris disks.
• Utilizing JWST's MIRI/MRS and NIRSpec IFU, the survey achieves high-resolution, spatially-resolved spectroscopy to measure key molecular lines and disk structures.
• The findings link volatile chemistry with dynamic processes such as disk winds and dust traps, offering vital insights into planet formation environments.

The MIRI Mid-Infrared Disk Survey (MINDS) is a large-scale Guaranteed Time program utilizing the unprecedented sensitivity and resolution of the James Webb Space Telescope (JWST) to conduct a comprehensive exploration of the physical and chemical structure of protoplanetary and early debris disks. By targeting a diverse sample of 52 disks—spanning Herbig Ae/Be, T Tauri, very low-mass stars (VLMS), and brown dwarfs—MINDS leverages the full capabilities of the Mid-Infrared Instrument’s Medium Resolution Spectrometer (MIRI/MRS), additional NIRSpec IFU spectroscopy, and high-contrast imaging to address fundamental questions of planet formation. The survey is uniquely positioned to probe the terrestrial planet-forming regions (within a few au) by resolving the key molecular and dust features required to link natal disk chemistry and structure to the outcome of planetary systems.

1. Survey Design, Scientific Objectives, and Target Selection

MINDS is structured around three synergistic scientific aims:

1. Chemical Inventory of the Inner Disk: The survey’s primary motivation is to construct a complete inventory of the molecular content (e.g., H₂O, CO₂, HCN, C₂H₂, larger hydrocarbons, and their isotopologues) of the terrestrial planet-forming zones. The molecular emission lines sampled in the 5–28 μm range with $R \sim$ 1500–3500 spectral resolution directly trace gas at temperatures and radii relevant for terrestrial planet assembly.
2. Gas Evolution and Disk Dispersal: By following key atomic and molecular fine-structure lines (e.g., [Ne II], [OI], H I recombination lines), MINDS systematically investigates processes such as photoevaporative dispersal, magnetohydrodynamic winds, and accretion-driven evolution in both full and depleted disks.
3. Mid-Infrared Structure and Spatial Diagnostics: Through IFU spectroscopy and high-contrast imaging (with MIRI coronagraphy and NIRCam), spatial structures—rings, gaps, cavities, spiral arms, and planet-induced asymmetries—are mapped, establishing the direct links between dynamical processes and disk chemistry in settings ranging from transition disks (with large dust cavities) to robust planet-forming disks like PDS 70.

The target sample covers a broad parameter space in stellar mass (Herbig Ae/Be to BDs), disk mass, evolutionary stage, and star-forming environment. Many sources are drawn from regions with existing ALMA continuum and molecular line data, facilitating multi-wavelength synthesis (Henning et al., 14 Mar 2024).

2. Observational Methodology and Spectroscopic Modeling

MINDS relies on JWST-MIRI/MRS’s integral field unit, providing simultaneous spectroscopy and spatial mapping at high signal-to-noise ($\mathrm{S/N}\sim100$–500). For select sources, complementary NIRSpec IFU data (2.87–5.27 μm) are used to cover ro-vibrational CO and H₂ lines. Advanced data reduction (including specialized routines for aperture extraction, PSF subtraction, and continuum normalization) and cross-dataset consistency checks are routine.

Spectral interpretation is based on 0D slab models under local thermodynamic equilibrium (LTE). For an emitting slab, the total molecular emission is characterized by the column density ($N$), excitation temperature ($T$), and emitting area ($A=\pi R^2$). The flux for optically thick slab emission is modeled as: $$F(\lambda) = \pi \left(\frac{R}{d}\right)^2 B_\nu(T) [1-\exp(-\tau(\lambda))]$$ Line profile fits and line-to-continuum ratios allow temperature gradients, chemical stratification, and spatial origin of emission to be inferred (Grant et al., 2022, Gasman et al., 2023, Temmink et al., 6 Jul 2024).

Key diagnostics include:

• Model fitting of Q-, P-, and R-branches for temperature and column density constraints, especially for CO₂, H₂O, and C₂H₂ (including isotopologues such as $^{13}$CO₂, $^{13}$CCH₂).
• Rotational diagrams for H₂ (and occasionally OH) to measure rotational temperatures, column densities, and total warm/cold gas mass (Franceschi et al., 18 Apr 2024, Schwarz et al., 17 Sep 2024).
• Flux ratios (e.g., $N_{\mathrm{CO}_2}/N_{\mathrm{H}_2O}$, $F_{^{13}\mathrm{CCH}_2}/F_{\mathrm{C}_2H_2}$) as proxies for abundance and physical conditions (Arabhavi et al., 3 Jun 2025).

3. Principal Findings: Diversity of Inner Disk Chemistry

MINDS reveals an extraordinary chemical diversity in the inner few au of disks:

• T Tauri disks typically show strong water emission with column densities $N(\mathrm{H}_2\mathrm{O})\sim10^{18}$–$10^{21}$ cm$^{-2}$, high enough for self-shielding. In several disks, $N(\mathrm{CO}_2)/N(\mathrm{H}_2O)\sim0.7$ is found—over two orders of magnitude above the canonical value measured with Spitzer, indicating localized enhancement, often attributed to unique disk structure or dynamical conditions (Grant et al., 2022, Dishoeck et al., 10 Dec 2024).
• The detection of $^{13}$CO₂ and at times CO$^{18}$O, optically thin isotopologues, enables accurate measurement of the total CO₂ column and reveals temperature gradients (e.g., $T_{12\mathrm{CO}_2}\sim$ 400 K, $T_{13\mathrm{CO}_2}\sim$ 325 K in GW Lup).
• Disks around VLMS ($M_*\lesssim0.3\,M_\odot$) and brown dwarfs frequently feature inner gas with C/O$>$1, signaled by strong C₂H₂, benzene (C₆H₆), and other hydrocarbons, and by weak-to-absent water. For example, (Kanwar et al., 19 Jul 2024, Arabhavi et al., 15 Apr 2025), and (Arabhavi et al., 3 Jun 2025) demonstrate that hydrocarbon-rich ("carbon-dominated") disks can coexist with significant, albeit veiled, water vapor—sometimes only revealed by stacking or cross-correlation analysis.
• Spatial and temperature gradients in water emission are routine. Multi-component models reveal hot ($T\gtrsim800$ K, $R<1$ au), intermediate ($T\approx400$–600 K), and cold ($T\approx200$ K, $R$ up to several au) water reservoirs (Temmink et al., 6 Jul 2024, Temmink et al., 21 May 2025). The emission is often consistent with a radial temperature gradient $T(R)\sim R^{-0.5}$ in disk surface layers.
• Disk winds and spatially resolved jets are identified via extended H₂ and forbidden line emission (Schwarz et al., 17 Sep 2024, Kurtovic et al., 4 Aug 2025), enabling direct estimates of wind mass loss rates ($\sim3\times10^{-9}\,M_\odot\,\text{yr}^{-1}$ in SY Cha) that can exceed or rival the measured accretion rates.

4. Interpretation: Disk Evolutionary State, Dynamics, and Chemical Segregation

MINDS findings are contextualized by dynamic disk evolution models:

• Radial drift of icy pebbles is a key process governing inner disk chemistry. Compact disks with strong radial drift can show enhanced cold ($T\sim200$ K) water reservoirs (Type E behavior) or, in some cases, a "CO₂-rich" spectrum if the water ice reservoir has already been exhausted via accretion or masked by dust (Vlasblom et al., 17 Dec 2024, Temmink et al., 21 May 2025).
• Dust traps and substructures modulate pebble flow, setting the supply of volatiles to the inner disk (Dishoeck et al., 10 Dec 2024). ALMA imaging is essential to constrain dust and gas morphology.
• Epoch-to-epoch variability is seen in both line and continuum emission, particularly in primaries of binary systems (Kurtovic et al., 4 Aug 2025). This is attributed to variable accretion, rapid disk heating, or evolving inner disk structure after dynamical truncation.
• Chemical stratification is often observed: optically thick main isotopologue emission (e.g., $^{12}$CO₂, C₂H₂) traces warmer surface layers, while optically thin isotopologues ($^{13}$CO₂, $^{13}$CCH₂, CO$^{18}$O) probe deeper, cooler disk regions, thus sampling vertical gradients in both temperature and composition.
• The appearance of carbon-dominated spectra in VLMS and BD disks is explained by high C/O ratios, rapid solid transport, grain growth, and a combination of chemical evolution and vertical/radial mixing. The presence of water, even in carbon-rich disks, implies that the hiding of emission lines due to strong hydrocarbon pseudo-continuum or line blending is common, rather than a true depletion of water (Arabhavi et al., 15 Apr 2025, Arabhavi et al., 3 Jun 2025, Morales-Calderón et al., 7 Aug 2025).

5. Comparison with Previous Surveys and Technical Advances

JWST/MIRI/MRS provides transformational advances over Spitzer/IRS:

• Spectral resolution (R ≈ 1500–3500 vs. R ≈ 600) enables the separation of blended features and the identification of minor and isotopic species previously inaccessible.
• High $\mathrm{S/N}$ ratios permit the detection of weak lines (e.g., $^{13}$CO₂) and improve measurement precision for temperature and column density, especially in heavily blended or optically thick regimes (Grant et al., 2022, Henning et al., 14 Mar 2024).
• Integral field spectroscopy and coronagraphy allow for spatial dissection of both continuum and line emission—essential for mapping structures such as spiral arms, gaps, and jet/wind emission (Henning et al., 14 Mar 2024, Christiaens et al., 7 Mar 2024, Kurtovic et al., 4 Aug 2025).
• The combination of MIRI and NIRCam imaging (e.g., in the paper of PDS 70) and the coordination with ALMA datasets provide an integrated, multi-scale understanding of disk structure and evolution (Christiaens et al., 7 Mar 2024).

6. Implications for Planet Formation and Future Directions

MINDS results have direct implications for planet formation theory:

• The observed chemical diversity in inner disk regions establishes widely varying initial conditions for planet assembly, particularly with respect to volatile inventory (H₂O, CO₂, organics) and the C/O ratio, which are crucial for exoplanet composition models (Dishoeck et al., 10 Dec 2024).
• The radial and vertical segregation of chemical species implies that the bulk and atmospheric compositions of planets formed in different disk CNS zones may vary strongly within and between systems.
• The detection of jets, disk winds, and turbulence not only connects the chemical and dynamical evolution but also determines the timescales and efficiency of disk dispersal, directly affecting planetary system architectures.
• Continued spectro-photometric monitoring, higher spatial resolution observations, and deep/multi-epoch campaigns are vital for characterizing variability and transient chemical phenomena, especially in complex systems (binaries or disks with strong dynamical interactions).
• A systematic extension of the MINDS modeling frameworks—including multi-component and non-LTE line radiative transfer, isotope-selective chemistry, and linkage to evolving disk dynamics—will be critical for extracting quantitative volatile budgets and constraining the inheritance of disk chemistry in exoplanets.

In total, MINDS sets the benchmark for physical and chemical characterization of planet-forming disks, providing the foundation for a comparative exoplanetology anchored in the resolved chemistry of protoplanetary environments.