How to Analyze the Surface Composition of a Rocky Exoplanet with JWST

Introduction

Understanding the surface composition of rocky exoplanets is a critical step toward characterizing their geology and potential habitability. In a landmark study, astronomers used the Mid-Infrared Instrument (MIRI) aboard the James Webb Space Telescope (JWST) to analyze the surface of LHS 3844 b, a nearby super-Earth. This guide walks you through the step-by-step process those researchers followed, from selecting the target to interpreting the final data. Whether you're a graduate student planning an observing proposal or a curious enthusiast, these steps outline the essential methodology.

What You Need

• Access to JWST – A successfully submitted and approved observing proposal for MIRI time.
• Target exoplanet – A rocky super-Earth with known orbital parameters (e.g., LHS 3844 b) that is bright enough in the mid-infrared.
• Knowledge of the planet's orbit – Precise ephemerides to schedule observations during secondary eclipse (when the planet passes behind its star).
• Data reduction pipeline – The official JWST pipeline or a custom Python-based tool (e.g., using jwst package and astropy).
• Atmospheric or surface model fitting software – For example, pysynphot, ExoTransmit, or PALEO to simulate and fit spectra.
• High-performance computing – For processing large MIRI data cubes and running Monte Carlo simulations.

Step-by-Step Guide

Step 1: Select and Characterize the Target

Begin by identifying a rocky exoplanet suitable for mid-infrared spectroscopy. LHS 3844 b meets the criteria: it is a super-Earth (about 1.3 times Earth's radius) orbiting an M-dwarf star, with a short orbital period (11 hours). The planet’s dayside temperature (~1000 K) ensures strong thermal emission at MIRI wavelengths (5–28 μm). Verify that the star is bright enough at these wavelengths (J magnitude ~11) to achieve a high signal-to-noise ratio in a single secondary eclipse observation.

Step 2: Propose and Schedule JWST Observations

Submit a proposal through the JWST proposal system, detailing the scientific objectives (measuring surface composition of a bare rocky planet). Once approved, schedule observations using the MIRI Low-Resolution Spectrometer (LRS) slitless mode, which covers 5–12 μm at R~100. Choose a time when the planet is in secondary eclipse (i.e., behind its host star) to isolate the planet's emission from the star's. The exact timing requires high-precision ephemerides, often obtained from previous TESS or ground-based transit measurements.

Step 3: Acquire and Reduce Raw Data

The MIRI instrument produces 3D data cubes (two spatial dimensions plus wavelength). Use the standard JWST pipeline (version 1.10.0 or later) to:

1. Correct for detector non-linearity and saturation using the calwebb_detector1 step.
2. Apply dark subtraction, flat-fielding, and cosmic ray rejection.
3. Extract 1D spectra from the 2D slitless frames using the calwebb_spec2 pipeline (or a custom extraction routine).
4. Perform background subtraction by fitting a polynomial to regions away from the target spectrum.

For LHS 3844 b, the team extracted spectra from both in-eclipse and out-of-eclipse images to compute the planet-to-star flux ratio. See tips below for handling fringing in MIRI data.

Step 4: Compute the Eclipse Depth Spectrum

For each wavelength channel, divide the spectrum obtained during secondary eclipse by the spectrum of the star (observed just before or after eclipse). This yields the relative flux, which directly corresponds to the planet's dayside emission. Correct for any systematic trends (e.g., ramp effects) using a linear or exponential model fitted to the out-of-eclipse baseline. The resulting eclipse depth spectrum shows how the planet's brightness varies with wavelength – a key diagnostic of surface composition.

Step 5: Model the Surface Emission

Interpret the measured spectrum using theoretical models of bare, rocky surfaces. The researchers considered several mineral compositions (basalt, granite, carbonates, etc.) and calculated their emissivity spectra. For each model:

1. Assume a surface temperature (derived from the planet's equilibrium temperature and any heat redistribution).
2. Convolve the model emissivity with the instrument's spectral response.
3. Compare the model eclipse depth to the observed spectrum using a chi-squared or Bayesian fitting routine.

For LHS 3844 b, the best fit pointed to a basalt-like composition (mafic rock) with no significant atmosphere. The flat spectrum from 5 to 12 μm ruled out thick clouds or strong molecular absorption.

Step 6: Validate and Interpret Results

Check robustness by performing injection-recovery tests (adding synthetic signals to the data) and testing alternative models (e.g., including a thin atmosphere). Calculate uncertainties using a Monte Carlo bootstrap. Finally, publish the findings – the LHS 3844 b study demonstrated that JWST can distinguish between different rocky compositions, opening a new frontier in exoplanetary geology.

Tips for Success

• Mitigate fringing: MIRI LRS data often exhibit periodic variations across the detector. Use the pipeline fringe correction or fit a sinusoidal baseline to remove these artifacts.
• Monitor systematic trends: JWST's ramp effect (increasing count rate with time) can bias eclipse depth measurements. Include a time-dependent quadratic term in your light-curve model.
• Combine with transit spectroscopy: If the planet has an atmosphere, simultaneous MIRI observations of primary transit can provide complementary constraints on gas composition.
• Use open-source tools: The JWST data analysis tools and ExoNexus simplify reduction and modeling. Many teams share their code for reproducibility (e.g., the Zieba & Kreidberg 2023 pipeline).
• Plan for calibration: Observe a standard star (e.g., an A0V star) with the same instrument setup to verify absolute flux calibration.
• Check for phase effects: For tidally locked planets, dayside emission may vary with orbital phase. If possible, obtain multiple eclipses to test for temporal changes.

Following these steps, astronomers successfully determined that LHS 3844 b has a dark, basalt-like surface – a first for a super-Earth-sized exoplanet. With JWST's sensitivity, similar analyses are now possible for dozens of other rocky worlds.