The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST

Brief

Using PandExo v1.1.2 and photochemically self-consistent atmospheric models from Lincowski et al. (2018), the authors simulate transmission and emission spectroscopy across all JWST bright-object time-series modes and all MIRI photometry filters for all seven TRAPPIST-1 planets. NIRSpec Prism (0.6–5.3 μm, R~100) emerges as the optimal instrument for detecting high mean molecular weight atmospheres, reaching ⟨SNR⟩ ≥ 5 on CO₂ features in fewer than 10 transits under clear-sky conditions, a number that inflates by up to 12× in the presence of Venus-like H₂SO₄ aerosols. Water vapor is flagged as prohibitively difficult to detect because it condenses deep in the atmosphere where transmission spectra lose sensitivity, and biogenic O₂ and O₃ are out of reach, though abiotically produced O₂ from ocean-loss scenarios may be accessible via O₂–O₂ CIA at 1.06 and 1.27 μm. The paper served as the pre-launch observational roadmap that structured actual JWST TRAPPIST-1 programs.

Metadata

Category

Search

Venue

The Astronomical Journal

Type

Peer-reviewed

Year

2019

Authors

Jacob Lustig-Yaeger, Victoria S. Meadows, Andrew P. Lincowski

DOI

10.3847/1538-3881/ab21e0

arXiv

1905.07070

Access

Open access

Length

3.0 M

Programs

NASA NExSS Virtual Planetary Laboratory

Instruments

JWST NIRSpec Prism, JWST NIRSpec G140H, JWST NIRSpec G140M, JWST NIRSpec G235H, JWST NIRSpec G235M, JWST NIRSpec G395H, JWST NIRSpec G395M, JWST NIRCam GRISM, JWST NIRISS SOSS, JWST MIRI LRS, JWST MIRI Imager, HST WFC3, Spitzer, Kepler

Data sources

PandExo v1.1.2, Pandeia v1.2.2, PHOENIX stellar models (Husser et al. 2013), Lincowski et al. 2018 photochemical-climate atmospheric models, SMART radiative transfer code (Meadows & Crisp 1996)

Tags

biosignature, astrobiology, exoplanet-atmosphere, TRAPPIST-1, JWST, atmospheric-characterization, habitability, false-positive-biosignature

Key points

• NIRSpec Prism transmission spectroscopy can detect CO₂-bearing atmospheres in fewer than 10 transits for all seven TRAPPIST-1 planets under clear-sky conditions, at a ⟨SNR⟩ ≥ 5 detection threshold.p.1
• Venus-like H₂SO₄ aerosol layers can require up to 12× more transits to detect an atmosphere compared to the aerosol-free baseline.p.1
• Water vapor is prohibitively difficult to detect in both Venus-like and habitable atmospheres because it is confined to the lower atmosphere where transmission spectra are least sensitive.p.1
• Biogenic O₂ and O₃ are extremely challenging to detect; abiotically produced O₂ from ocean loss may be accessible via O₂–O₂ collisionally induced absorption at 1.06 and 1.27 μm, or NIR O₃ features for the outer three planets.p.1
• A partially saturated NIRSpec Prism mode ('NIRSpec Prism*') improves observing duty cycle from 33.3% to 71.4% for the SUB512 subarray by increasing groups per integration from 2 to 6, accepting 47 saturated pixels at ramp end.p.4
• MIRI saturates on TRAPPIST-1 in the two shortest-wavelength filters (F560W and F770W) at minimum standard imaging exposure times.p.4
• Single-reflection quartz silicate emission features at ~8–10 μm and ~19–23 μm rival atmospheric CO₂ and O₃ features in emission contrast, creating a potential false-positive pathway for airless-body misidentification.p.7
• The study tested 15 distinct JWST instrument-mode-disperser-filter configurations spanning NIRCam, NIRISS, NIRSpec (six gratings plus prism), and MIRI LRS across 0.6–12 μm, covering all bright-object time-series spectroscopy modes.p.5

Verbatim

• “We find that transmission spectroscopy with NIRSpec Prism is optimal for detecting terrestrial, CO 2 containing atmospheres, potentially in fewer than 10 transits for all seven TRAPPIST-1 planets, if they lack high altitude aerosols.”

  p.1

• “If the TRAPPIST-1 planets possess Venus-like H 2 SO 4 aerosols, up to 12 times more transits may be required to detect an atmosphere.”

  p.1

• “We find that water may be prohibitively difficult to detect in both Venus-like and habitable atmospheres due to its presence lower in the atmosphere where transmission spectra are less sensitive.”

  p.1

• “Although the presence of biogenic O 2 and O 3 will be extremely challenging to detect, abiotically produced oxygen from past ocean loss may be detectable for all seven TRAPPIST-1 planets via O 2 -O 2 collisionally-induced absorption at 1.06 and 1.27 μ m, or via NIR O 3 features for the outer three planets.”

  p.1

• “Our results constitute a suite of hypotheses on the nature and detectability of highly-evolved terrestrial exoplanet atmospheres that may be tested with JWST.”

  p.1

Most interesting

• TRAPPIST-1's modeled stellar parameters (T_eff = 2511 K, K-band mag = 10.30, radius = 0.121 R☉) make it cool enough that MIRI saturates in its two bluest photometry filters even at minimum exposure time, eliminating those channels from practical atmosphere searches.
• Abiotic O₂ generated by ocean loss could amount to tens to thousands of bars and mimic a biogenic biosignature; the paper identifies O₂–O₂ CIA detection as the discriminating observable to flag this false-positive scenario.
• Planet densities from transit timing variations (Grimm et al. 2018) range from 0.6 to 1.0 ρ⊕, consistent with significant ice fractions, implying the TRAPPIST-1 planets may have started with larger volatile reservoirs than Earth and only subsequently lost them.
• The ⟨SNR⟩ statistic used throughout is explicitly an upper bound on detection confidence; the authors note that retrieval model complexity and additional noise sources will push the required transit counts above the reported figures.
• A partial-saturation strategy for NIRSpec Prism (6 groups per integration instead of 2) more than doubles the duty cycle from 33.3% to 71.4%, meaningfully compressing the total observatory time needed per experiment.
• Quartz silicate emissivity features at 8–10 μm could in principle be mistaken for atmospheric absorption in MIRI secondary-eclipse spectra of airless rocky worlds, a false-positive risk the paper quantifies and concludes diminishes rapidly when multiple minerals and multiple surface reflections are considered.