Living Worlds Working Group

• Living Worlds Working Group is a cross-disciplinary team that integrates atmospheric science, photochemical modeling, and instrument design to guide exoplanet biosignature detection.
• It establishes consensus science cases and key measurement requirements for detecting biological gases such as O₂, O₃, CH₄, and H₂O in exoplanet atmospheres.
• The group addresses critical research gaps in laboratory astrochemistry, host star characterization, and false-positive discrimination to enhance the robustness of life detection missions.

The Living Worlds Working Group is a cross-disciplinary, mission-defining research team assembled under the Science, Technology, and Architecture Review Team (START) for NASA’s Habitable Worlds Observatory (HWO). Its mandate is to synthesize consensus atmospheric biosignatures science cases, establish measurement requirements, articulate stellar and planetary system context necessary for biosignature interpretation, and identify priorities for filling critical laboratory, theoretical, and observational gaps required for robust life detection in exoplanetary atmospheres (Parenteau et al., 10 Jan 2026).

1. Scope and Mandate of the Living Worlds Working Group

The group encompasses researchers with expertise in atmospheric chemistry, photochemical modeling, radiative transfer, stellar astrophysics, planetary geology, and biosignature instrumentation. The fundamental aim is to define how HWO, as an astrophysics flagship mission, can most effectively characterize potentially habitable rocky exoplanets in the solar neighborhood and search for spectroscopic evidence of biology. Core responsibilities include:

• Identifying atmospheric biosignatures accessible in the UV–Visible–NIR domain most plausibly explained by biology.
• Defining baseline instrument requirements for biosignature gas detection in reflected-light spectroscopy.
• Determining auxiliary measurements necessary to interpret biosignature signals, including system-level context for ruling out false positives and negatives.
• Developing a catalogue of host star properties essential for anticipating biosignature detectability and atmospheric evolution.

This group operates collaboratively with parallel astrobiology and exoplanet science working groups to ensure mission-level coordination across NASA’s Science Mission Directorate (Parenteau et al., 10 Jan 2026).

2. Atmospheric Biosignatures: Core Targets and Spectroscopic Features

The Living Worlds Working Group’s biosignature portfolio is defined by gases whose sustained abundance or production on temperate rocky exoplanets is best explained by biological cycling, along with key spectroscopic features in the 0.3–2.0 μm window:

• O₂: A-band at 0.76 μm; O₂–O₂ collision-induced absorption (CIA) at 0.63 μm, 1.06 μm.
• O₃: Hartley band ~0.25 μm (broad UV); Chappuis band 0.5–0.7 μm (visible).
• CH₄: Near-IR bands at 1.0–1.2 μm, 1.65 μm.
• CO₂ and H₂O: Habitability indicators—CO₂ at 1.6, 2.0 μm; H₂O at 0.94, 1.13, 1.4, 1.9 μm.
• Additional Trace Biosignatures: Methyl halides (CH₃Cl, CH₃Br), organosulfur gases ((CH₃)₂S, CH₃SH), phosphine (PH₃), isoprene, methylselenides, acetylene—where opacity databases and laboratory measurements require expansion.

Spectroscopic measurement requirements:

• Continuous wavelength coverage from 0.3–2.0 μm.
• Spectral resolution R ≈ 50–150 (R ≥ 70 to resolve O₂ A-band, R ≈ 100 for CH₄ at 1.65 μm).
• Photometric stability and planet–star contrast of ~10⁻¹⁰.
• Detector read-noise <1 e⁻ per pixel; throughput >20% in 0.4–1.0 μm, >10% to 2.0 μm.
• Wavefront error <10 pm RMS (coronagraphic requirements).

These targets are developed in close consultation with photochemical modelers, laboratory spectroscopy groups, and optical engineers to ensure instrumental feasibility (Parenteau et al., 10 Jan 2026).

3. False-Positive and False-Negative Discrimination Frameworks

A rigorous false-positive/false-negative framework underpins all biosignature science cases. Recognizing pathways for abiotic O₂/O₃ accumulation (e.g., water photolysis and H escape), volcanic/serpentinization-driven CH₄, and photochemically induced trace species, the Working Group stresses:

• Accurate reaction rate coefficients for biosignature gases interacting with key radicals (OH, NO₃, O(¹D)) in non-Earthlike backgrounds (N₂, CO₂, H₂, He, 150–400 K).
• Detailed CIA parameters for O₂–O₂, O₂–N₂, O₂–CO₂ at exoplanet-relevant temperatures and pressures.
• Host star UV (NUV/FUV) spectrum and variability to constrain photochemical rates, especially for O₂, O₃, and CH₄ buildup.
• Stellar elemental abundance analysis (Fe, Mg, Si, CHNOPS, trace metals) at ≤10% precision for planetary interior and volatile inventories.

Robust interpretation of atmospheric data requires these constraints to assess the likelihood of biogenic versus abiotic origin for any observed gas or suite of gases (Parenteau et al., 10 Jan 2026).

4. Host Star and Planetary System Context

Host star properties are critical to biosignature detectability and interpretation:

• NUV/FUV flux (λ < 0.4 μm) modulates key photochemistry for biosignature gases.
• Stellar activity metrics (rotation, X-ray luminosity, flare rate) inform atmospheric erosion, false-positive rates, and temporal variability of atmospheric composition.
• Stellar age and elemental abundances situate the planet's evolutionary stage and context for expected atmospheric redox states.
• Planetary mass, radius, density, and orbital configuration determine scale height, pressure–temperature profiles, and geophysical cycling.

The group recommends advanced, targeted pre-mission campaigns to fully characterize candidate host stars, including space- and ground-based UV surveys, spectroscopic abundance determinations, and gyrochronology for age estimation (Parenteau et al., 10 Jan 2026).

5. Priority Research Gaps and Recommended Follow-On Studies

The Living Worlds Working Group highlights five critical gap areas, each tied to specific recommended actions:

1. Laboratory and Computational Astrochemistry: Line lists, cross-sections, and broadening parameters for biosignature gases (O₂, CH₄, CH₃Cl, PH₃) in the 0.4–2.0 μm regime; kinetic measurements for key reactions over 150–400 K; establishment of a community-standard opacity database with dedicated funding.
2. Target Stars and Systems: Complete UV SEDs and abundance catalogues for all HWO target hosts; refined stellar ages and evolutionary tracks.
3. Surface Biosignatures: Measurement of pigment and mineral reflectance (including full-Stokes polarization) under varied surface and atmospheric conditions; compilation of abiotic reflectance spectra for false-positive elimination; integration with 3D GCM–biogeochemical models.
4. Planetary Multiplicity: Systematic surveys of multi-star systems (e.g., α Centauri) for HZ planet occurrence rates; disk dynamics and planet formation models unique to binary configurations.
5. Alternative Metabolisms and Exotic Biosignatures: Modeling of biospheres under diverse stellar spectra and geochemical cycling; extension of systems biology frameworks to anticipate novel biosignature gases and gas mixtures.

Addressing these gaps demands coordination across astronomy, planetary science, laboratory spectroscopy, and computational modeling (Parenteau et al., 10 Jan 2026).

6. Cohesive Mission Science Narrative and Integration

The group positions atmospheric biosignatures as a cornerstone science goal for HWO—requiring a tightly integrated strategy linking wide-band, moderate-resolution spectroscopy at extreme contrast with photochemical and evolution modeling informed by precise host characterization. Recommended strategies are:

• Joint optimization of instrument design for 0.3–2.0 μm coverage, R ≈ 50–150, and planet–star contrast of ~10⁻¹⁰.
• Science return maximized by repeated, multi-epoch observations enabling time-domain biosignatures (i.e., seasonality, phase curves).
• Discovery framework hinges on closing laboratory/opacities gaps, obtaining host-star context, and leveraging advanced retrieval models incorporating all available atmospheric, planetary, and stellar priors.

The group’s output informs design trades, target selection, and mission time-allocation, ensuring HWO is tailored for a statistically robust, physically interpretable search for life across the nearby exoplanet population (Parenteau et al., 10 Jan 2026).

7. Role and Structure Within NASA's Strategic Astrobiology Initiative

The Living Worlds Working Group exemplifies NASA’s evolving Division-spanning astrobiology strategy, interfacing between astrophysics, planetary science, and biological sciences. Its work is central to the NASA Decadal Astrobiology Research and Exploration Strategy (DARES 2025), and its framework and science outputs are expected to guide future flagship mission architectures, interagency laboratory investments, and research priorities.

Coordination and integration with the Science Mission Directorate and other strategic mission design teams ensures that Living Worlds outputs both address immediate design needs and contribute to the long-term vision for biosignature characterization and interpretation (Parenteau et al., 10 Jan 2026).