Red Edge of Exoplanets: Models for HWO

# Detecting the Red Edge on Exoplanets with HWO: New Models

Chlorophyll in vegetation absorbs visible light for photosynthesis but sharply reflects near-infrared radiation starting at 700 nm. This creates a characteristic jump in reflectivity—the red edge—visible in Earth's spectrum from space. NASA's future Habitable Worlds Observatory (HWO) is designed to search for similar signals on exoplanets, but realistic conditions make the task challenging.

Scientists from JPL and Goddard Center developed models that account for surface and atmospheric heterogeneity. Traditional simulations assumed uniformity, overlooking the mosaic of oceans, forests, deserts, and glaciers, along with variable cloud cover.

Realistic 3D Models of Earth

The team led by Zachary Berra applied 3D models of Earth across nine time slices over the course of a day. This captured changes in the visible surface as the planet rotates.

Simulations were processed through the ExoReL system, adapted for spectrally dependent reflectivity. The models account for:

• Surface diversity: ocean, forests, deserts, ice.
• Cloud dynamics that distort the signal.
• Spectral averaging to simulate long-duration telescope observations.

This approach reflects real-world conditions, where the field of view includes a mix of biomes with varying albedo across different wavelengths.

Global photosynthesis distribution varies: oceanic phytoplankton and terrestrial vegetation produce peaks of activity in dark red and blue-green regions on composite images.

Signal Detection Results

Despite clouds and data averaging, the red edge is detectable if more than 50% of the visible surface is land. The reflectivity jump is pinpointed with ~70 nm accuracy, enabling distinction between biological signals (photosynthesis) and abiotic sources.

This is critical for HWO: the telescope will confirm biosignatures on Earth-like exoplanets in the habitable zone, minimizing false positives from geological processes.

70 nm accuracy suffices for spectral analysis in the near-IR range, where chlorophyll exhibits the anomaly. Models demonstrate signal robustness even under partial cloud cover.

Key Takeaways

• Red edge: a reflectivity jump at 700 nm due to chlorophyll, a key marker of photosynthesis.
• Realistic 3D Earth models with daily time slices and ExoReL enable ~70 nm detection accuracy.
• Signal persists with >50% land in the field of view, despite clouds and spectral averaging.
• HWO can distinguish biological from non-biological causes of the jump.
• Surface heterogeneity complicates but doesn't block the search for habitable worlds.

— Editorial Team