Detecting Exoplanet Biospheres by Reflection Spectroscopy

Now that we know many stars in the universe are accompanied by planets (“exoplanets”), and that many of these planets orbit in the region around their stars where liquid water can exist (the “habitable zone”), we can inquire whether any of these worlds are home to life—not necessarily alien civilisations, but perhaps something as simple as algae in oceans. Abundant life on a planet necessarily changes the composition of its atmosphere—the Earth’s atmosphere has free oxygen only because photosynthetic plants maintain it there, so it is possible in principle to detect the presence of life by observing the composition of an exoplanet’s atmosphere via spectroscopy.

With the advent of the James Webb Space Telescope, it is just barely possible to make such observations, but only for planets orbiting red dwarf stars which are very different from the Sun and may not be favourable to the origin of life. Future space telescopes, including the Nancy Grace Roman Space Telescope, scheduled for launch in 2026, will begin to make it possible to obtain reflection spectra for planets in the habitable zone of Sun-like stars. Bayesian analysis of these spectra may make it possible to detect vegetation on these planets, even if its metabolism and photosynthetic pathways are different from plants on Earth.

The research paper is “In Search of the Edge: A Bayesian Exploration of the Detectability of Red Edges in Exoplanet Reflection Spectra”. Here is the abstract.

Reflection spectroscopy holds great promise for characterizing the atmospheres and surfaces of potentially habitable terrestrial exoplanets. The surface of the modern Earth exhibits a sharp albedo change near 750 nm caused by vegetation - the red edge - which would leave a strong spectral signature if present on an exoplanet. However, the retrieval of wavelength-dependent surface properties from reflection spectra has seen relatively little study. Here, we propose a new surface albedo parameterization capable of retrieving the wavelength location of a priori unknown ‘edge-like’ features. We demonstrate that a wavelength-dependent surface albedo model achieves higher accuracy in retrieving atmospheric composition. Wavelength-dependent surfaces are also generally preferred over a uniform albedo model when retrieving simulated reflection spectra for a modern Earth analog, even for moderate signal-to-noise ratios (S/N = 10) and Earth-like clouds. Further, the location of the modern Earth’s red edge can be robustly and precisely constrained (within 70 nm for S/N = 10). Our results suggest that future space-based direct imaging missions have the potential to infer surface compositions for rocky exoplanets, including spectral edges similar to those caused by life on the modern Earth.

This implies that aliens with observational capabilities comparable to ours could have detected the presence of biosphere on Earth for hundreds of millions of years before the present.