Charles Keeling’s measurements of atmospheric carbon dioxide atop Hawaii’s Mauna Loa show that the concentration of the greenhouse gas has increased by 30% since the late 1950s. They also show a sinusoidal seasonal variation as trees and other plants in the Northern Hemisphere bloom, flourish, shed leaves, and lie dormant year after year. If an exoplanet harbors life and if its rotational axis is tilted, the composition of its atmosphere would likely vary with its seasons. Could those variations be detected by remote spectroscopy?

To answer the question, Stephanie Olson of the University of California, Riverside, and her collaborators first identified three gases whose seasonal variations, they first thought, might serve as indicators of life: CO2, methane, and oxygen. The researchers were pessimistic about CO2 because its observable signatures—absorption lines around 4.3 μm and 15 μm—are likely to be saturated throughout an exoplanet’s year. Methane’s suitability as...

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