Colors as a Tool to Search for Life in the Cosmos: Purple is the new Green

Biopigments, fundamental to the physiological makeup of organisms across diverse branches of the tree of life, play a pivotal role in coloring Earth’s landscapes. These molecules offer multifaceted benefits, acting as a shield against radiation, temperature fluctuations, resource scarcity, and aridity—the prevalent conditions in various earthly and extraterrestrial locations. Analogous mechanisms involving biopigments may be a universal adaptation of life to challenging environments and also in different planetary systems. These biopigments serve as distinctive signatures of life, manifesting unique reflectance spectral features that often correlate with specific environmental conditions. In the realm of exoplanetary exploration, biopigments emerge as prime candidates for direct biosignature observation. Current surface models predominantly focus on chlorophyll-covered planets, akin to modern Earth, neglecting the diverse landscapes that our planet has undergone throughout its evolution and how different Earth-like planets would look around different stars. A critical missing input is a spectral catalog of biopigments of the colors of life linking to specific environments. On Earth-like planets, purple bacteria could dominate the surface of anoxic environments, and colorful ice bacteria on the surface of frozen exoplanets. We present a comprehensive, freely accessible biopigment spectra library from the visible to the near-infrared and show its applications in simulating different Earth-like surfaces and their impact on the atmosphere. This library is a tool for modelers and observers to train retrieval algorithms, optimize search strategies, and inform models of Earth-like planets for ongoing and upcoming missions, like JWST, ELTs, Ariel, LIFE, and the Habitable World Observatory (HWO). The diverse environments that Earth-like planets in different planetary systems may harbor require a wider view of the characteristics and biopigments of life: Colors indicative of life on other planets, where orange, yellow, or even purple could be the new green.