Finding Signs of Life on Transiting Earthlike Planets: High-resolution Transmission Spectra of Earth through Time around FGKM Host Stars
Lisa Kaltenegger, Zifan Lin, Sarah Rugheimer
Abstract
Abstract The search for life in the universe mainly uses modern Earth as a template. However, we know that Earth’s atmospheric composition changed significantly through its geological evolution. Recent discoveries show that transiting, potentially Earthlike, exoplanets orbit a wide range of host stars, which strongly influence their atmospheric composition and remotely detectable spectra. Thus, a database for transiting terrestrial exoplanets around different host stars at different geological times is a crucial missing ingredient to support observational searches for signs of life in exoplanet atmospheres. Here, we present the first high-resolution transmission spectra database for Earthlike planets, orbiting a wide range of host stars, throughout four representative stages of Earth’s history. These correspond to a prebiotic high-CO 2 world—about 3.9 billion years ago in Earth’s history—and three epochs through the increase in oxygen from 0.2% to modern atmospheric levels of 21%. We demonstrate that the spectral biosignature pairs O 2 +CH 4 and O 3 +CH 4 in the atmosphere of a transiting Earthlike planet would show a remote observer that a biosphere exists for oxygen concentrations of about 1% of modern Earth’s—corresponding to about 1–2 billion years ago in Earth’s history—for all host stars. The full model and high-resolution transmission spectra database, covering 0.4–20 μ m, for transiting exoplanets—from young prebiotic worlds to modern Earth analogs orbiting a wide range of host stars—is available online. It is a tool to plan and optimize our observation strategy, train retrieval methods, and interpret upcoming observations with ground- and space-based telescopes.