When Planets Breathe: Models Constrain the Circumstances for Detection of Biomarker Gases on the Terrestrial Exoplanets of M Dwarfs

When it comes to worlds beyond our solar system, we rely on methods of remote observation to search for evidence of extraterrestrial life. One such method is transmission spectroscopy, which looks at the fraction of starlight transmitted through the gaseous envelope or atmosphere of an exoplanet during primary transit. Many of Earth’s atmospheric components are biogenic in origin. If we can observe a similar suite of biomarker gases in the atmosphere of an exoplanet, we may confirm an unambiguous detection of life on another world. In this paper, I present theoretical calculations which place constraints on the detection of information-rich biomarkers, with a specific focus on O2­. I use PHOENIX stellar spectra for stars with the characteristic size and temperature of M dwarfs of approximately 0.2-0.5Mʘ (Husser et al. 2013), and the absorption spectra of O2 in an Earth-like atmosphere, calculated by Rodler & López-Morales (2014), to simulate the transmission spectrum of an O­2-rich Earth-like exoplanet as observed from an Earth-based telescope. By simulating these observations with different values of instrumental resolution and the planet’s distance from Earth, I discovered that O2 features are resolvable at resolutions R=10000 to 100000 for star systems at distances out to 10 pc, for a star of radius 0.2 solar radii. Beyond about 10 pc, O2 absorption features are irretrievable with a 39.5-meter telescope, the largest yet under construction. O2 absorption features are also irretrievable for stars larger than 0.5 solar radii even at distances of 10 pc or less. With my models, these constraints can be recalculated for the absorption features of more explicit biomarkers fairly easily by inputting absorption spectra from the HITRAN database for these molecules, which I attempted with deuterated water (Rothman et al. 2013)