Sukrit Ranjan, PhD
CIERA Postdoctoral Fellow
Planetary Systems Biochemistry: Inferring the laws of life at a Planetary Scale
2.5 billion years ago, microbes learned to harness sunlight to reduce CO2 with H2O, producing O2 as waste. O2 production from this pathway (photosynthesis) was so vigorous that O2 saturated its photochemical sinks, permitting it to “runaway” and rapidly accumulate in the atmosphere despite its reactivity. O2 is not unique: diverse gases produced by life may experience a similar “runaway” effect. This runaway occurs because the ability of an atmosphere to photochemically cleanse itself of trace gases is generally finite. If produced at rates exceeding this finite limit, even reactive gases can rapidly accumulate to high concentrations and become detectable. Planets orbiting smaller, cooler M-dwarf stars, the prime targets for the upcoming James Webb Space Telescope (JWST), are especially favorable for runaway. We illustrate the dramatic implications of runaway for reactive biosignature gas detectability by simulating the runaway of NH3 on a habitable exoplanet with an H2-N2 atmosphere and net surface production of NH3 orbiting an M-dwarf. In runaway, an increase in surface production flux of 10x can increase NH3 concentrations by 1000x and render it detectable with JWST in just 2 transits. Our work suggests that diverse signs of life on exoplanets may be readily detectable even in the absence of oxygenic photosynthesis, so long as the life produces volatile products in sufficient yield.
Sukrit Ranjan is a planetary photochemist and a CIERA Postdoctoral Fellow at Northwestern University. Sukrit’s research focuses on applying photochemistry to questions related to the origin of life on Earth and the search for life on other worlds. These questions are coupled: efforts to understand the origin of life on Earth helps guide our search for it elsewhere in the cosmos, while observations of other planets help us test our theories of the prebiotic environment and of abiogenesis. To understand abiogenesis, Sukrit works to constrain the palette of environmental conditions from which life arose on Earth, to constrain and guide experimental studies of the origin of life. To search for life elsewhere, Sukrit works to determine observational tests by which life on other worlds may be remotely discriminated. In collaboration with experimental colleagues, Sukrit obtains the critical measurements of fundamental photochemical parameters required to build robust models in support of both goals. Prior to his role at CIERA, Sukrit completed a SCOL Fellowship at MIT and a PhD in Astronomy & Astrophysics at Harvard.