How unique is our rocky planet as a cradle of life in the universe?

When life emerged on Earth ~4 Gyr ago, the physical and chemical environments were vastly different from today. For example, the Sun was fainter, small bodies bombarded Earth’s surface, and the atmosphere was reducing. Yet, this seemingly lethal environment turned out to be beneficial for producing the building blocks of life, e.g. amino acids, sugars and nucleobases. The reaction chain to form such complex species can in theory begin with a simple molecule such as hydrogen cyanide (HCN) and take place on the planetary surface, in hypothesised warm little ponds. However, HCN itself is mainly produced in the atmosphere through photochemical reactions. It is therefore important to include the atmospheric production of HCN and its transport to the surface, through rain-out processes, and to understand how these processes are influenced by the physical environment. We use a 1-D photochemical kinetic code, named VULCAN, to study these processes on Early Earth and analogous exoplanets. By varying the physical environment and focusing on HCN chemistry we aim to answer the question: Do we live on a special planet or could (early) Earth be part of a larger population of rocky planets in the universe that has the potential to harbour life? As part of this work, we determine the important chemical pathways for a range of physical environments so they can be used later in 3D climate-chemistry model simulations.