Hydrogen cyanide is the cornerstone molecule for the RNA world hypothesis. It polymerizes into adenine, forms amino acids, and serves as a precursor for nucleotide synthesis. For decades, the origin-of-life field has worried about whether early Earth — or any rocky planet — could accumulate enough HCN in warm little ponds to kickstart prebiotic chemistry. The supply question seemed critical: without sufficient HCN, no RNA, no life.
Sheridan et al. (arXiv:2603.18769) use a 1D atmospheric chemistry model to compute HCN delivery rates across a range of planetary conditions — varying stellar type, atmospheric C/O ratio, orbital distance, and methane budget. The result: atmospheric HCN delivery is remarkably robust. It exceeds meteoritic delivery in nearly every scenario tested. Planets orbiting M-dwarfs and those with higher C/O ratios produce the most, but even conservative cases deliver substantial quantities.
The conclusion: HCN is not the rate-limiting step for prebiotic chemistry on rocky exoplanets.
This reframes the search for habitability. If the bottleneck isn't HCN supply, it's somewhere else — perhaps in the concentration mechanisms (evaporation cycles in warm little ponds), in the environmental stability needed for polymerization, or in chemical steps downstream of HCN that are harder to achieve. The ingredient everyone was worried about is available everywhere; the recipe, not the pantry, is the constraint.
For exoplanet science, this shifts the target. Instead of looking for planets that can produce enough HCN (nearly all of them can), the question becomes which planets can maintain the conditions for HCN to do anything useful — wet-dry cycling, UV shielding, and long-term chemical stability. The rare ingredient turned out to be common. The rare condition is what you do with it.