Rare Earth: P and N

The idea that Planet Earth is rare and special for being able to host life has a very readable book-length argument, Rare Earth by Brownlee and Ward. Technically it argues why complex life is rare, while simpler life may be more achievable over a broader range of conditions. Given that we only have a sample size of one for life-harboring planets, who knows if life proliferates beyond our star? You can plug a range of numbers into the Drake equation to convince yourself either way.

 

The idea that Earth sits in a Goldilocks habitable zone most often refers to whether water exists as a liquid on the surface of a planet. This assumes that H₂O is crucial to life as a liquid, and that other liquids (ammonia, hydrocarbons, formamide) may not have the same versatility. It’s hard to say otherwise with a sample size of one.  We also assume that carbon-based molecules are crucial for life, which is reasonable from a chemical point of view (diversity, bond energies, or as a carrier). That takes care of carbon, hydrogen, and oxygen. What about nitrogen and phosphorus?

 

The idea that our rare Earth may be rarer than we previously thought comes from a paper published last month in Nature Astronomy (DOI: 10.1038/s41550-026-02775-z). It examines core formation of rocky planets and estimates the availability of nitrogen and phosphorus under different conditions. Things that are important are the relative redox state of the mantle. Our planet apparently sits in a zone that optimizes decent amounts of N and P (although much less abundant than C, H, O). It’s a tricky balance. If the redox situation is too reducing, availability of P plummets; too oxidizing and N might be lost to outer space by significant degassing. Simulated origin-of-life chemistry in the lab has always worked better under reducing conditions. Thus, the authors conclude: “there is a plausible case to make that only moderately oxidizing planets have both sufficient mantle P and sufficient reducing power to sustain prebiotic chemistry and then life”.

 

Why do we need N? Amino acids. Catalysts. Chemical versatility. We would not have fine-tuning of the thermodynamics and kinetics of (bio)chemical reactions without nitrogen. Why do we need P? I’m not sure. Arguments can be made for its crucial role in nucleic acids (Westheimer’s famous paper). But it’s possible some other backbone might work. Bioenergetic currency relies on phosphates today, but it’s possible that sulfur could have played the early role of energy transduction. Sulfate bacteria are also fierce competitors. I’d be very curious what is known about sulfur availability during planetary formation. We know that outgassing of volcanoes on the early Earth is a source. And I’m also clearly biased because I have a current research grant to study the role of sulfur in origins-of-life chemistry.

 

Final tidbit of the paper that I enjoyed: the authors use oxygen fugacity as their redox measure. When I teach P-Chem, I try to pepper in examples of why the math-and-models are useful. Fugacity is one of those bewildering topics to students, because the math looks like a merry-go-round and having a hypothetical reference state seems strange. Now I can point to another example why learning about fugacity is useful and interesting. The paper makes reference to Mars and other exoplanets in the search for life in the universe.