When Did Life First Emerge in the Universe?

Avi Loeb: One way to determine how early life started in the cosmos is to examine whether it formed on planets around the oldest stars. Such stars are expected to be deficient in elements heavier than helium, which astrophysicists call “metals.” (in our language, unlike that of most people, oxygen, for example, is considered a metal). Indeed, metal-poor stars have been discovered in the periphery of the Milky Way, and have been recognized as potential members of the earliest generation of stars in the universe. These stars often exhibit an enhanced abundance of carbon, making them “carbon enhanced metal poor” (CEMP) stars. My former student Natalie Mashian and I suggested that planets around CEMP stars might be made mostly of carbon, so their surfaces could provide a rich foundation for nourishing early life. We could therefore search for planets that transit, or pass in front of, CEMP stars and show biosignatures in their atmospheric composition. This would allow us to determine observationally how far back in time life may have started in the cosmos, based on the ages of these stars.

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