How to Find the Mean Density of an Exoplanet

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Lisa Kaltenegger is the founding director of the Carl Sagan Institute at Cornell.

In 2009, Kaltenegger realized that a telescope like JWST would see only tiny signals from atmospheric gases during each transit, so in order to achieve any statistical certainty, astronomers would need to observe dozens or even hundreds of transits, which would take years. Acting on this insight, astronomers started to seek Earths in close orbits around dimmer, colder red dwarf stars, where atmospheric signals will be less drowned out by starlight and transits repeat more frequently.

In 2017, astronomers announced the discovery of seven rocky planets around a red dwarf star called TRAPPIST-1. Then in September, the SPECULOOS-2 system emerged as a backup. These stars are close. They’re dim and red. They each have multiple rocky planets that transit. And as of the summer, the JWST is up and running even better than expected. It will spend a sizable fraction of the next five years staring hard at these messy globes of rock and chemicals spinning around their strange stars. For theoreticians like Kaltenegger who went from daydreaming of alternate Earths to churning out predictions about their atmospheric chemistry, decades of anticipation have given way to a slow fade-in of squiggly spectra on computer monitors.

The goal at the time was to compare spectra from rocky, temperate planets to what Earth’s spectrum would look like from far away, seeking conspicuous signals like a surplus of oxygen due to widespread photosynthesis. Kaltenegger’s objection was that, for the first 2 billion years of Earth’s existence, its atmosphere had no oxygen. Then it took another billion years for oxygen to build up to high levels. And this biosignature hit its highest concentration not in Earth’s present-day spectrum, but during a short window in the late Cretaceous Period when proto-birds chased giant insects through the skies.

Without a good theoretical model for how Earth’s own spectrum has changed, Kaltenegger feared, the big planet-finding missions could easily miss a living world that didn’t match a narrow temporal template. She needed to envision Earth as an exoplanet evolving through time. To do this, she adapted one of the first global climate models, developed by the geoscientist James Kasting, which still includes references to the 1970s magnetic-tape era it originated in. Kaltenegger developed this code into a bespoke tool that can analyze not only Earth through time but also radically alien scenarios, and it remains her lab’s workhorse.

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• Join the Carl Sagan Inst: https://carlsaganinstitute.cornell.edu
• Read How to Characterize Habitable Worlds and Signs of Life https://www.annualreviews.org/doi/abs/10.1146/annurev-astro-082214-122238

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