Audible's new audio drama "Exoplanet," written by Benjamin Percy and executive produced by Andy Weir, follows three crew members stranded on an alien planet. The story uses this fictional scenario to explore real science about what makes exoplanets habitable.
Astronomers determine habitability using several factors. They employ spectroscopy to analyze starlight, splitting it into wavelengths to measure a star's temperature and composition. A planet's habitability depends on both the star's luminosity and the planet's orbital distance. Worlds must sit in the habitable zone, where temperatures allow liquid water to exist on the surface with a suitable atmosphere. A planet orbiting a cool red dwarf needs to be much closer to its star than Earth is to the Sun to reach this zone.
But orbital position alone does not guarantee habitability. Stars emit ultraviolet radiation and X-rays that interact with planetary atmospheres, potentially altering chemistry and eroding atmospheres. Violent stellar flares produce bursts of high-energy radiation. Young stars tend to be more active and can damage a planet's atmosphere over time. As stars age, their changing luminosities can drastically alter planetary conditions.
Atmospheric composition presents another challenge. Even if oxygen makes up 21 percent of an atmosphere like Earth's, atmospheric pressure matters equally. At lower pressures, less oxygen reaches lungs. Volcanic planets release gases such as sulfur dioxide and hydrogen sulfide, which are hazardous at high concentrations. An oxygen-rich atmosphere could still prove lethal.
Detecting signs of life adds complexity. Probes in the audio drama find dimethyl sulfide, a molecule associated with biological activity on Earth. Astronomers have investigated possible signs of this molecule in K2-18 b, an exoplanet orbiting a red dwarf. However, both the detection itself and the biological interpretation remain heavily disputed. Detecting a single molecule is not definitive evidence of life, as its presence can result from atmospheric chemistry and stellar radiation rather than biology.