UV Environments and Atmospheres of Earth-like Planets Orbiting White Dwarfs

A transiting Earth-sized planet orbiting in the habitable zone of a white dwarf would produce a large signal-to-noise ratio, which has prompted their inclusion in exoplanet searches. However, such planets would be exposed to different UV environments compared to planets hosted by main sequence stars of the same temperature, impacting both ground UV flux and atmospheric photochemistry.

Using a coupled 1D climate-photochemistry code we model atmospheres of planets orbiting white dwarfs at the 1 AU equivalent distance. White dwarfs gradually cool down over time due to a lack of internal heat sources. We mimic this by using white dwarf stellar models for a range of temperatures (6000-4000 K), placing our model planet at the same orbital distance during each simulation. This keeps the planet in the continuously habitable zone, causing the incident flux to decrease in successive model runs as we use cooler white dwarf models.

We compare our models to planets orbiting main sequence stars of the same host temperature. We consider atmospheric photochemistry, ground UV, and surface temperature conditions. We place particular focus on changes in abundances for species that are thought to indicate habitability. Differences in planetary atmospheric photochemistry are driven by the strength of far-UV wavelength fluxes determined by the level of the host’s chromospheric activity. We conclude by using the modern and early Earth as references to evaluate our ground UV flux results.

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