Figure 1: Artist’s impression of an Earth-like planet. Credit: Scott Richard.
In the search for Earth-like planets around other stars, the presence of life on these worlds can be determined by looking for various biomarker gases in the planet’s atmosphere. Two promising biomarker gases are oxygen, which is produced almost entirely by photosynthesis on Earth, and ozone, which is produced in the Earth’s stratosphere when ultraviolet (UV) light splits oxygen into individual oxygen atoms where they then combine with other oxygen molecules to form ozone. Ozone is a good indicator of photosynthetic life because even a small amount of atmospheric oxygen can result in a significant concentration of ozone.
Segura et al. (2003) developed photochemical and radiative/convective atmospheric models of Earth-like planets around 3 different types of stars: F-type, G-type (Sun) and K-type. This is to see how an Earth-like planet might differ from a planet circling our Sun. The models assume a present-day Earth-analogue planet with an atmospheric oxygen concentration at the present atmospheric level (PAL). Also, the planet’s distance from its host star is scaled according to the star’s luminosity such that the planet’s average surface temperature is 288 K, which is similar to the average surface temperature of present-day Earth.
Figure 2: A comparison of the types of stars from M-type to B-type.
On Earth, the stratosphere is a layer of the atmosphere where temperature increases as altitude increases, reaching a peak at 45 km altitude. This heating is caused by absorption of UV flux by ozone. In the atmospheric models by Segura et al. (2003), stratospheric temperatures are warmest for the planet around an F-type star and coolest for the planet around a K-type star (Figure 3). This is because an F-type star is hotter and produces a higher UV flux, resulting in the greater absorption of UV flux by ozone. Furthermore, the higher UV flux also causes the planet around the F-type star to have a thicker ozone layer (Figure 4).
Figure 3: Atmospheric temperature profile results for an Earth-like planet with 1 PAL of oxygen and around different types of stars. (Segura et al., 2003)
Figure 4: Atmospheric ozone concentration results for an Earth-like planet with 1 PAL of oxygen and around different types of stars. (Segura et al., 2003)
The amount of UV flux reaching a planet’s surface is of concern due to its ability to damage cells and even DNA. On Earth, most damage caused by UV radiation is from UV-A (315-400 nm) and UV-B (280-315 nm), with UV-B being somewhat more dangerous. Fortunately, almost no UV-C (< 280 nm) penetrates the Earth’s atmosphere. UV-C is the most energetic and most dangerous form of UV radiation. The atmospheric models for an Earth-like planet with 1 PAL of oxygen show, in the UV range of 200 to 400 nm, the surface of the planet around a K-type star receives 0.44 times the UV flux that the Earth’s surface receives, while the surface of the planet around an F-type star receives 1.61 times the UV flux.
For UV-B alone, the planet around a K-type star receives 0.43 times the amount Earth receives, while the planet around an F-type star receives 0.68 times the amount. This shows that planets around K-type and F-type stars exhibit significantly better UV protection than Earth at 1 PAL of oxygen, despite an F-type star being hotter than our Sun. For a K-type star, it is simply due to it being cooler and emitting less UV flux. For the F-type star, it is a consequence of its higher UV flux which results in the formation of a much thicker ozone layer. Planets around all 3 types of stars also show negligible amounts of UV-C reaching the surface.
Figure 5: Incoming and surface UV fluxes for Earth-like planets with different UV fluxes and orbiting around different stars. (Segura et al., 2003)
Figure 6: Normalized surface UV dose rates relative to present-day Earth for skin cancer (erythema) and DNA damage on Earth-like planets with different UV fluxes and orbiting around different stars. (Segura et al., 2003)
The atmospheric models by Segura et al. (2003) can be extended to oxygen levels lower than 1 PAL. For 0.1 PAL of oxygen and in the UV range of 200 to 400 nm, the planet around a K-type star receives 0.45 times the UV flux that present-day Earth receives, while the planet around an F-type star receives 1.62 times the UV flux. For UV-B alone, the values are 0.61 (planet around K-type star) and 0.85 (planet around F-type star) times the UV flux that present-day Earth gets. It is also clear that below ~0.01 PAL of oxygen, the ozone layer becomes too thin to provide significant UV shielding regardless of the type of star the planet circles around.
Segura et al., “Ozone Concentrations and Ultraviolet Fluxes on Earth-Like Planets Around Other Stars”, Astrobiology Volume 3, Number 4, 2003