How Stars Influence Habitability Of Their Planets? (Planetary Science)

From birth to death, the evolution of the nature of a planet (i.e. of its interior, surface and atmosphere) is intrinsically linked to that of its host star. A first naive thing that comes to mind when thinking of the effects that a star may have on the habitability of its planets is the following thought experiment: what would happen to a planet without a star? Science fiction provides ideas of the situations in which this could happen. Although a planet around a black hole seems like an attractive configuration (see e.g., the Interstellar movie), black holes may lead to several warming mechanisms (e.g., by the blueshifting and beaming of incident radiation of the background stars on the planet) so that the system is not equivalent to a planet with no star.

Now, Tubet and Selsis in their recent paper, presented brief overview of the main effects by which a star will have an impact (positive or negative) on the surface habitability of planets in orbit around it.

Firstly, spectral and spatial variations of the bolometric (i.e. visible and near-infrared) stellar emission can affect the way a planet’s surface, atmosphere and clouds will absorb or reflect incident light. These changes can affect the climate of a planet (especially the surface temperature) which can thus impact its habitability.

— told Martin Turbet, first author of the study

At first, they showed how the climate and thus the habitability of a planet can be directly affected by the spectral type of its host star. The first naive effect is that the spectral type of the star changes its luminosity (for example, the luminosity of Proxima Cen is 0.15% that of the Sun) and thus the incident bolometric flux received by a planet at a fixed distance from the star. However, for a fixed amount of incident bolometric radiation received by the planet, the way in which stellar bolometric emission is distributed spectrally and spatially directly affects the way in which the surface, atmosphere and clouds absorb and reflect incident light, which changes the planet’s surface temperature and thus the conditions of habitability.

“Secondly, variations in UV (far and midUV) incident fluxes on a planet can drive different photochemistry. This can potentially cause significant changes in the atmospheric composition, and thus the radiation balance of a planet, which can again strongly impact its habitability.”

— told Franck Selsis, second author of the study

They also showed that stellar Hydrogen Lyman-α (at 121.6 nm), far-ultraviolet (∼ 122-200 nm) and mid-ultraviolet (∼ 200-300 nm) spectral emissions are the main drivers of planetary photochemistry. UV (∼ 100-300 nm) photons can in fact photolize (i.e. break) atmospheric molecules, forming radicals capable of generating multiple, complex chemical reactions. These photochemical reactions can deeply affect the atmospheric composition of a planet in a way that depends on initial composition and the total amount and spectral distribution of the UV stellar emission. Eventually, the photochemically-driven destruction or buildup of greenhouse gases in the atmosphere will affect planetary surface temperature and thus the surface habitability of the planet.

“Thirdly, cumulative variations in XUV (X and extreme-UV) incident fluxes on a planet can lead to various degrees of atmospheric erosion, potentially leading to dramatic effects (e.g. through the partial or complete loss of the atmosphere and the water) on the habitability.”

— told Martin Turbet, first author of the study

Finally, they showed that XUV incident flux on a planet can strongly vary depending on the type of host star, e.g. between 2-5 orders of magnitude depending on the exact wavelength between solar-type and low-mass stars (see Fig. 1a ; for a fixed bolometric incident flux), and depending also on the level of activity of the low-mass star. The total XUV flux of Proxima Cen is ∼ 200 times higher than that of the Sun (see Fig. 1b), relatively to their total bolometric emission. This means that the atmospheric escape rate can be significantly higher for planets around late M-stars than for Sun-like stars.

Fig. 1. First panel: Incident flux spectra (at the top of the atmosphere) on a planet receiving a total of 1362 W m¯² (i.e. the solar constant on Earth). The black curve is based on solar spectra reconstructed for λ < 3µm and for λ ≥ 3µm. The red curve is based on Proxima Centauri spectra. Second panel: Cumulative incident flux spectra. Third panel: UV cross-sections for common atmospheric species (H2O, N2, CO2, O2, O3, CH4) in terrestrial planetary atmospheres. © Turbet and Selsis

XUV heating, i.e. the main process by which the planet is likely to lose its atmosphere and its water, is much more efficient during the pre-main sequence phase of star, due to this reason, water loss is potentially much stronger on planets around late M-stars than around sun-like stars, which can have serious consequences for their habitability.

told Franck Selsis, second author of the study

Reference: Martin Turbet, Franck Selsis, “Main ways in which stars influence the climate and surface habitability of their planets”, ArXiv, pp. 1-20, 2021.

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