Extreme hydrodynamic losses of Earth-like atmospheres in the habitable zones of very active stars

Abstract

Aims. In this letter, we calculate for the first time the full transonic hydrodynamic escape of an Earth-like atmosphere. We consider the case of an Earth-mass planet with an atmospheric composition identical to that of the current Earth orbiting at 1 AU around a young and very active solar mass star. Methods. To model the upper atmosphere, we used the Kompot Code, which is a first-principles model that calculates the physical structures of the upper atmospheres of planets, taking into account hydrodynamics and the main chemical and thermal processes taking place in the upper atmosphere of a planet. This model enabled us to calculate the 1D vertical structure of the atmosphere using as input the high-energy spectrum of a young and active Sun.Results. The atmosphere has the form of a transonic hydrodynamic Parker wind, which has an outflow velocity at the upper boundary of our computational domain that exceeds the escape velocity. The outflowing gas is dominated by atomic nitrogen and oxygen and their ion equivalents and has a maximum ionization fraction of 20%. The mass outflow rate is found to be 1.8 × 10 9 g s −1 , which would erode the modern Earth's atmosphere in less than 0.1 Myr. Conclusions. This extreme mass loss rate suggests that an Earth-like atmosphere cannot form when the planet is orbiting within the habitable zone of a very active star. Instead, such an atmosphere can only form after the activity of the star has decreased to a much lower level. This happened in the early atmosphere of the Earth, which was likely dominated by other gases such as CO2. Since the time it takes for the activity of a star to decay is highly dependent on its mass, this is important for understanding possible formation timescales for planets orbiting low-mass stars.

The Role of N2as a Geo-Biosignature for the Detection and Characterization of Earth-like Habitats

“…Moreover, Airapetian et al (2017b) found that the escape time of a 1 bar atmosphere on a terrestrial-type planet in the habitable zone of Proxima Cen b is expected to be about 10 Myr. In agreement with these results, a more recent study by Johnstone et al (2019) found extreme hydrodynamic losses of Earth-like at-mospheres in the habitable zones of very active stars, resulting in the complete evaporation of a modern Earth atmosphere in £ 0.1 Myr. One can conclude that atmospheres of M and K star Earth-like habitable zone planets, which could maintain liquid water oceans on their surface, are unlikely to build up N 2 -dominated atmospheres.…”

A census of coronal mass ejections on solar-like stars

“…As known from the Sun there is interaction between CMEs and the Earth's atmosphere. Model results for exoplanet systems have shown that frequent and energetic CMEs together with a high short-wavelength (EUV) radiation environment lead to very efficient planetary atmospheric mass loss (Lammer et al 2007;Khodachenko et al 2007;Cohen et al 2011;Cherenkov et al 2017;Johnstone et al 2019). In a worst-case scenario frequent and energetic CME impacts may lead to the loss of planetary atmospheres and therefore CMEs endanger planetary habitability.…”

The UV surface habitability of Proxima b: first experiments revealing probable life survival to stellar flares

“…at the equator, in order to provide the case of maximum irradiance. Very recently, it was predicted that Earth-like atmospheres can not form in the habitable zones of very active stars (Johnstone et al 2019), and atmospheres containing CO 2 are likely stable on waterdepleted planets orbiting M dwarfs (Gao et al 2015). Atmospheres dominated by N 2 tend to be unstable under high XUV fluxes and experience efficient atmospheric escape (Tian et al 2008;Lammer et al 2011).…”

“…The effectiveness of UVA to induce lethal damage depends also on mechanisms that in the presence of oxygen produce reactive oxygen species (Kielbassa et al 1997;Oppezzo et al 2011). According to several studies O-rich atmospheres are not expected to exist in planets orbiting M-type stars (Garcia-Sage et al 2017;Johnstone et al 2019), therefore, this would reduce the damage induced by UVA in this context.…”

“…: O 2 rich Earth-like atmospheric compositions are unfeasible in planets orbiting M dwarfs, e.g. : Johnstone et al 2019) (ii) an inaccurate estimation of the biological impact of UV by using extrapolations of biologically relevant UV fluences 1 based on empirical data from the literature, such as the UV biological action spectra (UV-BAS; a measure of biological UV damage at a given UV wavelength under a given constant fluence), or the UV "lethal dose" (UV-LD; the estimated amount of UVR required to kill a determined percentage of the population of the life forms under consideration). Even though UV-BAS or UV-LD can provide information about biological effectiveness of radiation or survival of the population, respectively, there are some drawbacks related to their use.…”