Galactic Cosmic Ray–Induced Radiation Dose on Terrestrial Exoplanets

Abstract

This past decade has seen tremendous advancements in the study of extrasolar planets. Observations are now made with increasing sophistication from both ground and space based instruments, and exoplanets are characterized with increasing precision. There is a class of particularly interesting exoplanets, falling in the habitable zone, which is defined as the area around a star where the planet is capable of supporting liquid water on its surface. Planetary systems around M dwarfs are considered to be prime candidates to search for life beyond the solar system. Such planets are likely to be tidally locked and have close-in habitable zones. Theoretical calculations also suggest that close-in exoplanets are more likely to have weaker planetary magnetic fields, especially in case of super earths. Such exoplanets are subjected to a high flux of Galactic Cosmic Rays (GCRs) due to their weak magnetic moments.GCRs are energetic particles of astrophysical origin, which strike the planetary atmosphere and produce secondary particles, including muons, which are highly penetrating. Some of these particles reach the planetary surface and contribute to the radiation dose. Along with the magnetic field, another factor governing the radiation dose is the depth of the planetary atmosphere. The higher the depth of the planetary atmosphere, the lower the flux of secondary particles will be on the surface. If the secondary particles are energetic enough, and their flux is sufficiently high, the radiation from muons can also impact the sub-surface regions, such as in the case of Mars. If the radiation dose is too high, the chances of sustaining a long-term biosphere on the planet are very low. We explore the dependence of the GCR induced radiation dose on the strength of the planetary magnetic field and its atmospheric depth, finding that the latter is the decisive factor for the protection of a planetary biosphere.

Galactic cosmic rays on extrasolar Earth-like planets

“…The surface radiation calculation thus is similar to the calculations of Atri et al (2013). However, we extend the primary particle energy range and increase the range of planetary magnetic fields.…”

“…The electromagnetic interactions enhance the atmospheric ionization rate and change the atmospheric chemistry (Atri et al 2010). For energies of the primary particles above 8 GeV, hadronic interactions produce particles (such as muons and neutrons), some of which reach the ground and contribute to the radiation dose (Atri et al 2011(Atri et al , 2013.…”

“…3.2. First results have been presented by Atri et al (2013); for the current work, the range of planetary magnetic moments has been extended. Figure 9 and Table 2 show the total biological radiation dose rate as a function of the planetary magnetic field, measured in mSv/yr.…”

“…In addition to the new cosmic-ray fluxes from Paper I, the main differences with respect to the approach of previous work (Grenfell et al 2007a;Atri et al 2013) are the following:…”

“…The resulting surface radiation dose was evaluated by Atri et al (2013). For an Earth-like atmosphere with a surface pressure of 1033 hPa, they find that the absence of magnetospheric shielding can increase the surface biological dose rate by up to a factor of ∼2.…”