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Register a new userThe Galactic cosmic ray intensity at the evolving Earth and young exoplanets
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Cosmic rays may have contributed to the start of life on Earth. Here, we investigate the evolution of the Galactic cosmic ray spectrum at Earth from ages $t = 0.6-6.0,$Gyr. We use a 1D cosmic ray transport model and a 1.5D stellar wind model to derive the evolving wind properties of a solar-type star. At $t=1,$Gyr, approximately when life is thought to have begun on Earth, we find that the intensity of $sim$GeV Galactic cosmic rays would have been $sim10$ times smaller than the present-day value. At lower kinetic energies, Galactic cosmic ray modulation would have been even more severe. More generally, we find that the differential intensity of low energy Galactic cosmic rays decreases at younger ages and is well described by a broken power-law in solar rotation rate. We provide an analytic formula of our Galactic cosmic ray spectra at Earths orbit for different ages. Our model is also applicable to other solar-type stars with exoplanets orbiting at different radii. Specifically, we use our Galactic cosmic ray spectrum at 20$,$au for $t=600,$Myr to estimate the penetration of cosmic rays in the atmosphere of HR$,$2562b, a directly imaged exoplanet orbiting a young solar-type star. We find that the majority of particles $<0.1$GeV are attenuated at pressures $gtrsim10^{-5},$bar and thus do not reach altitudes below $sim100,$km. Observationally constraining the Galactic cosmic ray spectrum in the atmosphere of a warm Jupiter would in turn help constrain the flux of cosmic rays reaching young Earth-like exoplanets.
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Energetic particles, such as stellar cosmic rays, produced at a heightened rate by active stars (like the young Sun) may have been important for the origin of life on Earth and other exoplanets. Here we compare, as a function of stellar rotation rate ($Omega$), contributions from two distinct populations of energetic particles: stellar cosmic rays accelerated by impulsive flare events and Galactic cosmic rays. We use a 1.5D stellar wind model combined with a spatially 1D cosmic ray transport model. We formulate the evolution of the stellar cosmic ray spectrum as a function of stellar rotation. The maximum stellar cosmic ray energy increases with increasing rotation i.e., towards more active/younger stars. We find that stellar cosmic rays dominate over Galactic cosmic rays in the habitable zone at the pion threshold energy for all stellar ages considered ($t_*=0.6-2.9,$Gyr). However, even at the youngest age, $t_*=0.6,$Gyr, we estimate that $gtrsim,80$MeV stellar cosmic ray fluxes may still be transient in time. At $sim1,$Gyr when life is thought to have emerged on Earth, we demonstrate that stellar cosmic rays dominate over Galactic cosmic rays up to $sim$4$,$GeV energies during flare events. Our results for $t_*=0.6,$Gyr ($Omega = 4Omega_odot$) indicate that $lesssim$GeV stellar cosmic rays are advected from the star to 1$,$au and are impacted by adiabatic losses in this region. The properties of the inner solar wind, currently being investigated by the Parker Solar Probe and Solar Orbiter, are thus important for accurate calculations of stellar cosmic rays around young Sun-like stars.
Energetic particles may have been important for the origin of life on Earth by driving the formation of prebiotic molecules. We calculate the intensity of energetic particles, in the form of stellar and Galactic cosmic rays, that reach Earth at the time when life is thought to have begun ($sim$3.8Gyr ago), using a combined 1.5D stellar wind model and 1D cosmic ray model. We formulate the evolution of a stellar cosmic ray spectrum with stellar age, based on the Hillas criterion. We find that stellar cosmic ray fluxes are larger than Galactic cosmic ray fluxes up to $sim$4 GeV cosmic ray energies $sim$3.8Gyr ago. However, the effect of stellar cosmic rays may not be continuous. We apply our model to HR 2562b, a young warm Jupiter-like planet orbiting at 20au from its host star where the effect of Galactic cosmic rays may be observable in its atmosphere. Even at 20au, stellar cosmic rays dominate over Galactic cosmic rays.
Galactic cosmic rays are energetic particles important in the context of life. Many works have investigated the propagation of Galactic cosmic rays through the Suns heliosphere. However, the cosmic ray fluxes in M dwarf systems are still poorly known. Studying the propagation of Galactic cosmic rays through the astrospheres of M dwarfs is important to understand the effect on their orbiting planets. Here, we focus on the planetary system GJ 436. We perform simulations using a combined 1D cosmic ray transport model and 1D Alfven-wave-driven stellar wind model. We use two stellar wind set-ups: one more magnetically-dominated and the other more thermally-dominated. Although our stellar winds have similar magnetic field and velocity profiles, they have mass-loss rates two orders of magnitude different. Because of this, they give rise to two different astrosphere sizes, one ten times larger than the other. The magnetically-dominated wind modulates the Galactic cosmic rays more at distances < 0.2 au than the thermally-dominated wind due to a higher local wind velocity. Between 0.2 and 1 au the fluxes for both cases start to converge. However, for distances > 10 au, spatial diffusion dominates, and the flux of GeV cosmic rays is almost unmodulated. We find, irrespective of the wind regime, that the flux of Galactic cosmic rays in the habitable zone of GJ 436 (0.2 - 0.4 au) is comparable with intensities observed at Earth. On the other hand, around GJ 436 b (0.028 au), both wind regimes predict Galactic cosmic ray fluxes that are approximately $10^4$ times smaller than the values observed at Earth.
We have studied long-term variations of galactic cosmic ray (GCR) intensity in relation to the sunspot number (SSN) during the most recent solar cycles. This study analyses the time-lag between the GCR intensity and SSN, and hysteresis plots of the GCR count rate against SSN for solar activity cycles 20-23 to validate a methodology against previous results in the literature, before applying the method to provide a timely update on the behaviour of cycle 24. Cross-plots of SSN vs GCR show a clear difference between the odd-numbered and even-numbered cycles. Linear and elliptical models have been fit to the data with the linear fit and elliptical model proving the more suitable model for even-numbered and odd-numbered solar activity cycles respectively, in agreement with previous literature. Through the application of these methods for the 24th solar activity cycle, it has been shown that cycle 24 experienced a lag of 2-4 months and follows the trend of the preceding activity cycles albeit with a slightly longer lag than previous even-numbered cycles. It has been shown through the hysteresis analysis that the linear fit is a better representative model for cycle 24, as the ellipse model doesnt show a significant improvement, which is also in agreement with previous even-numbered cycles.
In order to understand the atmospheres as well as the formation mechanism of giant planets formed outside our solar system, the next decade will require an investment in studies of isolated young brown dwarfs. In this white paper we summarize the opportunity for discovery space in the coming decade of isolated brown dwarfs with planetary masses in young stellar associations within 150 pc. We suggest that next generation telescopes and beyond need to invest in characterizing young brown dwarfs in order to fully understand the atmospheres of sibling directly imaged exoplanets as well as the tail end of the star formation process.
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