Revisiting the Biological Ramifications of Variations in Earth's Magnetic Field

An Earth-like planetary magnetic field has been widely invoked as a requirement for habitability as it purportedly mitigates the fluxes of ionizing radiation reaching the surface and the escape of neutrals and ions from the atmosphere. Recent paleomagnetic evidence indicates that the nucleation of Earth's inner core, followed perhaps by an increase in geomagnetic field strength, might have occurred close to the Ediacaran period. Motivated by this putative discovery, we explore the ensuing ramifications from the growth or reversals of Earth's dynamo. By reviewing and synthesizing emerging quantitative models, it is proposed that neither the biological radiation dose rates nor the atmospheric escape rates would vary by more than a factor of $\sim 2$ under these circumstances. Hence, we suggest that hypotheses seeking to explain the Cambrian radiation or mass extinctions via changes in Earth's magnetic field intensity are potentially unlikely. We also briefly discuss how variations in the planetary magnetic field may have impacted early Mars and could influence exoplanets orbiting M-dwarfs.Comment: Published in The Astrophysical Journal Letters; 7 pages; 0 figure

Analytical solutions for weak black hole kicks

The black hole is modeled by a combined gravitational potential of the bulge, disk and halo and is subjected to an initial weak kick. The resulting differential equations are set up, and shown to possess analytical solutions. The effects of black hole accretion and dynamical friction are also incorporated into this analytical framework. The resultant frequencies and amplitudes are computed, and are compared with the ones obtained from numerical simulations. Within the valid range of parameters of the analytical model, the two sets of results are shown to be in reasonable agreement. It is shown that this model reproduces the linear dependence of the amplitude on the initial kick velocity, and the constant of proportionality is close to that obtained from the simulations. The analytical treatment presented is quite general, and its applications to other areas are also indicated.Comment: 11 pages, 3 figures, 1 table; accepted for publication in Astrophysics and Space Scienc

Dependence of Biological Activity on the Surface Water Fraction of Planets

One of the unique features associated with the Earth is that the fraction of its surface covered by land is comparable to that spanned by its oceans and other water bodies. Here, we investigate how extraterrestrial biospheres depend on the ratio of the surficial land and water fractions. We find that worlds that are overwhelmingly dominated by landmasses or oceans are likely to have sparse biospheres. Our analysis suggests that major evolutionary events such as the buildup of O$_2$ in the atmosphere and the emergence of technological intelligence might be relatively feasible only on a small subset of worlds with surface water fractions ranging approximately between $30\%$ and $90\%$. We also discuss how our predictions can be evaluated by future observations, and the implications for the prevalence of microbial and technological species in the Universe.Comment: Published in The Astronomical Journal; 14 pages; 3 figure

Brown Dwarf Atmospheres as the Potentially Most Detectable and Abundant Sites for Life

We show that the total habitable volume in the atmospheres of cool brown dwarfs with effective temperatures of $\sim 250$-$350$ K is possibly larger by two orders of magnitude than that of Earth-like planets. We also study the role of aerosols, nutrients and photosynthesis in facilitating life in brown dwarf atmospheres. Our predictions might be testable through searches for spectral edges in the near-infrared and chemical disequilibrium in the atmospheres of nearby brown dwarfs that are either free-floating or within several AU of stars. For the latter category, we find that the James Webb Space Telescope (JWST) may be able to achieve a signal-to-noise ratio of $\sim 5$ after a few hours of integration time per source for the detection of biogenic spectral features in $\sim 10^3$ cool brown dwarfs.Comment: Published in The Astrophysical Journal; 19 pages; 3 figure

Optimal Target Stars in the Search for Life

The selection of optimal targets in the search for life represents a highly important strategic issue. In this Letter, we evaluate the benefits of searching for life around a potentially habitable planet orbiting a star of arbitrary mass relative to a similar planet around a Sun-like star. If recent physical arguments implying that the habitability of planets orbiting low-mass stars is selectively suppressed are correct, we find that planets around solar-type stars may represent the optimal targets.Comment: Published in The Astrophysical Journal Letters; 6 pages, 3 figure

Physical constraints on the likelihood of life on exoplanets

One of the most fundamental questions in exoplanetology is to determine whether a given planet is habitable. We estimate the relative likelihood of a planet's propensity towards habitability by considering key physical characteristics such as the role of temperature on ecological and evolutionary processes, and atmospheric losses via hydrodynamic escape and stellar wind erosion. From our analysis, we demonstrate that Earth-sized exoplanets in the habitable zone around M-dwarfs seemingly display much lower prospects of being habitable relative to Earth, owing to the higher incident ultraviolet fluxes and closer distances to the host star. We illustrate our results by specifically computing the likelihood (of supporting life) for the recently discovered exoplanets, Proxima b and TRAPPIST-1e, which we find to be several orders of magnitude smaller than that of Earth.Comment: published in International Journal of Astrobiology; 31 pages; 3 figure

Risks for life on habitable planets from superflares of their host stars

We explore some of the ramifications arising from superflares on the evolutionary history of Earth, other planets in the Solar system, and exoplanets. We propose that the most powerful superflares can serve as plausible drivers of extinction events, and that their periodicity could correspond to certain patterns in the terrestrial fossil diversity record. On the other hand, weaker superflares may play a positive role in enabling the origin of life through the formation of key organic compounds. Superflares could also prove to be quite detrimental to the evolution of complex life on present-day Mars and exoplanets in the habitable zone of M- and K-dwarfs. We conclude that the risk posed by superflares has not been sufficiently appreciated, and that humanity might potentially witness a superflare event in the next $\sim 10^3$ years leading to devastating economic and technological losses. In light of the many uncertainties and assumptions associated with our analysis, we recommend that these results should be viewed with due caution.Comment: 18 pages; 0 figures; published in The Astrophysical Journa

Impact and mitigation strategy for future solar flares

It is widely established that extreme space weather events associated with solar flares are capable of causing widespread technological damage. We develop a simple mathematical model to assess the economic losses arising from these phenomena over time. We demonstrate that the economic damage is characterized by an initial period of power-law growth, followed by exponential amplification and eventual saturation. We outline a mitigation strategy to protect our planet by setting up a magnetic shield to deflect charged particles at the Lagrange point L$_1$, and demonstrate that this approach appears to be realizable in terms of its basic physical parameters. We conclude our analysis by arguing that shielding strategies adopted by advanced civilizations will lead to technosignatures that are detectable by upcoming missions.Comment: 7 pages; 2 figures; minor typos fixe

Physical constraints for the evolution of life on exoplanets

Recently, many Earth-sized planets have been discovered around stars other than the Sun that might possess appropriate conditions for life. The development of theoretical methods for assessing the putative habitability of these worlds is of paramount importance, since it serves the dual purpose of identifying and quantifying what types of biosignatures may exist and determining the selection of optimal target stars and planets for subsequent observations. This Colloquium discusses how a multitude of physical factors act in tandem to regulate the propensity of worlds for hosting detectable biospheres. The focus is primarily on planets around low-mass stars, as they are most readily accessible to searches for biosignatures. This Colloquium outlines how factors such as stellar winds, the availability of ultraviolet and visible light, the surface water and land fractions, stellar flares, and associated phenomena place potential constraints on the evolution of life on these planets.Comment: Published in Reviews of Modern Physics; 24 pages; 1 figur

What's in a name: The etymology of astrobiology

Astrobiology has been gaining increasing scientific prominence and public attention as the search for life beyond Earth continues to make significant headway on multiple fronts. In view of these recent developments, the fascinating and dynamic etymology of astrobiology is elucidated, and thus shown to encompass a plethora of vivid characters drawn from different continents, religions, ideologies and centuries.Comment: Accepted for publication in the International Journal of Astrobiology; 16 pages; 0 figure