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PostPosted: Sat Mar 30, 2019 2:01 am 
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Posts: 273
Habitability of Earth-like stagnant lid planets: Climate evolution and recovery from snowball states

"... For a "hard" snowball with no [atmosphere-ocean chemical] exchange, recovery is unlikely, as most CO2 outgassing takes place via metamorphic decarbonation of the crust, which occurs below the ice layer. However, for a "soft" snowball where there is exchange between atmosphere and ocean, planets can readily recover. For both hard and soft snowball states, there is a minimum CO2 budget needed for recovery; below this limit any snowball state would be permanent. ..."

An axisymmetric limit for the width of the Hadley cell on planets with large obliquity and long seasonality

"... In order to understand the characteristics of the Hadley circulation in case of extreme planetary characteristics, we show both theoretically, using axisymmetric theory, and numerically, using a set of idealized GCM simulations, that the thermal Rossby number dictates the character of the circulation. Given the possible variation of thermal Rossby number parameters, the rotation rate is found to be the most critical factor controlling the circulation characteristics. ..."


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PostPosted: Tue Apr 09, 2019 4:03 pm 
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Posts: 273
Advanced Aspects of the Galactic Habitability

"... Our aim is to numerically investigate the dynamics of opposed processes of expansion (panspermia, colonization) and extinction (catastrophic mechanisms) of life in the Galaxy. ... Results. While the largest part of the examined parameter space shows very low habitability values, as expected, the remaining part has some observationally appealing features that imply, among other things, a reduction in the amount of fine-tuning necessary for resolving the Fermi paradox. ..."


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

"... 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 ∼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. ..."


Lessons from early Earth: UV surface radiation should not limit the habitability of active M star systems

"... Here we show the first models of the surface UV environments of the four closest potentially habitable exoplanets: Proxima-b, TRAPPIST-1e, Ross-128b, and LHS-1140b assuming different atmospheric compositions, spanning Earth-analogue to eroded and anoxic atmospheres and compare them to levels for Earth throughout its geological evolution. Even for planet models with eroded and anoxic atmospheres, surface UV radiation remains below early Earth levels, even during flares. Given that the early Earth was inhabited, we show that UV radiation should not be a limiting factor for the habitability of planets orbiting M stars. ..."


Last edited by thrash on Wed Apr 17, 2019 8:07 pm, edited 1 time in total.

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PostPosted: Wed Apr 10, 2019 12:47 pm 
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Faster Water Escape on High Obliquity Planets

"... Three complexities that only exist under high obliquity are found to be causally relevant. 1) Seasonal variation under high obliquity causes extremely high surface temperatures to occur during polar days, moistening the polar air that may eventually enter the stratosphere. 2) Unlike the low obliquity scenario where the cold trap efficiently freezes out water vapor, the high obliquity stratosphere gets most of its moisture input from high latitudes, and thus largely bypasses the cold trap. 3) A high obliquity climate tends to be warmer than its low obliquity equivalent, thus moistening the atmosphere as a whole. ... The water escape could cause a high obliquity planet to loss [sic] habitability before the runaway greenhouse takes place."


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PostPosted: Mon May 06, 2019 2:22 pm 
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Posts: 273
Accounting for Multiplicity in Calculating Eta Earth

"Using the updated exoplanet population parameters of our previous study, which includes the planetary radius updates from Gaia DR2 and an inferred multiplicity distribution, we provide a revised η⊕ calculation. This is achieved by sampling planets from our derived population model and determining which planets meet our criterion for habitability. To ensure robust results, we provide probabilities calculated over a range of upper radius limits. Our most optimistic criterion for habitability provides an η⊕ value of 0.34±0.02planetsstar. We also consider the effects of multiplicity and the number of habitable planets each system may contain. Our calculation indicates that 6.4±0.5% of GK dwarfs have more than one planet within their habitable zone. This optimistic habitability criterion also suggests that 0.036±0.009% of solar-like stars will harbor 5 or more habitable planets. These tightly packed highly habitable system should be extremely rare, but still possible. Even with our most pessimistic criterion we still expect that 1.8±0.2% of solar-like stars harbor more than one habitable planet."

This paper cites a broad sample of the current literature on habitability, and so may serve as a useful entry point.


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PostPosted: Tue May 14, 2019 1:16 pm 
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Impact of Space Weather on Climate and Habitability of Terrestrial Type Exoplanets

"... The assessment of the impacts of host stars on the climate and habitability of terrestrial (exo)planets will significantly expand the current definition of the habitable zone to the biogenic zone and provide new observational strategies for searching for signatures of life. The major goal of this paper is to describe and discuss the current status and recent progress in this interdisciplinary field and to provide a new roadmap for the future development of the emerging field of exoplanetary science and astrobiology."


Discovery of the first Earth-sized planets orbiting a star other than our Sun in the Kepler-20 system

"... Kepler-20e and 20f orbit their Sun-like star every 6.1 and 19.6 days, respectively, and are most likely of rocky composition. Here we review the history of how they were found, and present an overview of the methodology that was used to validate them."


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PostPosted: Mon May 20, 2019 3:39 pm 
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Does the evolution of complex life depend on the stellar spectral energy distribution?

"... The proportional evolutionary time hypothesis predicts that late K- and M-dwarf stars (M < 0.7 M⊙) are too young to host any complex life at the present age of the universe. F-, G-, and early K-dwarf stars (M > 0.7 M⊙) represent the best targets for the next generation of space telescopes to search for spectroscopic biosignatures indicative of complex life."


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PostPosted: Mon May 20, 2019 9:36 pm 
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Location: the Retirement Coast, NSW, Australia
thrash wrote:
Does the evolution of complex life depend on the stellar spectral energy distribution?

"... The proportional evolutionary time hypothesis predicts that late K- and M-dwarf stars (M < 0.7 M⊙) are too young to host any complex life at the present age of the universe. F-, G-, and early K-dwarf stars (M > 0.7 M⊙) represent the best targets for the next generation of space telescopes to search for spectroscopic biosignatures indicative of complex life."

Aha! I have been wondering about that for years. Good find!

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— Brett Evill

My SF RPG setting, Flat Black


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PostPosted: Fri May 24, 2019 1:40 am 
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The Exoplanet Population Observation Simulator. II - Population Synthesis in the Era of Kepler

From p. 12:

"The populations that match the overall Kepler statistics ... predicts an occurrence rate of 0.8−1.7 R⊕ planets of ... 18%.
"More importantly, the model predicts which planets in this size range are rocky and which planets have substantial hydrogen atmospheres. The fraction of true rocky planets is only ... 7%, though we caution that this number is likely to change as the models become better calibrated to the observed exoplanet populations. However, future direct imaging missions should contend with the possibility that a large fraction of observable exo-earths could have substantial hydrogen/helium atmospheres."


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PostPosted: Fri May 24, 2019 1:06 pm 
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Enhanced Habitability on High Obliquity Bodies near the Outer Edge of the Habitable Zone of Sun-like Stars

"... We show that high obliquity planets are hotter than their low obliquity counterparts due to ice-albedo feedbacks for cold climates, and water vapor in warm climates. We suggest that the water vapor greenhouse trapping is greater on high obliquity bodies due to the different dynamical regimes that occur between the two states.
"While equatorial ice-belts are stable at high obliquity in some climate regimes, it is harder to achieve global glaciation than for a low obliquity planet. Temperate polar conditions can be present at high obliquity at forcings for which low obliquity planets would be in a hard snowball state. We suggest the conditions on high obliquity planets are likely to be more favorable for a robust biosphere to develop approaching the outer edge of the HZ. However, the influence of obliquity diminishes for dense atmospheres, in agreement with calculations from 1-D Energy Balance Models."


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PostPosted: Sat Jun 15, 2019 2:17 pm 
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Effects of a Binary Companion Star on Habitability of Tidally Locked Planets around an M-type Host Star

"... we investigate the effects of irradiation from a G-type companion star on the climate of a tidally locked planet around an M-type star using the 2D energy balance model. ... Tidally locked ocean planets with < ~ 0.3 bar atmospheres or land planets with < ~ 3 bar atmospheres can realize temperate climate with surface liquid water when they are also irradiated by a companion star with a separation of 1 - 4 au...."


Atmospheric Dynamics on Terrestrial Planets: The Seasonal Response to changes in Orbital, Rotational and Radiative Timescales

"... Using an idealized general circulation model with a seasonal cycle, we study the seasonal response of the surface temperature and Hadley circulation to changes in the orbital, rotational and radiative timescales...."


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