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PostPosted: Fri Feb 09, 2018 2:32 pm 
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The Chemical Homogeneity of Sun-like Stars in the Solar Neighborhood

"We ... reveal that stars with similar ages and metallicities have nearly identical abundance patterns. Contrary to previous results, we find that the ratios of carbon-to-oxygen and magnesium-to-silicon are homogeneous to within 10% throughout the solar neighborhood, implying that exoplanets may exhibit much less compositional diversity than previously thought. Finally, we demonstrate that the Sun has a subtle deficiency in refractory material relative to ~95% of solar twins, suggesting a possible signpost for planetary systems like our own."


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PostPosted: Thu Feb 15, 2018 1:39 pm 
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Cometary impactors on the TRAPPIST-1 planets can destroy all planetary atmospheres and rebuild secondary atmospheres on planets f, g, h

What it says.


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PostPosted: Mon Feb 19, 2018 12:35 pm 
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Impact of Gas Giant Instabilities on Habitable Planets

"Planetary systems consisting of two giant planets are fairly benign to terrestrial planets, whereas six giant planets very often lead to a complete clearing of the habitable zone. Systems with initial distances of five Hill Radii between the giant planets have a high chance to harbour a habitable planet, although more compact systems are very destructive. The giant planet masses have a smaller impact on the stability of habitable worlds. ... As a rule of thumb, observed gas giants with eccentricities higher than 0.4 and inclinations higher than 20 degrees have experienced strong planet-planet scatterings and are unlikely to have a habitable planet in its system. Furthermore, it was found that habitable planets surrounding a K or M-star have a higher survival rate than those surrounding a G-star."


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PostPosted: Mon Mar 05, 2018 12:54 pm 
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Habitable Snowballs: Generalizing the Habitable Zone

"Habitable planetary are commonly imagined to be temperate planets like Earth, with areas of open ocean and warm land. In contrast, planets with colder surfaces and permanent snowball states, where oceans are entirely ice-covered, are believed to be inhospitable. However, we show using a general circulation model that terrestrial habitable zone planets are able to support large unfrozen areas of land even while in a snowball state. These unfrozen regions reach summer temperatures in excess of 10 ∘Celsius and develop their own hydrological cycles. Such conditions permit substantial carbon dioxide weathering, allowing these snowballs to become stable climate states, rather than transient as is commonly assumed. Glaciated planets can thus be habitable, which represents a generalization of the habitable zone concept."

The continued importance of habitability studies

"We summarize recent advances in theoretical habitability studies ..."


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PostPosted: Mon Mar 19, 2018 12:54 pm 
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Earth: Atmospheric Evolution of a Habitable Planet

"Our present-day atmosphere is often used as an analog for potentially habitable exoplanets, but Earth's atmosphere has changed dramatically throughout its 4.5 billion year history... Understanding Earth's momentous changes and its enduring habitability is essential as a guide to the diversity of habitable planetary environments that may exist beyond our solar system and for ultimately recognizing spectroscopic fingerprints of life elsewhere in the Universe. Here, we review long-term trends in the composition of Earth's atmosphere as it relates to both planetary habitability and inhabitation."


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PostPosted: Tue Mar 20, 2018 1:18 pm 
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Accretion Processes

Dynamical Evolution of Planetary Systems

Two review articles by Alessandro Morbidelli, one of the big names in planetary science, particularly dynamical studies. Required reading for anyone creating their own worldbuilding system.


Habitability from Tidally-Induced Tectonics

"The stability of Earth's climate on geological timescales is enabled by the carbon-silicate cycle that acts as a negative feedback mechanism stabilizing surface temperatures via the intake and outgas of atmospheric carbon. On Earth, this thermostat is enabled by plate tectonics that sequesters outgassed CO2 back into the mantle via weathering and subduction at convergent margins. Here we propose a separate tectonic mechanism -- vertical recycling -- that can serve as the vehicle for CO2 outgassing and sequestration over long timescales. The mechanism requires continuous tidal heating, which makes it particularly relevant to planets in the habitable zone of M stars."

The methods used are based in part on observations of Io, but there doesn't appear to be an consideration of large moons in the habitable zone.


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PostPosted: Wed Mar 21, 2018 8:23 am 
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Thanks for posting all these links, Thrash. I read along avidly.

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PostPosted: Sat Mar 24, 2018 3:52 pm 
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Detection of the closest Jovian exoplanet in the Epsilon Indi triple system

"We confirm the trend in the radial velocity data for Epsilon Indi A suggesting a long-period planetary companion and find ... Epsilon Indi Ab as a cold Jupiter with a minimum mass of 2.71+2.19−0.44 MJup on a nearly circular orbit with a semi-major axis of 12.82+4.18−0.71 au and an orbital period of 52.62+27.70−4.12 yr... Thus the Epsilon Indi system comprises of at least Epsilon Indi A, Ab as well as a long period brown dwarf binary Ba and Bb; so it provides a benchmark case for our understanding of the formation of gas giants and brown dwarfs."


Gaian bottlenecks and planetary habitability maintained by evolving model biospheres: The ExoGaia model

"We seed multiple model planets with life while their atmospheres are still forming and find that the microbial biospheres are, under suitable conditions, generally able to prevent the host planets from reaching inhospitable temperatures, as would happen on a lifeless planet. We find that the underlying geochemistry plays a strong role in determining long-term habitability prospects of a planet. We find five distinct classes of model planets, including clear examples of 'Gaian bottlenecks' - a phenomenon whereby life either rapidly goes extinct leaving an inhospitable planet, or survives indefinitely maintaining planetary habitability."

This paper uses a highly simplified ("toy") model to look at qualitative behavior of the system, rather than try to make real-world predictions. The five classes are:

Extreme - Planets that never reach habitable temperatures
Doomed - Planets that reach habitable temperatures but are unable to support life.
Critical - Planets that have a higher colonisation success than long-term habitability success.
Bottleneck - Planets that if successfully colonised enjoy long-term habitability.
Abiding - Planets that are always successfully colonised and always have long-term habitability.

Note the ABCDE mnemonic, and that there are cases where planets achieved a biosphere at some point only to have it die out over time.

A key feature that distinguishes the classes (within the context of the model) seems to be the availability of robust geochemical feedback loops, pointing to plate tectonics as a potentially necessary condition for long-term habitability.


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PostPosted: Mon Mar 26, 2018 1:56 pm 
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OOHH, I like the ABCDE categorization. For a simplified world definition that could be very useful! :)

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PostPosted: Tue Mar 27, 2018 4:20 am 
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Formation of Terrestrial Planets

"We present formation models for close-in super-Earths... and for terrestrial exoplanets in gas giant systems. We explain why super-Earth systems cannot form in-situ but rather may be the result of inward gas-driven migration followed by the disruption of compact resonant chains. The Solar System is unlikely to have harbored an early system of super-Earths; rather, Jupiter's early formation may have blocked the ice giants' inward migration. Finally, we present a chain of events that may explain why our Solar System looks different than more than 99% of exoplanet systems."

Another chapter for the same book on Exoplanets as the two Morbidelli articles, above.


A comprehensive understanding of planet formation is required for assessing planetary habitability and for the search for life

"Many parameters and processes influence habitability, ranging from the orbit through detailed composition including volatiles and organics, to the presence of geological activity and plate tectonics. While some properties will soon be directly observable, others cannot be probed by remote sensing for the foreseeable future. Thus, statistical understanding of planetary systems' formation and evolution is a key supplement to the direct measurements of planet properties."

An overview of planetary system formation, with an emphasis on observable processes. Mentions both Proxima Centauri b and the Trappist-1 system as examples.


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