2,619 research outputs found
Why do we find ourselves around a yellow star instead of a red star?
M-dwarf stars are more abundant than G-dwarf stars, so our position as
observers on a planet orbiting a G-dwarf raises questions about the suitability
of other stellar types for supporting life. If we consider ourselves as
typical, in the anthropic sense that our environment is probably a typical one
for conscious observers, then we are led to the conclusion that planets
orbiting in the habitable zone of G-dwarf stars should be the best place for
conscious life to develop. But such a conclusion neglects the possibility that
K-dwarfs or M-dwarfs could provide more numerous sites for life to develop,
both now and in the future. In this paper we analyze this problem through
Bayesian inference to demonstrate that our occurrence around a G-dwarf might be
a slight statistical anomaly, but only the sort of chance event that we expect
to occur regularly. Even if M-dwarfs provide more numerous habitable planets
today and in the future, we still expect mid G- to early K-dwarfs stars to be
the most likely place for observers like ourselves. This suggests that
observers with similar cognitive capabilities as us are most likely to be found
at the present time and place, rather than in the future or around much smaller
stars.Comment: Published in International Journal of Astrobiolog
The Effect of Land Albedo on the Climate of Land-Dominated Planets in the TRAPPIST-1 System
Variations in the reflective properties of the bulk material that comprises
the surface of land-dominated planets will affect the planetary energy balance
by interacting differently with incident radiation from the host star.
Furthermore, low-mass cool stars, such as nearby M8V dwarf TRAPPIST-1, emit a
significant fraction of their flux in longer wavelengths relative to the Sun in
regions where terrestrial materials may exhibit additional variability in
albedo. Using the Community Earth System Model (CESM) we investigate the effect
of the composition of the land surface and its albedo on planetary climate in
the context of spatially homogeneous, entirely land-covered planets with dry
atmospheres at the orbital separation of TRAPPIST-1d, TRAPPIST-1e, and
TRAPPIST-1f. We use empirically derived spectra of four terrestrial
compositional endmembers (granite, calcite, aridisol, and dune sand) and a
composite spectrum of TRAPPIST-1 for these simulations and compare these model
output to an aquaplanet and several Sol-spectrum control cases. We report a
difference of approximately 50 K in global mean surface temperature, variations
in atmospheric rotational features, and a reduction in cross-equatorial heat
transport between scenarios in which materials with higher albedo in the
infrared (calcite and dune sand) were used and those with more absorptive
crustal material, such as granite or dry soils. An aquaplanet TRAPPIST-1d
scenario results in an unstable runaway greenhouse regime. Therefore, we
demonstrate that determining the composition and albedo of continental
landmasses is crucial for making accurate determinations of the climate of
terrestrial exoplanets.Comment: 18 pages, 11 figures. Accepted for publication in the Astrophysical
Journal (ApJ
Variation in Vital-rate Sensitivity Between Populations of Texas Horned Lizards
Demographic studies of imperiled populations can aid managers in planning conservation actions. However, applicability of findings for a single population across a species’ range is sometimes questionable. We conducted long-term studies (8 and 9 years, respectively) of 2 populations of the lizard Phrynosoma cornutum separated by 1000 km within the historical distribution of the species. The sites were a 15-ha urban wildlife reserve on Tinker Air Force Base (TAFB) in central Oklahoma and a 6000-ha wildland site in southern Texas, the Chaparral Wildlife Management Area (CWMA). We predicted a trade-off between the effect of adult survival and fecundity on population growth rate (λ), leading to population-specific contributions of individual vital rates to λ and individualized strategies for conservation and management of this taxon. The CWMA population had lower adult survival and higher fecundity than TAFB. As predicted, there was a trade-off in the effects of adult survival and fecundity on λ between the two sites; fecundity affected λ more at CWMA than at TAFB. However, adult survival had the smallest effect on λ in both populations. We found that recruitment in P. cornutum most affected λ at both sites, with hatchling survival having the strongest influence on λ. Management strategies focusing on hatchling survival would strongly benefit both populations. As a consequence, within the constraint of the need to more accurately estimate hatchling survival, managers across the range of species such as P. cornutum could adopt similar management priorities with respect to stage classes, despite intra-population differences in population vital rates
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