23 research outputs found
Hydrohalite Salt-albedo Feedback Could Cool M-dwarf Planets
A possible surface type that may form in the environments of M-dwarf planets
is sodium chloride dihydrate, or "hydrohalite" (NaCl 2HO), which
can precipitate in bare sea ice at low temperatures. Unlike salt-free water
ice, hydrohalite is highly reflective in the near-infrared, where M-dwarf stars
emit strongly, making the effect of the interaction between hydrohalite and the
M-dwarf SED necessary to quantify. We carried out the first exploration of the
climatic effect of hydrohalite-induced salt-albedo feedback on extrasolar
planets, using a three-dimensional global climate model. Under fixed CO
conditions, rapidly-rotating habitable-zone M-dwarf planets receiving 65% or
less of the modern solar constant from their host stars exhibit cooler
temperatures when an albedo parameterization for hydrohalite is included in
climate simulations, compared to simulations without such a parameterization.
Differences in global mean surface temperature with and without this
parameterization increase as the instellation is lowered, which may increase
CO build-up requirements for habitable conditions on planets with active
carbon cycles. Synchronously-rotating habitable-zone M-dwarf planets appear
susceptible to salt-albedo feedback at higher levels of instellation (90% or
less of the modern solar constant) than planets with Earth-like rotation
periods, due to their cooler minimum day-side temperatures. These instellation
levels where hydrohalite seems most relevant correspond to several
recently-discovered potentially habitable M-dwarf planets, including Proxima
Centauri b, TRAPPIST-1e, and LHS 1140b, making an albedo parameterization for
hydrohalite of immediate importance in future climate simulations.Comment: 12 pages, 4 figures, 1 table, accepted for publication in the
Astrophysical Journa
Habitability and Water Loss Limits on Eccentric Planets Orbiting Main Sequence Stars
A planet's climate can be strongly affected by its orbital eccentricity and
obliquity. Here we use a 1-dimensional energy balance model modified to include
a simple runaway greenhouse (RGH) parameterization to explore the effects of
these two parameters on the climate of Earth-like aqua planets - completely
ocean-covered planets - orbiting F-, G-, K-, and M-dwarf stars. We find that
the range of instellations for which planets exhibit habitable surface
conditions throughout an orbit decreases with increasing eccentricity. However,
the appearance of temporarily habitable conditions during an orbit creates an
eccentric habitable zone (EHZ) that is sensitive to orbital eccentricity and
obliquity, planetary latitude, and host star spectral type. We find that the
fraction of a planet's orbit over which it exhibits habitable surface
conditions is larger on eccentric planets orbiting M-dwarf stars, due to the
lower broadband planetary albedos of these planets. Planets with larger
obliquities have smaller EHZs, but exhibit warmer climates if they do not enter
a snowball state during their orbits. We also find no transient runaway
greenhouse state on planets at all eccentricities. Rather, planets spend their
entire orbits either in a RGH or not. For G-dwarf planets receiving 100% of the
modern solar constant and with eccentricities above 0.55, an entire Earth ocean
inventory can be lost in 3.6 Gyr. M-dwarf planets, due to their larger incident
XUV flux, can become desiccated in only 690 Myr with eccentricities above 0.38.
This work has important implications for eccentric planets that may exhibit
surface habitability despite technically departing from the traditional
habitable zone as they orbit their host stars.Comment: 22 pages, 9 figures, 2 tables, accepted for publication in the
Astrophysical Journa
Energy Budgets for Terrestrial Extrasolar Planets
The pathways through which incoming energy is distributed between the surface
and atmosphere has been analyzed for the Earth. However, the effect of the
spectral energy distribution of a host star on the energy budget of an orbiting
planet may be significant given the wavelength-dependent absorption properties
of atmospheric CO2 and water vapor, and surface ice and snow. We have
quantified the flow of energy on aqua planets orbiting M-, G-, and F-dwarf
stars, using a 3D Global Climate Model with a static ocean. The atmosphere and
surface of an M-dwarf planet receiving an instellation equal to 88% of the
modern solar constant at the top of the atmosphere absorb 12% more incoming
stellar radiation than those of a G-dwarf planet receiving 100% of the modern
solar constant, and 17% more radiation than a F-dwarf planet receiving 108% of
the modern solar constant, resulting in climates similar to modern-day Earth on
all three planets, assuming a 24-hr rotation period and fixed CO2. At 100%
instellation, a synchronously-rotating M-dwarf planet exhibits smaller flux
absorption in the atmosphere and on the surface of the dayside, and a dayside
mean surface temperature that is 37 K colder than its rapidly-rotating
counterpart. Energy budget diagrams are included to illustrate the variations
in global energy budgets as a function of host star spectral class, and can
contribute to habitability assessments of planets as they are discovered.Comment: 10 pages, 3 figures, 2 tables. Accepted for publication in The
Astrophysical Journal Letter
The effect of land fraction and host star spectral energy distribution on the planetary albedo of terrestrial worlds
The energy balance and climate of planets can be affected by the reflective properties of their land, ocean, and frozen surfaces. Here we investigate the effect of host star spectral energy distribution (SED) on the albedo of these surfaces using a one-dimensional (1-D) energy balance model (EBM). Incorporating spectra of M-, K-, G- and F-dwarf stars, we determined the effect of varying fractional and latitudinal distribution of land and ocean surfaces as a function of host star SED on the overall planetary albedo, climate, and ice-albedo feedback response. While noting that the spatial distribution of land masses on a given planet will have an effect on the overall planetary energy balance, we find that terrestrial planets with higher average land/ocean fractions are relatively cooler and have higher albedo regardless of star type. For Earth-like planets orbiting M-dwarf stars the increased absorption of water ice in the near-infrared (NIR), where M-dwarf stars emit much of their energy, resulted in warmer global mean surface temperatures, ice lines at higher latitudes, and increased climate stability as the ice-albedo feedback became negative at high land fractions. Conversely, planets covered largely by ocean, and especially those orbiting bright stars, had a considerably different energy balance due to the contrast between the reflective land and the absorptive ocean surface, which in turn resulted in warmer average surface temperatures than land-covered planets and a stronger potential ice-albedo feedback. While dependent on the properties of individual planetary systems, our results place so constraints on a range of climate states of terrestrial exoplanets based on albedo and incident flux
Terminator Habitability: the Case for Limited Water Availability on M-dwarf Planets
Rocky planets orbiting M-dwarf stars are among the most promising and
abundant astronomical targets for detecting habitable climates. Planets in the
M-dwarf habitable zone are likely synchronously rotating, such that we expect
significant day-night temperature differences, and potentially limited
fractional habitability. Previous studies have focused on scenarios where
fractional habitability is confined to the substellar or "eye" region, but in
this paper we explore the possibility of planets with terminator habitability,
defined by the existence of a habitable band at the transition between a
scorching dayside and a glacial nightside. Using a global climate model, we
show that for water-limited planets it is possible to have scorching
temperatures in the "eye" and freezing temperatures on the nightside, while
maintaining a temperate climate in the terminator region, due to a reduced
atmospheric energy transport. Whereas on water-rich planets, increasing stellar
flux leads to increased atmospheric energy transport and a reduction in
day-night temperature differences, such that the terminator does not remain
habitable once the dayside temperatures approach runaway or moist greenhouse
limits. We also show that, while water-abundant simulations may result in
larger fractional habitability, they are vulnerable to water loss through
cold-trapping on the nightside surface or atmospheric water vapor escape,
suggesting that even if planets were formed with abundant water, their climates
could become water-limited and subject to terminator habitability.Comment: 16 pages, 9 figures, 3 tables, Accepted for publication in The
Astronomical Journa
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
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The Spitzer Bibliography Database: Bibliographic Statistics
The Spitzer Science Center maintains a database of peer-refereed publications utilizing observations made by the Spitzer Space Telescope5. Originally intended as a way to easily track these publications with limited resources, the database has grown in scope to provide more services for investigators. The design and population of the system and some interesting insights into the use of Spitzer data are presented
The Effect of Orbital Configuration on the Possible Climates and Habitability of Kepler-62f
Abstract As lower-mass stars often host multiple rocky planets, gravitational interactions among planets can have significant effects on climate and habitability over long timescales. Here we explore a specific case, Kepler-62f (Borucki et al., 2013), a potentially habitable planet in a five-planet system with a K2V host star. N-body integrations reveal the stable range of initial eccentricities for Kepler-62f is 0.00 ≤ e ≤ 0.32, absent the effect of additional, undetected planets. We simulate the tidal evolution of Kepler-62f in this range and find that, for certain assumptions, the planet can be locked in a synchronous rotation state. Simulations using the 3-D Laboratoire de Météorologie Dynamique (LMD) Generic global climate model (GCM) indicate that the surface habitability of this planet is sensitive to orbital configuration. With 3 bar of CO2 in its atmosphere, we find that Kepler-62f would only be warm enough for surface liquid water at the upper limit of this eccentricity range, providing it has a high planetary obliquity (between 60° and 90°). A climate similar to that of modern-day Earth is possible for the entire range of stable eccentricities if atmospheric CO2 is increased to 5 bar levels. In a low-CO2 case (Earth-like levels), simulations with version 4 of the Community Climate System Model (CCSM4) GCM and LMD Generic GCM indicate that increases in planetary obliquity and orbital eccentricity coupled with an orbital configuration that places the summer solstice at or near pericenter permit regions of the planet with above-freezing surface temperatures. This may melt ice sheets formed during colder seasons. If Kepler-62f is synchronously rotating and has an ocean, CO2 levels above 3 bar would be required to distribute enough heat to the nightside of the planet to avoid atmospheric freeze-out and permit a large enough region of open water at the planet's substellar point to remain stable. Overall, we find multiple plausible combinations of orbital and atmospheric properties that permit surface liquid water on Kepler-62f. Key Words: Extrasolar planets—Habitability—Planetary environments. Astrobiology 16, 443–464