254 research outputs found
Characterizing Habitable Extrasolar Planets using Spectral Fingerprints
The detection and characterization of Earth-like planet is approaching
rapidly thanks to radial velocity surveys (HARPS), transit searches (Corot,
Kepler) and space observatories dedicated to their characterization are already
in development phase (James Webb Space Telescope), large ground based
telescopes (ELT, TNT, GMT), and dedicated space-based missions like Darwin,
Terrestrial Planet Finder, New World Observer). In this paper we discuss how we
can read a planets spectrum to assess its habitability and search for the
signatures of a biosphere. Identifying signs of life implies understanding how
the observed atmosphere physically and chemically works and thus to gather
information on the planet in addition to the observing its spectral
fingerprint.Comment: 14pg, 4 figures, Accepted CRAS (Proceedings of the National Academy
of Science France), Palevol serie
3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability and habitability
The inner edge of the classical habitable zone is often defined by the
critical flux needed to trigger the runaway greenhouse instability. This 1D
notion of a critical flux, however, may not be so relevant for inhomogeneously
irradiated planets, or when the water content is limited (land planets).
Here, based on results from our 3D global climate model, we find that the
circulation pattern can shift from super-rotation to stellar/anti stellar
circulation when the equatorial Rossby deformation radius significantly exceeds
the planetary radius. Using analytical and numerical arguments, we also
demonstrate the presence of systematic biases between mean surface temperatures
or temperature profiles predicted from either 1D or 3D simulations.
Including a complete modeling of the water cycle, we further demonstrate that
for land planets closer than the inner edge of the classical habitable zone,
two stable climate regimes can exist. One is the classical runaway state, and
the other is a collapsed state where water is captured in permanent cold traps.
We identify this "moist" bistability as the result of a competition between the
greenhouse effect of water vapor and its condensation. We also present
synthetic spectra showing the observable signature of these two states.
Taking the example of two prototype planets in this regime, namely Gl581c and
HD85512b, we argue that they could accumulate a significant amount of water ice
at their surface. If such a thick ice cap is present, gravity driven ice flows
and geothermal flux should come into play to produce long-lived liquid water at
the edge and/or bottom of the ice cap. Consequently, the habitability of
planets at smaller orbital distance than the inner edge of the classical
habitable zone cannot be ruled out. Transiting planets in this regime represent
promising targets for upcoming observatories like EChO and JWST.Comment: Accepted for publication in Astronomy and Astrophysics, complete
abstract in the pdf, 18 pages, 18 figure
Habitable planets around the star Gl 581?
Radial velocity surveys are now able to detect terrestrial planets at
habitable distance from M-type stars. Recently, two planets with minimum masses
below 10 Earth masses were reported in a triple system around the M-type star
Gliese 581. Using results from atmospheric models and constraints from the
evolution of Venus and Mars, we assess the habitability of planets Gl 581c and
Gl 581d and we discuss the uncertainties affecting the habitable zone (HZ)
boundaries determination. We provide simplified formulae to estimate the HZ
limits that may be used to evaluate the astrobiological potential of
terrestrial exoplanets that will hopefully be discovered in the near future.
Planets Gl 581c and 'd' are near, but outside, what can be considered as the
conservative HZ. Planet 'c' receives 30% more energy from its star than Venus
from the Sun, with an increased radiative forcing caused by the spectral energy
distribution of Gl 581. Its habitability cannot however be positively ruled out
by theoretical models due to uncertainties affecting cloud properties.
Irradiation conditions of planet 'd' are comparable with those of early Mars.
Thanks to the warming effect of CO2-ice clouds planet 'd' might be a better
candidate for the first exoplanet known to be potentially habitable. A mixture
of various greenhouse gases could also maintain habitable conditions on this
planet.Comment: Astronomy and Astrophysics (2007) accepted for publicatio
Gaseous Planets, Protostars And Young Brown Dwarfs : Birth And Fate
We review recent theoretical progress aimed at understanding the formation
and the early stages of evolution of giant planets, low-mass stars and brown
dwarfs. Calculations coupling giant planet formation, within a modern version
of the core accretion model, and subsequent evolution yield consistent
determinations of the planet structure and evolution. Because of the
uncertainties in the initial conditions, however, it is not possible to say
whether young planets are faint or bright compared with low-mass young brown
dwarfs. We review the effects of irradiation and evaporation on the evolution
of short period planets and argue that substantial mass loss may have occurred
for these objects. Concerning star formation, geometrical effects in protostar
core collapse are examined by comparing 1D and 3D calculations. Spherical
collapse is shown to overestimate the core inner density and temperature and
thus to yield incorrect initial conditions for PMS or young brown dwarf
evolution. Accretion is also shown to occur over a very limited fraction of the
protostar surface. Accretion affects the evolution of young brown dwarfs and
yields more compact structures for a given mass and age, thus fainter
luminosities. This can lead to severe misinterpretations of the mass and/or age
of young accreting objects from their location in the HR diagram. We argue that
newborn stars and brown dwarfs should appear rapidly over an extended area in
the HR diagram, depending on their accretion history, rather than on a well
defined birth line. Finally, we suggest that the distinction between planets
and brown dwarfs be based on an observational diagnostic, reflecting the
different formation mechanisms between these two distinct populations, rather
than on an arbitrary, confusing definition.Comment: Invited Review, Protostars and Planets V (Hawai, October 2005
Primary and secondary eclipse spectroscopy with JWST: exploring the exoplanet parameter space
Eclipse exoplanet spectroscopy has yielded detection of H_2O, CH_4, CO_2 and
CO in the atmosphere of hot jupiters and neptunes. About 40 large terrestrial
planets are announced or confirmed, two of which are transiting, and another
deemed habitable. Hence the potential for eclipse spectroscopy of terrestrial
planets with James Webb Space Telescope (JWST) has become an active field of
study. We explore the parameter space (type of stars, planet orbital periods
and types, and instruments/wavelengths) in terms of the signal-to-noise ratio
(S/N) achievable on the detection of spectroscopic features. We use analytic
formula and model data for both the astrophysical scene and the instrument, to
plot S/N contour maps, while indicating how the S/N scales with the fixed
parameters. We systematically compare stellar photon noise-only figures with
ones including detailed instrumental and zodiacal noises. Likelihood of
occurring targets is based both on model and catalog star population of the
solar neighborhood. The 9.6 micron ozone band is detectable (S/N = 3) with
JWST, for a warm super-earth 6.7 pc away, using ~2% of the 5-year nominal
mission time (summing observations, M4V and lighter host star for primary
eclipses, M5V for secondary). If every star up to this mass limit and distance
were to host a habitable planet, there should be statistically ~1 eclipsing
case. Investigation of systematic noises in the co-addition of 5 years worth-,
tens of days separated-, hours-long observations is critical, complemented by
dedicated characterisation of the instruments, currently in integration phase.
The census of nearby transiting habitable planets must be complete before the
beginning of science operations.Comment: Accepted for publication in A&A, 16 pages, 19 figure
The effect of rotation and tidal heating on the thermal lightcurves of Super Mercuries
Short period (<50 days) low-mass (<10Mearth) exoplanets are abundant and the
few of them whose radius and mass have been measured already reveal a diversity
in composition. Some of these exoplanets are found on eccentric orbits and are
subjected to strong tides affecting their rotation and resulting in significant
tidal heating. Within this population, some planets are likely to be depleted
in volatiles and have no atmosphere. We model the thermal emission of these
"Super Mercuries" to study the signatures of rotation and tidal dissipation on
their infrared light curve. We compute the time-dependent temperature map at
the surface and in the subsurface of the planet and the resulting
disk-integrated emission spectrum received by a distant observer for any
observation geometry. We calculate the illumination of the planetary surface
for any Keplerian orbit and rotation. We include the internal tidal heat flow,
vertical heat diffusion in the subsurface and generate synthetic light curves.
We show that the different rotation periods predicted by tidal models
(spin-orbit resonances, pseudo-synchronization) produce different photometric
signatures, which are observable provided that the thermal inertia of the
surface is high, like that of solid or melted rocks (but not regolith). Tidal
dissipation can also directly affect the light curves and make the inference of
the rotation more difficult or easier depending on the existence of hot spots
on the surface. Infrared light curve measurement with the James Webb Space
Telescope and EChO can be used to infer exoplanets' rotation periods and
dissipation rates and thus to test tidal models. This data will also constrain
the nature of the (sub)surface by constraining the thermal inertia.Comment: 15 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
Photometric Variability of the Disk Integrated Infrared Emission of the Earth
We present an analysis of the global-integrated mid-infrared emission flux of
the Earth based on data derived from satellite measurements. We have studied
the photometric annual, seasonal, and rotational variability of the thermal
emission of the Earth to determine which properties can be inferred from the
point-like signal. We find that the analysis of the time series allows us to
determine the 24 hr rotational period of the planet for most observing
geometries, due to large warm and cold areas, identified with geographic
features, which appear consecutively in the observer's planetary view. However,
the effects of global-scale meteorology can effectively mask the rotation for
several days at a time. We also find that orbital time series exhibit a
seasonal modulation, whose amplitude depends strongly on the latitude of the
observer but weakly on its ecliptic longitude. As no systematic difference of
brightness temperature is found between the dayside and nightside, the phase
variations of the Earth in the infrared range are negligible. Finally, we also
conclude that the phase variation of a spatially unresolved Earth-Moon system
is dominated by the lunar signal.Comment: 11 pages, 10 figures. Accepted for publication in Ap
Is Gliese 581d habitable? Some constraints from radiative-convective climate modeling
The recently discovered exoplanet Gl581d is extremely close to the outer edge
of its system's habitable zone, which has led to much speculation on its
possible climate. We have performed a range of simulations to assess whether,
given simple combinations of chemically stable greenhouse gases, the planet
could sustain liquid water on its surface. For best estimates of the surface
gravity, surface albedo and cloud coverage, we find that less than 10 bars of
CO2 is sufficient to maintain a global mean temperature above the melting point
of water. Furthermore, even with the most conservative choices of these
parameters, we calculate temperatures above the water melting point for CO2
partial pressures greater than about 40 bar. However, we note that as Gl581d is
probably in a tidally resonant orbit, further simulations in 3D are required to
test whether such atmospheric conditions are stable against the collapse of CO2
on the surface.Comment: 9 pages, 11 figures. Accepted for publication in Astronomy &
Astrophysic
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