539 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
The atmospheric chemistry of the warm Neptune GJ 3470b: influence of metallicity and temperature on the CH4/CO ratio
Current observation techniques are able to probe the atmosphere of some giant
exoplanets and get some clues about their atmospheric composition. However, the
chemical compositions derived from observations are not fully understood, as
for instance in the case of the CH4/CO abundance ratio, which is often inferred
different from what has been predicted by chemical models. Recently, the warm
Neptune GJ3470b has been discovered and because of its close distance from us
and high transit depth, it is a very promising candidate for follow up
characterisation of its atmosphere. We study the atmospheric composition of
GJ3470b in order to compare with the current observations of this planet, to
prepare the future ones, but also as a typical case study to understand the
chemical composition of warm (sub-)Neptunes. The metallicity of such
atmospheres is totally uncertain, and vary probably to values up to 100x solar.
We explore the space of unknown parameters to predict the range of possible
atmospheric compositions. Within the parameter space explored we find that in
most cases methane is the major carbon-bearing species. We however find that in
some cases, typically for high metallicities with a sufficiently high
temperature the CH4/CO abundance ratio can become lower than unity, as
suggested by some multiwavelength photometric observations of other warm
(sub-)Neptunes, such as GJ1214b and GJ436b. As for the emission spectrum of
GJ3470b, brightness temperatures at infrared wavelengths may vary between 400
and 800K depending on the thermal profile and metallicity. Combined with a hot
temperature profile, a substantial enrichment in heavy elements by a factor of
100 with respect to the solar composition can shift the carbon balance in
favour of carbon monoxide at the expense of CH4. Nevertheless, current
observations of this planet do not allow yet to determine which model is more
accurate.Comment: 12 pages, 8 figures, accepted in Astronomy & Astrophysic
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
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
From Protoplanets to Protolife: The Emergence and Maintenance of Life
Despite great advances in our understanding of the formation of the Solar
System, the evolution of the Earth, and the chemical basis for life, we are not
much closer than the ancient Greeks to an answer of whether life has arisen and
persisted on any other planet. The origin of life as a planetary phenomenon
will probably resist successful explanation as long as we lack an early record
of its evolution and additional examples. It is widely thought that the
geologic record shows that life emerged quickly after the end of prolonged
bombardment of the Earth. New data and simulations contradict that view and
suggest that more than half a billion years of unrecorded Earth history may
have elapsed between the origin of life and LUCA. The impact-driven exchange of
material between the inner planets may have allowed earliest life to be more
cosmopolitan. Indeed, terrestrial life may not have originated on the Earth, or
even on any planet. Smaller bodies, e.g. the parent bodies of primitive
meteorites, offer alternative environments for the origin of life in our Solar
System. The search for past or present life on Mars is an obvious path to
greater enlightenment. The subsurface oceans of some icy satellites of the
outer planets represent the best locales to search for an independent origin of
life in the Solar System because of the high dynamical barriers for transfer,
intense radiation at their surfaces, and thick ice crusts. The ``ultimate''
answer to the abundance of life in the Cosmos will remain the domain of
speculation until we develop observatories capable of detecting habitable
planets - and signs of life - around the nearest million or so stars.Comment: Protostars and Planets V Conference, Hawai
Terrestrial Planet Formation in Extra-Solar Planetary Systems
Terrestrial planets form in a series of dynamical steps from the solid
component of circumstellar disks. First, km-sized planetesimals form likely via
a combination of sticky collisions, turbulent concentration of solids, and
gravitational collapse from micron-sized dust grains in the thin disk midplane.
Second, planetesimals coalesce to form Moon- to Mars-sized protoplanets, also
called "planetary embryos". Finally, full-sized terrestrial planets accrete
from protoplanets and planetesimals. This final stage of accretion lasts about
10-100 Myr and is strongly affected by gravitational perturbations from any gas
giant planets, which are constrained to form more quickly, during the 1-10 Myr
lifetime of the gaseous component of the disk. It is during this final stage
that the bulk compositions and volatile (e.g., water) contents of terrestrial
planets are set, depending on their feeding zones and the amount of radial
mixing that occurs. The main factors that influence terrestrial planet
formation are the mass and surface density profile of the disk, and the
perturbations from giant planets and binary companions if they exist. Simple
accretion models predicts that low-mass stars should form small, dry planets in
their habitable zones. The migration of a giant planet through a disk of rocky
bodies does not completely impede terrestrial planet growth. Rather, "hot
Jupiter" systems are likely to also contain exterior, very water-rich
Earth-like planets, and also "hot Earths", very close-in rocky planets. Roughly
one third of the known systems of extra-solar (giant) planets could allow a
terrestrial planet to form in the habitable zone.Comment: 19 pages, 5 figures. To appear in the proceedings of IAU Symposium
249: Exoplanets: Detection, Formation and Dynamics, held in Suzhou, China,
Oct 22-26 200
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
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