94 research outputs found
The radius anomaly in the planet/brown dwarf overlapping mass regime
The recent detection of the transit of very massive substellar companions
(CoRoT-3b, Deleuil et al. 2008; CoRoT-15b, Bouchy et al. 2010; WASP-30b,
Anderson et al. 2010; Hat-P-20b, Bakos et al. 2010) provides a strong
constraint to planet and brown dwarf formation and migration mechanisms.
Whether these objects are brown dwarfs originating from the gravitational
collapse of a dense molecular cloud that, at the same time, gave birth to the
more massive stellar companion, or whether they are planets that formed through
core accretion of solids in the protoplanetary disk can not always been
determined unambiguously and the mechanisms responsible for their short orbital
distances are not yet fully understood.
In this contribution, we examine the possibility to constrain the nature of a
massive substellar object from the various observables provided by the
combination of Radial Velocity and Photometry measurements (e.g. M_p, R_p, M_s,
Age, a, e...).
In a second part, developments in the modeling of tidal evolution at high
eccentricity and inclination - as measured for HD 80 606 with e=0.9337 (Naef et
al. 2001), XO-3 with a stellar obliquity >37.3+-3.7 deg (H\'ebrard et al. 2008;
Winn et al. 2009) and several other exoplanets - are discussed along with their
implication in the understanding of the radius anomaly problem of extrasolar
giant planets.Comment: Proceedings of the conference: "Detection and dynamics of transiting
exoplanets" held at the OHP, 23-27 August 2010. 7 pages, 3 figure
Understanding exoplanet formation, structure and evolution in 2010
In this short review, we summarize our present understanding (and
non-understanding) of exoplanet formation, structure and evolution, in the
light of the most recent discoveries. Recent observations of transiting massive
brown dwarfs seem to remarkably confirm the predicted theoretical mass-radius
relationship in this domain. This mass-radius relationship provides, in some
cases, a powerful diagnostic to distinguish planets from brown dwarfs of same
mass, as for instance for Hat-P-20b. If confirmed, this latter observation
shows that planet formation takes place up to at least 8 Jupiter masses.
Conversely, observations of brown dwarfs down to a few Jupiter masses in young,
low-extinction clusters strongly suggest an overlapping mass domain between
(massive) planets and (low-mass) brown dwarfs, i.e. no mass edge between these
two distinct (in terms of formation mechanism) populations. At last, the large
fraction of heavy material inferred for many of the transiting planets confirms
the core-accretion scenario as been the dominant one for planet formation.Comment: Invited review, IAU Symposium No. 276, The Astrophysics of Planetary
Systems: Formation, Structure, and Dynamical Evolutio
A generic frequency dependence for the atmospheric tidal torque of terrestrial planets
Thermal atmospheric tides have a strong impact on the rotation of terrestrial
planets. They can lock these planets into an asynchronous rotation state of
equilibrium. We aim at characterizing the dependence of the tidal torque
resulting from the semidiurnal thermal tide on the tidal frequency, the planet
orbital radius, and the atmospheric surface pressure. The tidal torque is
computed from full 3D simulations of the atmospheric climate and mean flows
using a generic version of the LMDZ general circulation model (GCM) in the case
of a nitrogen-dominated atmosphere. Numerical results are discussed with the
help of an updated linear analytical framework. Power scaling laws governing
the evolution of the torque with the planet orbital radius and surface pressure
are derived. The tidal torque exhibits i) a thermal peak in the vicinity of
synchronization, ii) a resonant peak associated with the excitation of the Lamb
mode in the high frequency range, and iii) well defined frequency slopes
outside these resonances. These features are well explained by our linear
theory. Whatever the star-planet distance and surface pressure, the torque
frequency spectrum -- when rescaled with the relevant power laws -- always
presents the same behaviour. This allows us to provide a single and easily
usable empirical formula describing the atmospheric tidal torque over the whole
parameter space. With such a formula, the effect of the atmospheric tidal
torque can be implemented in evolutionary models of the rotational dynamics of
a planet in a computationally efficient, and yet relatively accurate way.Comment: Accepted for publication in Astronomy & Astrophysics, 23 pages, 9
figure
Toward a multidimensional analysis of transmission spectroscopy. Part III: Modelling 2D effects in retrievals with TauREx
New-generation spectrographs dedicated to the study of exoplanetary
atmospheres require a high accuracy in the atmospheric models to better
interpret the input spectra. Thanks to space missions, the observed spectra
will cover a large wavelength range from visible to mid-infrared with an higher
precision compared to the old-generation instrumentation, revealing complex
features coming from different regions of the atmosphere. For hot and ultra hot
Jupiters (HJs and UHJs), the main source of complexity in the spectra comes
from thermal and chemical differences between the day and the night sides. In
this context, one-dimensional plane parallel retrieval models of atmospheres
may not be suitable to extract the complexity of such spectra. In addition,
Bayesian frameworks are computationally intensive and prevent us from using
complete three-dimensional self-consistent models to retrieve exoplanetary
atmospheres. We propose the TauREx 2D retrieval code, which uses
two-dimensional atmospheric models as a good compromise between computational
cost and model accuracy to better infer exoplanetary atmospheric
characteristics for the hottest planets. TauREx 2D uses a 2D parametrization
across the limb which computes the transmission spectrum from an exoplanetary
atmosphere assuming azimuthal symmetry. It also includes a thermal dissociation
model of various species. We demonstrate that, given an input observation,
TauREx 2D mitigates the biases between the retrieved atmospheric parameters and
the real atmospheric parameters. We also show that having a prior knowledge on
the link between local temperature and composition is instrumental in inferring
the temperature structure of the atmosphere. Finally, we apply such a model on
a synthetic spectrum computed from a GCM simulation of WASP-121b and show how
parameter biases can be removed when using two-dimensional forward models
across the limb.Comment: 16 pages, 16 figures. Accepted for publication in Astronomy &
Astrophysic
Differences in Water Vapor Radiative Transfer among 1D Models Can Significantly Affect the Inner Edge of the Habitable Zone
An accurate estimate of the inner edge of the habitable zone is critical for determining which exoplanets are potentially habitable and for designing future telescopes to observe them. Here, we explore differences in estimating the inner edge among seven one-dimensional radiative transfer models: two line-by-line codes (SMART and LBLRTM) as well as five band codes (CAM3, CAM4_Wolf, LMDG, SBDART, and AM2) that are currently being used in global climate models. We compare radiative fluxes and spectra in clear-sky conditions around G and M stars, with fixed moist adiabatic profiles for surface temperatures from 250 to 360 K. We find that divergences among the models arise mainly from large uncertainties in water vapor absorption in the window region (10 μm) and in the region between 0.2 and 1.5 μm. Differences in outgoing longwave radiation increase with surface temperature and reach 10–20 W m^(−2); differences in shortwave reach up to 60 W m^(−2), especially at the surface and in the troposphere, and are larger for an M-dwarf spectrum than a solar spectrum. Differences between the two line-by-line models are significant, although smaller than among the band models. Our results imply that the uncertainty in estimating the insolation threshold of the inner edge (the runaway greenhouse limit) due only to clear-sky radiative transfer is ≈10% of modern Earth's solar constant (i.e., ≈34 W m^(−2) in global mean) among band models and ≈3% between the two line-by-line models. These comparisons show that future work is needed that focuses on improving water vapor absorption coefficients in both shortwave and longwave, as well as on increasing the resolution of stellar spectra in broadband models
Layered convection as the origin of Saturn's luminosity anomaly
As they keep cooling and contracting, Solar System giant planets radiate more
energy than they receive from the Sun. Applying the first and second principles
of thermodynamics, one can determine their cooling rate, luminosity, and
temperature at a given age. Measurements of Saturn's infrared intrinsic
luminosity, however, reveal that this planet is significantly brighter than
predicted for its age. This excess luminosity is usually attributed to the
immiscibility of helium in the hydrogen-rich envelope, leading to "rains" of
helium-rich droplets. Existing evolution calculations, however, suggest that
the energy released by this sedimentation process may not be sufficient to
resolve the puzzle. Here, we demonstrate using planetary evolution models that
the presence of layered convection in Saturn's interior, generated, like in
some parts of Earth oceans, by the presence of a compositional gradient,
significantly reduces its cooling. It can explain the planet's present
luminosity for a wide range of configurations without invoking any additional
source of energy. This suggests a revision of the conventional homogeneous
adiabatic interior paradigm for giant planets, and questions our ability to
assess their heavy element content. This reinforces the possibility for layered
convection to help explaining the anomalously large observed radii of
extrasolar giant planets.Comment: Published in Nature Geoscience. Online publication date: April 21st,
2013. Accepted version before journal editing and with Supplementary
Informatio
- …