154 research outputs found
Constraining the volatile fraction of planets from transit observations
The determination of the abundance of volatiles in extrasolar planets is very
important as it can provide constraints on transport in protoplanetary disks
and on the formation location of planets. However, constraining the internal
structure of low-mass planets from transit measurements is known to be a
degenerate problem. Using planetary structure and evolution models, we show how
observations of transiting planets can be used to constrain their internal
composition, in particular the amount of volatiles in the planetary interior,
and consequently the amount of gas (defined in this paper to be only H and He)
that the planet harbors. We show for low-mass gas-poor planets that are located
close to their central star that assuming evaporation has efficiently removed
the entire gas envelope, it is possible to constrain the volatile fraction of
close-in transiting planets. We illustrate this method on the example of 55 Cnc
e and show that under the assumption of the absence of gas, the measured mass
and radius imply at least 20 % of volatiles in the interior. For planets at
larger distances, we show that the observation of transiting planets at
different evolutionary ages can be used to set statistical constraints on the
volatile content of planets. These results can be used in the context of future
missions like PLATO to better understand the internal composition of planets.Comment: accepted in Astronomy and Astrophysic
On the radius of habitable planets
The conditions that a planet must fulfill to be habitable are not precisely
known. However, it is comparatively easier to define conditions under which a
planet is very likely not habitable. Finding such conditions is important as it
can help select, in an ensemble of potentially observable planets, which ones
should be observed in greater detail for characterization studies. Assuming, as
in the Earth, that the presence of a C-cycle is a necessary condition for
long-term habitability, we derive, as a function of the planetary mass, a
radius above which a planet is likely not habitable. We compute the maximum
radius a planet can have to fulfill two constraints: surface conditions
compatible with the existence of liquid water, and no ice layer at the bottom
of a putative global ocean. We demonstrate that, above a given radius, these
two constraints cannot be met. We compute internal structure models of planets,
using a five-layer model (core, inner mantle, outer mantle, ocean, and
atmosphere), for different masses and composition of the planets (in
particular, the Fe/Si ratio of the planet). Our results show that for planets
in the Super-Earth mass range (1-12 Mearth), the maximum that a planet, with a
composition similar to that of the Earth, can have varies between 1.7 and 2.2
Rearth. This radius is reduced when considering planets with higher Fe/Si
ratios and taking radiation into account when computing the gas envelope
structure. These results can be used to infer, from radius and mass
determinations using high-precision transit observations like those that will
soon be performed by the CHaracterizing ExOPlanet Satellite (CHEOPS), which
planets are very likely not habitable, and therefore which ones should be
considered as best targets for further habitability studies.}Comment: 8 pages, 5 figures, accepted in Astronomy and Astrophysic
The maximum mass of planetary embryos formed in core-accretion models
We compute the maximum mass a growing planetary embryo can reach depending on
the size of accreted planetesimals or pebbles, to infer the possibility of
growing the cores of giant planets, and giant planets themselves. We compute
the internal structure of the gas envelope of planetary embryos, to determine
the core mass that is necessary to bind an envelope large enough to destroy
planetesimals or pebbles while they are gravitationally captured. We also
consider the effect of the advection wind originating from the protoplanetary
disk, following the results of Ormel et al. (2015). We show that for low mass
pebbles, once the planetary embryo is larger than ~1 Mearth, the envelope is
large enough to destroy and vaporize pebbles completely before they can reach
the core. The material constituting pebbles is therefore released in the
planetary envelope, and later on dispersed in the protoplanetary disk, if the
advection wind is strong enough. As a consequence the growth of the planetary
embryo is stopped at a mass that is so small that Kelvin-Helmholtz accretion
cannot lead to the accretion of significant amounts of gas. For larger
planetesimals, a similar process occurs but at much larger mass, of the order
of ten Earth masses, and is followed by rapid accretion of gas. If the effect
of the advection is as efficient as described in Ormel al. (2015), the combined
effect of the vaporization of accreted solids in the envelope of forming
planetary embryos, and of this advection wind, prevents the growth of the
planets at masses smaller or similar to the Earth mass in the case of formation
by pebble accretion, up to a distance of the order of 10 AU. In the case of
formation by accretion of large mass planetesimals, the growth of the planetary
core is limited at masses ~10 Mearth but further growth of the planet can
proceed by gas accretion.Comment: accepted in Astronomy and Astrophysic
Modeling the Jovian subnebula: II - Composition of regular satellites ices
We use the evolutionary turbulent model of Jupiter's subnebula described by
Alibert et al. (2005a) to constrain the composition of ices incorporated in its
regular icy satellites. We consider CO2, CO, CH4, N2, NH3, H2S, Ar, Kr, and Xe
as the major volatile species existing in the gas-phase of the solar nebula.
All these volatile species, except CO2 which crystallized as a pure condensate,
are assumed to be trapped by H2O to form hydrates or clathrate hydrates in the
solar nebula. Once condensed, these ices were incorporated into the growing
planetesimals produced in the feeding zone of proto-Jupiter. Some of these
solids then flowed from the solar nebula to the subnebula, and may have been
accreted by the forming Jovian regular satellites. We show that ices embedded
in solids entering at early epochs into the Jovian subdisk were all vaporized.
This leads us to consider two different scenarios of regular icy satellites
formation in order to estimate the composition of the ices they contain. In the
first scenario, icy satellites were accreted from planetesimals that have been
produced in Jupiter's feeding zone without further vaporization, whereas, in
the second scenario, icy satellites were accreted from planetesimals produced
in the Jovian subnebula. In this latter case, we study the evolution of carbon
and nitrogen gas-phase chemistries in the Jovian subnebula and we show that the
conversions of N2 to NH3, of CO to CO2, and of CO to CH4 were all inhibited in
the major part of the subdisk. Finally, we assess the mass abundances of the
major volatile species with respect to H2O in the interiors of the Jovian
regular icy satellites. Our results are then compatible with the detection of
CO2 on the surfaces of Callisto and Ganymede and with the presence of NH3
envisaged in subsurface oceans within Ganymede and Callisto.Comment: 9 pages, A&A, in pres
Formation and composition of planets around very low mass stars
The recent detection of planets around very low mass stars raises the
question of the formation, composition and potential habitability of these
objects. We use planetary system formation models to infer the properties, in
particular their radius distribution and water content, of planets that may
form around stars ten times less massive than the Sun. Our planetary system
formation and composition models take into account the structure and evolution
of the protoplanetary disk, the planetary mass growth by accretion of solids
and gas, as well as planet-planet, planet-star and planet-disk interactions. We
show that planets can form at small orbital period in orbit about low mass
stars. We show that the radius of the planets is peaked at about 1 rearth and
that they are, in general, volatile rich especially if proto-planetary discs
orbiting this type of stars are long-lived. Close-in planets orbiting low-mass
stars similar in terms of mass and radius to the ones recently detected can be
formed within the framework of the core accretion paradigm as modeled here. The
properties of protoplanetary disks, and their correlation with the stellar
type, are key to understand their composition.Comment: to appear in Astronomy and Astrophysics Letter
Planetesimal formation starts at the snow line
Planetesimal formation stage represents a major gap in our understanding of
the planet formation process. The late-stage planet accretion models typically
make arbitrary assumptions about planetesimals and pebbles distribution while
the dust evolution models predict that planetesimal formation is only possible
at some orbital distances. We want to test the importance of water snow line
for triggering formation of the first planetesimals during the gas-rich phase
of protoplanetary disk, when cores of giant planets have to form. We connect
prescriptions for gas disk evolution, dust growth and fragmentation, water ice
evaporation and recondensation, as well as transport of both solids and water
vapor, and planetesimal formation via streaming instability into a single,
one-dimensional model for protoplanetary disk evolution. We find that processes
taking place around the snow line facilitate planetesimal formation in two
ways. First, due to the change of sticking properties between wet and dry
aggregates, there is a "traffic jam" inside of the snow line that slows down
the fall of solids onto the star. Second, ice evaporation and outward diffusion
of water followed by its recondensation increases the abundance of icy pebbles
that trigger planetesimal formation via streaming instability just outside of
the snow line. Planetesimal formation is hindered by growth barriers and radial
drift and thus requires particular conditions to take place. Snow line is a
favorable location where planetesimal formation is possible for a wide range of
conditions, but still not in every protoplanetary disk model. This process is
particularly promoted in large, cool disks with low intrinsic turbulence and
increased initial dust-to-gas ratio.Comment: Accepted for publication in Astronomy & Astrophysic
Using Deep Neural Networks to compute the mass of forming planets
Computing the mass of planetary envelopes and the critical mass beyond which
planets accrete gas in a runaway fashion is important when studying planet
formation, in particular for planets up to the Neptune mass range. This
computation requires in principle solving a set of differential equations, the
internal structure equations, for some boundary conditions (pressure,
temperature in the protoplanetary disk where a planet forms, core mass and
accretion rate of solids by the planet). Solving these equations in turn proves
being time consuming and sometimes numerically unstable. We developed a method
to approximate the result of integrating the internal structure equations for a
variety of boundary conditions. We compute a set of planet internal structures
for a very large number (millions) of boundary conditions, considering two
opacities,(ISM and reduced). This database is then used to train Deep Neural
Networks in order to predict the critical core mass as well as the mass of
planetary envelopes as a function of the boundary conditions. We show that our
neural networks provide a very good approximation (at the level of percents) of
the result obtained by solving interior structure equations, but with a much
smaller required computer time. The difference with the real solution is much
smaller than the one obtained using some analytical formulas available in the
literature which at best only provide the correct order of magnitude. We
compare the results of the DNN with other popular machine learning methods
(Random Forest, Gradient Boost, Support Vector Regression) and show that the
DNN outperforms these methods by a factor of at least two. We show that some
analytical formulas that can be found in various papers can severely
overestimate the mass of planets, therefore predicting the formation of planets
in the Jupiter-mass regime instead of the Neptune-mass regime.Comment: accepted in A&A. Animations visible at
http://nccr-planets.ch/research/phase2/domain2/project5/machine-learning-and-advanced-statistical-analysis/
and code available at https://github.com/yalibert/DNN_internal_structur
Giant Planet Formation by Core Accretion
We present a review of the standard paradigm for giant planet formation, the
core accretion theory. After an overview of the basic concepts of this model,
results of the original implementation are discussed. Then, recent improvements
and extensions, like the inclusion of planetary migration and the resulting
effects are discussed. It is shown that these improvement solve the timescale
problem. Finally, it is shown that by means of generating synthetic populations
of (extrasolar) planets, core accretion models are able to reproduce in a
statistically significant way the actually observed planetary population.Comment: 8 pages, 3 figures, invited review, to appear in "Extreme Solar
Systems" ASP Conference Series, eds. Debra Fischer, Fred Rasio, Steve
Thorsett and Alex Wolszcza
Metallicity effect and planet mass function in pebble-based planet formation models
One of the main scenarios of planet formation is the core accretion model
where a massive core forms first and then accretes a gaseous envelope. This
core forms by accreting solids, either planetesimals, or pebbles. A key
constraint in this model is that the accretion of gas must proceed before the
dissipation of the gas disc. Classical planetesimal accretion scenario predicts
that the time needed to form a giant planets core is much longer than the time
needed to dissipate the disc. This difficulty led to the development of another
accretion scenario, in which cores grow by accretion of pebbles, which are much
smaller and thus more easily accreted, leading to a more rapid formation. The
aim of this paper is to compare our updated pebble-based planet formation model
with observations, in particular the well studied metallicity effect. We adopt
the Bitsch et al. 2015a disc model and the Bitsch et al. 2015b pebble model and
use a population synthesis approach to compare the formed planets with
observations. We find that keeping the same parameters as in Bitsch et al.
2015b leads to no planet growth due to a computation mistake in the pebble flux
(Bitsch et al. 2017). Indeed a large fraction of the heavy elements should be
put into pebbles (Zpeb/Ztot = 0.9) in order to form massive planets using this
approach. The resulting mass functions show a huge amount of giants and a lack
of Neptune mass planets, which are abundant according to observations. To
overcome this issue we include the computation of the internal structure for
the planetary atmosphere to our model. This leads to the formation of Neptune
mass planets but no observable giants. Reducing the opacity of the planetary
envelope finally matches observations better. We conclude that modeling the
internal structure for the planetary atmosphere is necessary to reproduce
observations.Comment: 13 pages, 22 figure
On the composition of ices incorporated in Ceres
We use the clathrate hydrate trapping theory and gas drag formalism to calculate the composition of ices incorporated in the interior of Ceres. Utilizing a time-dependent solar nebula model, we show that icy solids can drift from beyond 5 au to the present location of the asteroid and be preserved from vaporization. We argue that volatiles were trapped in the outer solar nebula in the form of clathrate hydrates, hydrates and pure condensates prior to having been incorporated in icy solids and subsequently in Ceres. Under the assumption that most of volatiles were not vaporized during the accretion phase and the thermal evolution of Ceres, we determine the per mass abundances with respect to H2O of CO2, CO, CH4, N2, NH3, Ar, Xe and Kr in the interior of the asteroid. The Dawn space mission, scheduled to explore Ceres in August 2014, may have the capacity to test some predictions. We also show that an in situ measurement of the D/H ratio in H2O in Ceres could constrain the distance range in the solar nebula where its icy planetesimals were produce
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