2,250 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
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
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
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
Nebular water depletion as the cause of Jupiter's low oxygen abundance
Motivated by recent spectroscopic observations suggesting that atmospheres of
some extrasolar giant-planets are carbon-rich, i.e. carbon/oxygen ratio (C/O)
1, we find that the whole set of compositional data for Jupiter is
consistent with the hypothesis that it be a carbon-rich giant planet. We show
that the formation of Jupiter in the cold outer part of an oxygen-depleted disk
(C/O 1) reproduces the measured Jovian elemental abundances at least as
well as the hitherto canonical model of Jupiter formed in a disk of solar
composition (C/O = 0.54). The resulting O abundance in Jupiter's envelope is
then moderately enriched by a factor of 2 solar (instead of
7 solar) and is found to be consistent with values predicted by
thermochemical models of the atmosphere. That Jupiter formed in a disk with C/O
1 implies that water ice was heterogeneously distributed over several AU
beyond the snow line in the primordial nebula and that the fraction of water
contained in icy planetesimals was a strong function of their formation
location and time. The Jovian oxygen abundance to be measured by NASA's Juno
mission en route to Jupiter will provide a direct and strict test of our
predictions.Comment: Accepted for publication in Astrophysical Journal Letter
Modeling the Jovian subnebula: I - Thermodynamical conditions and migration of proto-satellites
We have developed an evolutionary turbulent model of the Jovian subnebula
consistent with the extended core accretion formation models of Jupiter
described by Alibert et al. (2005b) and derived from Alibert et al.
(2004,2005a). This model takes into account the vertical structure of the
subnebula, as well as the evolution of the surface density as given by an
-disk model and is used to calculate the thermodynamical conditions in
the subdisk, for different values of the viscosity parameter. We show that the
Jovian subnebula evolves in two different phases during its lifetime. In the
first phase, the subnebula is fed through its outer edge by the solar nebula as
long as it has not been dissipated. In the second phase, the solar nebula has
disappeared and the Jovian subdisk expands and gradually clears with time as
Jupiter accretes the remaining material. We also demonstrate that early
generations of satellites formed during the beginning of the first phase of the
subnebula cannot survive in this environment and fall onto the proto-Jupiter.
As a result, these bodies may contribute to the enrichment of Jupiter in heavy
elements. Moreover, migration calculations in the Jovian subnebula allow us to
follow the evolution of the ices/rocks ratios in the proto-satellites as a
function of their migration pathways. By a tempting to reproduce the distance
distribution of the Galilean satellites, as well as their ices/rocks ratios, we
obtain some constraints on the viscosity parameter of the Jovian subnebula.Comment: Accepted in Astronomy and Astrohpysic
Migration and giant planet formation
We extend the core-accretion model of giant gaseous planets by Pollack et al.
(\cite{P96}) to include migration, disc evolution and gap formation. Starting
with a core of a fraction of an Earth's mass located at 8 AU, we end our
simulation with the onset of runaway gas accretion when the planet is at 5.5 AU
1 Myr later. This timescale is about a factor ten shorter than the one found by
Pollack et al. (\cite{P96}) even though the disc was less massive initially and
viscously evolving. Other initial conditions can lead to even shorter
timescales. The reason for this speed-up is found to result from the fact that
a moving planet does not deplete its feeding zone to the extend of a static
planet. Thus, the uncomfortably long formation timescale associated with the
core-accretion scenario can be considerably reduced and brought in much better
agreement with the typical disc lifetimes inferred from observations of young
circumstellar discs.Comment: 9 pages, 2 figures, published in A&A Letter
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