843 research outputs found
Critical core mass for enriched envelopes: the role of H2O condensation
Context. Within the core accretion scenario of planetary formation, most
simulations performed so far always assume the accreting envelope to have a
solar composition. From the study of meteorite showers on Earth and numerical
simulations, we know that planetesimals must undergo thermal ablation and
disruption when crossing a protoplanetary envelope. Once the protoplanet has
acquired an atmosphere, the primordial envelope gets enriched in volatiles and
silicates from the planetesimals. This change of envelope composition during
the formation can have a significant effect in the final atmospheric
composition and on the formation timescale of giant planets.
Aims. To investigate the physical implications of considering the envelope
enrichment of protoplanets due to the disruption of icy planetesimals during
their way to the core. Particular focus is placed on the effect on the critical
core mass for envelopes where condensation of water can occur.
Methods. Internal structure models are numerically solved with the
implementation of updated opacities for all ranges of metallicities and the
software CEA to compute the equation of state. CEA computes the chemical
equilibrium for an arbitrary mixture of gases and allows the condensation of
some species, including water. This means that the latent heat of phase
transitions is consistently incorporated in the total energy budget.
Results. The critical core mass is found to decrease significantly when an
enriched envelope composition is considered in the internal structure
equations. A particular strong reduction of the critical core mass is obtained
for planets whose envelope metallicity is larger than Z=0.45 when the outer
boundary conditions are suitable for condensation of water to occur in the top
layers of the atmosphere. We show that this effect is qualitatively preserved
when the atmosphere is out of chemical equilibrium.Comment: Accepted for publication in A&
Global Models of Planet Formation and Evolution
Despite the increase in observational data on exoplanets, the processes that
lead to the formation of planets are still not well understood. But thanks to
the high number of known exoplanets, it is now possible to look at them as a
population that puts statistical constraints on theoretical models. A method
that uses these constraints is planetary population synthesis. Its key element
is a global model of planet formation and evolution that directly predicts
observable planetary properties based on properties of the natal protoplanetary
disk. To do so, global models build on many specialized models that address one
specific physical process. We thoroughly review the physics of the sub-models
included in global formation models. The sub-models can be classified as models
describing the protoplanetary disk (gas and solids), the (proto)planet (solid
core, gaseous envelope, and atmosphere), and finally the interactions
(migration and N-body interaction). We compare the approaches in different
global models and identify physical processes that require improved
descriptions in future. We then address important results of population
synthesis like the planetary mass function or the mass-radius relation. In
these results, the global effects of physical mechanisms occurring during
planet formation and evolution become apparent, and specialized models
describing them can be put to the observational test. Due to their nature as
meta models, global models depend on the development of the field of planet
formation theory as a whole. Because there are important uncertainties in this
theory, it is likely that global models will in future undergo significant
modifications. Despite this, they can already now yield many testable
predictions. With future global models addressing the geophysical
characteristics, it should eventually become possible to make predictions about
the habitability of planets.Comment: 30 pages, 16 figures. Accepted for publication in the International
Journal of Astrobiology (Cambridge University Press
Impacts of planet migration models on planetary populations. Effects of saturation, cooling and stellar irradiation
Context: Several recent studies have found that planet migration in adiabatic
discs differs significantly from migration in isothermal discs. Depending on
the thermodynamic conditions, i.e., the effectiveness of radiative cooling, and
the radial surface density profile, planets migrate inward or outward. Clearly,
this will influence the semimajor axis - mass distribution of planets as
predicted by population synthesis simulations. Aims: Our goal is to study the
global effects of radiative cooling, viscous torque desaturation and gap
opening as well as stellar irradiation on the tidal migration of a synthetic
planet population. Methods: We combine results from several analytical studies
and 3D hydrodynamic simulations in a new semi-analytical migration model for
the application in our planet population synthesis calculations. Results: We
find a good agreement of our model with torques obtained in a 3D radiative
hydrodynamic simulations. We find three convergence zones in a typical disc,
towards which planets migrate from the in- and outside, affecting strongly the
migration behavior of low-mass planets. Interestingly, this leads to slow type
II like migration behavior for low-mass planets captured in those zones even
without an ad hoc migration rate reduction factor or a yet to be defined
halting mechanism. This means that the new prescription of migration including
non-isothermal effects makes the preciously widely used artificial migration
rate reduction factor obsolete. Conclusions: Outward migration in parts of a
disc makes some planets survive long enough to become massive. The convergence
zones lead to a potentially observable accumulations of low-mass planets at
certain semimajor axes. Our results indicate that further studies of the mass
where the corotation torque saturates will be needed since its value has a
major impact on the properties of planet populations.Comment: 18 pages, 15 figures. Accepted for A&
Planet formation models: the interplay with the planetesimal disc
According to the sequential accretion model, giant planet formation is based
first on the formation of a solid core which, when massive enough, can
gravitationally bind gas from the nebula to form the envelope. In order to
trigger the accretion of gas, the core has to grow up to several Earth masses
before the gas component of the protoplanetary disc dissipates. We compute the
formation of planets, considering the oligarchic regime for the growth of the
solid core. Embryos growing in the disc stir their neighbour planetesimals,
exciting their relative velocities, which makes accretion more difficult. We
compute the excitation state of planetesimals, as a result of stirring by
forming planets, and gas-solid interactions. We find that the formation of
giant planets is favoured by the accretion of small planetesimals, as their
random velocities are more easily damped by the gas drag of the nebula.
Moreover, the capture radius of a protoplanet with a (tiny) envelope is also
larger for small planetesimals. However, planets migrate as a result of
disc-planet angular momentum exchange, with important consequences for their
survival: due to the slow growth of a protoplanet in the oligarchic regime,
rapid inward type I migration has important implications on intermediate mass
planets that have not started yet their runaway accretion phase of gas. Most of
these planets are lost in the central star. Surviving planets have either
masses below 10 ME or above several Jupiter masses. To form giant planets
before the dissipation of the disc, small planetesimals (~ 0.1 km) have to be
the major contributors of the solid accretion process. However, the combination
of oligarchic growth and fast inward migration leads to the absence of
intermediate mass planets. Other processes must therefore be at work in order
to explain the population of extrasolar planets presently known.Comment: Accepted for publication in Astronomy and Astrophysic
Application of recent results on the orbital migration of low mass planets: convergence zones
Previous models of the combined growth and migration of protoplanets needed
large ad hoc reduction factors for the type I migration rate as found in the
isothermal approximation. In order to eliminate these factors, a simple
semi-analytical model is presented that incorporates recent results on the
migration of low mass planets in non-isothermal disks. It allows for outward
migration. The model is used to conduct planetary populations synthesis
calculations. Two points with zero torque are found in the disks. Planets
migrate both in- and outward towards these convergence zones. They could be
important for accelerating planetary growth by concentrating matter in one
point. We also find that the updated type I migration models allow the
formation of both close-in low mass planets, but also of giant planets at large
semimajor axes. The problem of too rapid migration is significantly mitigated.Comment: 4 pages, 3 figures. Proceedings of the IAU Symposium 276, 2010: The
Astrophysics of Planetary Systems: Formation, Structure, and Dynamical
Evolution, ed. A. Sozzetti, M. G. Lattanzi, and A. P. Bos
Chemical composition of Earth-like planets
Models of planet formation are mainly focused on the accretion and dynamical
processes of the planets, neglecting their chemical composition. In this work,
we calculate the condensation sequence of the different chemical elements for a
low-mass protoplanetary disk around a solar-type star. We incorporate this
sequence of chemical elements (refractory and volatile elements) in our
semi-analytical model of planet formation which calculates the formation of a
planetary system during its gaseous phase. The results of the semi-analytical
model (final distributions of embryos and planetesimals) are used as initial
conditions to develope N-body simulations that compute the post-oligarchic
formation of terrestrial-type planets. The results of our simulations show that
the chemical composition of the planets that remain in the habitable zone has
similar characteristics to the chemical composition of the Earth. However,
exist differences that can be associated to the dynamical environment in which
they were formed.Comment: 3 pages, 4 figures - Accepted for publication in the Bolet\'in de la
Asociaci\'on Argentina de Astronom\'ia, vol.5
Formation and structure of the three Neptune-mass planets system around HD69830
Since the discovery of the first giant planet outside the solar system in
1995 (Mayor & Queloz 1995), more than 180 extrasolar planets have been
discovered. With improving detection capabilities, a new class of planets with
masses 5-20 times larger than the Earth, at close distance from their parent
star is rapidly emerging. Recently, the first system of three Neptune-mass
planets has been discovered around the solar type star HD69830 (Lovis et al.
2006). Here, we present and discuss a possible formation scenario for this
planetary system based on a consistent coupling between the extended core
accretion model and evolutionary models (Alibert et al. 2005a, Baraffe et al.
2004,2006). We show that the innermost planet formed from an embryo having
started inside the iceline is composed essentially of a rocky core surrounded
by a tiny gaseous envelope. The two outermost planets started their formation
beyond the iceline and, as a consequence, accrete a substantial amount of water
ice during their formation. We calculate the present day thermodynamical
conditions inside these two latter planets and show that they are made of a
rocky core surrounded by a shell of fluid water and a gaseous envelope.Comment: Accepted in AA Letter
Dynamics of Planetesimals due to Gas Drag from an Eccentric Precessing Disk
We analyze the dynamics of individual kilometer-size planetesimals in
circumstellar orbits of a tight binary system. We include both the
gravitational perturbations of the secondary star and a non-linear gas drag
stemming from an eccentric gas disk with a finite precession rate. We consider
several precession rates and eccentricities for the gas, and compare the
results with a static disk in circular orbit.
The disk precession introduces three main differences with respect to the
classical static case: (i) The equilibrium secular solutions generated by the
gas drag are no longer fixed points in the averaged system, but limit cycles
with frequency equal to the precession rate of the gas. The amplitude of the
cycle is inversely dependent on the body size, reaching negligible values for
km size planetesimals. (ii) The maximum final eccentricity attainable
by small bodies is restricted to the interval between the gas eccentricity and
the forced eccentricity, and apsidal alignment is no longer guaranteed for
planetesimals strongly coupled with the gas. (iii) The characteristic
timescales of orbital decay and secular evolution decrease significantly with
increasing precession rates, with values up to two orders of magnitude smaller
than for static disks.
Finally, we apply this analysis to the -Cephei system and estimate
impact velocities for different size bodies and values of the gas eccentricity.
For high disk eccentricities, we find that the disk precession decreases the
velocity dispersion between different size planetesimals, thus contributing to
accretional collisions in the outer parts of the disk. The opposite occurs for
almost circular gas disks, where precession generates an increase in the
relative velocities.Comment: 11 pages, 9 figures. Accepted in MNRA
An extrasolar planetary system with three Neptune-mass planets
Over the past two years, the search for low-mass extrasolar planets has led
to the detection of seven so-called 'hot Neptunes' or 'super-Earths' around
Sun-like stars. These planets have masses 5-20 times larger than the Earth and
are mainly found on close-in orbits with periods of 2-15 days. Here we report a
system of three Neptune-mass planets with periods of 8.67, 31.6 and 197 days,
orbiting the nearby star HD 69830. This star was already known to show an
infrared excess possibly caused by an asteroid belt within 1 AU (the Sun-Earth
distance). Simulations show that the system is in a dynamically stable
configuration. Theoretical calculations favour a mainly rocky composition for
both inner planets, while the outer planet probably has a significant gaseous
envelope surrounding its rocky/icy core; the outer planet orbits within the
habitable zone of this star.Comment: 17 pages, 3 figures, preprint of the paper published in Nature on May
18, 200
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