1,091 research outputs found
The Survival Rate of Ejected Terrestrial Planets with Moons
During planet formation, a gas giant will interact with smaller protoplanets
that stray within its sphere of gravitational influence. We investigate the
outcome of interactions between gas giants and terrestrial-sized protoplanets
with lunar-sized companions. An interaction between a giant planet and a
protoplanet binary may have one of several consequences, including the delivery
of volatiles to the inner system, the capture of retrograde moons by the giant
planet, and the ejection of one or both of the protoplanets. We show that an
interesting fraction of terrestrial-sized planets with lunar sized companions
will likely be ejected into interstellar space with the companion bound to the
planet. The companion provides an additional source of heating for the planet
from tidal dissipation of orbital and spin angular momentum. This heat flux
typically is larger than the current radiogenic heating of the Earth for up to
the first few hundred million years of evolution. In combination with an
atmosphere of sufficient thickness and composition, the heating can provide the
conditions necesary for liquid water to persist on the surface of the
terrestrial mass planet, making it a potential site for life. We also determine
the possibility for directly detecting such systems through all-sky infrared
surveys or microlensing surveys. Microlensing surveys in particular will
directly measure the frequency of this phenomenon.Comment: 4 pages, 2 figures, Accepted to ApJ
Building the Terrestrial Planets: Constrained Accretion in the Inner Solar System
To date, no accretion model has succeeded in reproducing all observed
constraints in the inner Solar System. These constraints include 1) the orbits,
in particular the small eccentricities, and 2) the masses of the terrestrial
planets -- Mars' relatively small mass in particular has not been adequately
reproduced in previous simulations; 3) the formation timescales of Earth and
Mars, as interpreted from Hf/W isotopes; 4) the bulk structure of the asteroid
belt, in particular the lack of an imprint of planetary embryo-sized objects;
and 5) Earth's relatively large water content, assuming that it was delivered
in the form of water-rich primitive asteroidal material. Here we present
results of 40 high-resolution (N=1000-2000) dynamical simulations of late-stage
planetary accretion with the goal of reproducing these constraints, although
neglecting the planet Mercury. We assume that Jupiter and Saturn are
fully-formed at the start of each simulation, and test orbital configurations
that are both consistent with and contrary to the "Nice model." We find that a
configuration with Jupiter and Saturn on circular orbits forms low-eccentricity
terrestrial planets and a water-rich Earth on the correct timescale, but Mars'
mass is too large by a factor of 5-10 and embryos are often stranded in the
asteroid belt. A configuration with Jupiter and Saturn in their current
locations but with slightly higher initial eccentricities (e = 0.07-0.1)
produces a small Mars, an embryo-free asteroid belt, and a reasonable Earth
analog but rarely allows water delivery to Earth. None of the configurations we
tested reproduced all the observed constraints. (abridged)Comment: Accepted to Icarus. 21 pages, 12 figures, 5 tables in emulateapj
format. Figures 3 and 4 degraded. For full-resolution see
http://casa.colorado.edu/~raymonsn/ms_emulateapj.pd
The Complex History of Trojan Asteroids
The Trojan asteroids provide a unique perspective on the history of Solar
System. As a large population of small bodies, they record important
gravitational interactions and dynamical evolution of the Solar System. In the
past decade, significant advances have been made in understanding physical
properties, and there has been a revolution in thinking about the origin of
Trojans. The ice and organics generally presumed to be a significant part of
Trojan compositions have yet to be detected directly, though low density of the
binary system Patroclus (and possibly low density of the binary/moonlet system
Hektor) is consistent with an interior ice component. By contrast, fine-grained
silicates that appear to be similar to cometary silicates in composition have
been detected, and a color bimodality may indicate distinct compositional
groups among the Trojans. Whereas Trojans had traditionally been thought to
have formed near 5 AU, a new paradigm has developed in which the Trojans formed
in the proto-Kuiper Belt, and they were scattered inward and captured in the
Trojan swarms as a result of resonant interactions of the giant planets.
Whereas the orbital and population distributions of current Trojans are
consistent with this origin scenario, there are significant differences between
current physical properties of Trojans and those of Kuiper Belt objects. These
differences may be indicative of surface modification due to the inward
migration of objects that became the Trojans, but understanding of appropriate
modification mechanisms is poor and would benefit from additional laboratory
studies. Many open questions remain, and the future promises significant
strides in our understanding of Trojans. The time is ripe for a spacecraft
mission to the Trojans, to turn these objects into geologic worlds that can be
studied in detail to unravel their complex history.Comment: Chapter for Asteroids IV book (UA Press), accepted for publication,
33 pages, 10 figure
Water Delivery and Giant Impacts in the 'Grand Tack' Scenario
A new model for terrestrial planet formation (Hansen 2009, Walsh et al. 2011)
has explored accretion in a truncated protoplanetary disk, and found that such
a configuration is able to reproduce the distribution of mass among the planets
in the Solar System, especially the Earth/Mars mass ratio, which earlier
simulations have generally not been able to match. Walsh et al. tested a
possible mechanism to truncate the disk--a two-stage, inward-then-outward
migration of Jupiter and Saturn, as found in numerous hydrodynamical
simulations of giant planet formation. In addition to truncating the disk and
producing a more realistic Earth/Mars mass ratio, the migration of the giant
planets also populates the asteroid belt with two distinct populations of
bodies--the inner belt is filled by bodies originating inside of 3 AU, and the
outer belt is filled with bodies originating from between and beyond the giant
planets (which are hereafter referred to as `primitive' bodies).
We find here that the planets will accrete on order 1-2% of their total mass
from primitive planetesimals scattered onto planet-crossing orbits during the
formation of the planets. For an assumed value of 10% for the water mass
fraction of the primitive planetesimals, this model delivers a total amount of
water comparable to that estimated to be on the Earth today. While the radial
distribution of the planetary masses and the dynamical excitation of their
orbits are a good match to the observed system, we find that the last giant
impact is typically earlier than 20 Myr, and a substantial amount of mass is
accreted after that event. However, 5 of the 27 planets larger than half an
Earth mass formed in all simulations do experience large late impacts and
subsequent accretion consistent with the dating of the Moon-forming impact and
the estimated amount of mass accreted by Earth following that event
On the connection between the Nekhoroshev theorem and Arnold Diffusion
The analytical techniques of the Nekhoroshev theorem are used to provide
estimates on the coefficient of Arnold diffusion along a particular resonance
in the Hamiltonian model of Froeschl\'{e} et al. (2000). A resonant normal form
is constructed by a computer program and the size of its remainder
at the optimal order of normalization is calculated as a function
of the small parameter . We find that the diffusion coefficient
scales as , while the size of the optimal remainder
scales as in the range
. A comparison is made with the numerical
results of Lega et al. (2003) in the same model.Comment: Accepted in Celestial Mechanics and Dynamical Astronom
Gas accretion onto Jupiter mass planets in discs with laminar accretion flows
(Abridged) Studies have shown that a Jovian mass planet embedded in a viscous
protoplanetary disc (PPD) can accrete gas efficiently through the gap and
doubles its mass in Myr. The planet also migrates inwards on a
timescale of Myr. These timescales are short compared to PPD
lifetimes, and raise questions about the origins of cold giant exoplanets.
However, PPDs are unlikely to be globally turbulent, and instead they may
launch magnetised winds such that accretion towards the star occurs in laminar
accretion flows located in narrow layers near the surfaces of the disc. The aim
of this study is to examine the rate at which gas accretes onto Jovian mass
planets that are embedded in layered PPDs. We use 3D hydrodynamical simulations
of planets embedded in PPDs, in which a constant radial mass flux towards the
star of M yr is sustained. We consider a
classical viscous alpha model, and also models in which an external torque is
applied in narrow surface layers to mimic the effects of a magnetised wind. The
accreting layers are parameterised by their column densities ,
and we consider values in the range 0.1 to 10 g cm. The viscous model
gives results in agreement with previous studies. We find the accretion rate
onto the planet in the layered models crucially depends on the planet's ability
to block the wind-induced mass flow. For g cm, the
planet torque can block the mass flow through the disc, accretion onto the
planet is slow, and a mass doubling time of 10 Myr is obtained. For
g cm, accretion is fast and the mass doubling time
is 0.2 Myr. Although the radial mass flow through the layered disc models is
always M yr, adopting different values of
leads to very different gas accretion rates onto gas giant
planets.Comment: 14 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
Massive planet migration: Theoretical predictions and comparison with observations
We quantify the utility of large radial velocity surveys for constraining
theoretical models of Type II migration and protoplanetary disk physics. We
describe a theoretical model for the expected radial distribution of extrasolar
planets that combines an analytic description of migration with an empirically
calibrated disk model. The disk model includes viscous evolution and mass loss
via photoevaporation. Comparing the predicted distribution to a uniformly
selected subsample of planets from the Lick / Keck / AAT planet search
programs, we find that a simple model in which planets form in the outer disk
at a uniform rate, migrate inward according to a standard Type II prescription,
and become stranded when the gas disk is dispersed, is consistent with the
radial distribution of planets for orbital radii 0.1 AU < a < 2.5 AU and planet
masses greater than 1.65 Jupiter masses. Some variant models are disfavored by
existing data, but the significance is limited (~95%) due to the small sample
of planets suitable for statistical analysis. We show that the favored model
predicts that the planetary mass function should be almost independent of
orbital radius at distances where migration dominates the massive planet
population. We also study how the radial distribution of planets depends upon
the adopted disk model. We find that the distribution can constrain not only
changes in the power-law index of the disk viscosity, but also sharp jumps in
the efficiency of angular momentum transport that might occur at small radii.Comment: ApJ, in press. References updated to match published versio
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