339 research outputs found
Kepler's Multiple Planet Systems
More than one-third of the 4700 planet candidates found by NASAs Kepler spacecraft are associated with target stars that have more than one planet candidate, and such multis account for the vast majority of candidates that have been verified as true planets.The large number of multis tells us that flat multiplanet systems like our Solar System are common. Virtually all of the candidate planetary systems are stable, as tested by numerical integrations that assume a physically motivated mass-radius relationship. Statistical studies performed on these candidate systems reveal a great deal about the architecture of planetary systems, including the typical spacing of orbits and flatness. The characteristics of several of the most interesting confirmed Kepler & K2 multi-planet systems will also be discussed
Composition of Massive Giant Planets
The two current models for giant planet formation are core accretion and disk
instability. We discuss the core masses and overall planetary enrichment in
heavy elements predicted by the two formation models, and show that both models
could lead to a large range of final compositions. For example, both can form
giant planets with nearly stellar compositions. However, low-mass giant
planets, enriched in heavy elements compared to their host stars, are more
easily explained by the core accretion model. The final structure of the
planets, i.e., the distribution of heavy elements, is not firmly constrained in
either formation model.Comment: 6 pages, Proceedings of IAU Symposium 276 (Invited talk), The
Astrophysics of Planetary Systems: Formation, Structure, and Dynamical
Evolution. Turin, Italy, Oct. 201
Terrestrial Planet Formation Surrounding Close Binary Stars
Disk material has been observed around both components of some young close
binary star systems. It has been shown that if planets form at the right places
within such disks, they can remain dynamically stable for very long times.
Herein, we numerically simulate the late stages of terrestrial planet growth in
circumbinary disks around 'close' binary star systems with stellar separations
between 0.05 AU and 0.4 AU and binary eccentricities up to 0.8. In each
simulation, the sum of the masses of the two stars is 1 solar mass, and giant
planets are included. Our results are statistically compared to a set of planet
formation simulations in the Sun-Jupiter-Saturn system that begin with
essentially the same initial disk of protoplanets. The planetary systems formed
around binaries with apastron distances less than ~ 0.2 AU are very similar to
those around single stars, whereas those with larger maximum separations tend
to be sparcer, with fewer planets, especially interior to 1 AU. We also provide
formulae that can be used to scale results of planetary accretion simulations
to various systems with different total stellar mass, disk sizes, and
planetesimal masses and densities.Comment: 60 pages, 4 tables, and 11 low resolution eps figures. Article with
high resolution figures is available at
http://www-personal.umich.edu/~equintan/publications.html . Accepted for
publication in Icaru
A scaling law for accretion zone sizes
Current theories of runaway planetary accretion require small random velocities of the accreted particles. Two body gravitational accretion cross sections which ignore tidal perturbations of the Sun are not valid for the slow encounters which occur at low relative velocities. Wetherill and Cox have studied accretion cross sections for rocky protoplanets orbiting at 1 AU. Using analytic methods based on Hill's lunar theory, one can scale these results for protoplanets that occupy the same fraction of their Hill sphere as does a rocky body at 1 AU. Generalization to bodies of different sizes is achieved here by numerical integrations of the three-body problem. Starting at initial positions far from the accreting body, test particles are allowed to encounter the body once, and the cross section is computed. A power law is found relating the cross section to the radius of the accreting body (of fixed mass)
Formation of Giant Planets and Brown Dwarves
According to the prevailing core instability model, giant planets begin their growth by the accumulation of small solid bodies, as do terrestrial planets. However, unlike terrestrial planets, the growing giant planet cores become massive enough that they are able to accumulate substantial amounts of gas before the protoplanetary disk dissipates. Models predict that rocky planets should form in orbit about most stars. It is uncertain whether or not gas giant planet formation is common, because most protoplanetary disks may dissipate before solid planetary cores can grow large enough to gravitationally trap substantial quantities of gas. Ongoing theoretical modeling of accretion of giant planet atmospheres, as well as observations of protoplanetary disks, will help decide this issue. Observations of extrasolar planets around main sequence stars can only provide a lower limit on giant planet formation frequency . This is because after giant planets form, gravitational interactions with material within the protoplanetary disk may cause them to migrat~ inwards and be lost to the central star. The core instability model can only produce planets greater than a few jovian masses within protoplanetary disks that are more viscous than most such disks are believed to be. Thus, few brown dwarves (objects massive enough to undergo substantial deuterium fusion, estimated to occur above approximately 13 jovian masses) are likely to be formed in this manner. Most brown dwarves, as well as an unknown number of free-floating objects of planetary mass, are probably formed as are stars, by the collapse of extended gas/dust clouds into more compact objects
Architectures of Kepler's Multi-Transiting Planetary Systems
More than one-third of the 4700 planet candidates found by NASA's Kepler spacecraft are associated with target stars that have more than one planet candidate, and such "multis" account for the vast majority of candidates that have been verified as true planets. The large number of multis tells us that flat multi-planet systems like our Solar System are common. Virtually all of the candidate planetary systems are stable, as tested by numerical integrations that assume a physically motivated mass-radius relationship. Statistical studies performed on these candidate systems reveal a great deal about the architecture and dynamics of planetary systems, including the typical spacing of orbits and flatness. The characteristics of several of the most interesting confirmed Kepler, K2 & TESS multi-planet systems will also be discussed
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