Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Formation Mechanism of Gas Giants on Wide Orbits   The recent discoveries of massive planets on ultra-wide orbits of HR 8799
(Marois et al. 2008) and Fomalhaut (Kalas et al. 2008) present a new challenge
for planet formation theorists. Our goal is to figure out which of three giant
planet formation mechanisms--core accretion (with or without migration),
scattering from the inner disk, or gravitational instability--could be
responsible for Fomalhaut b, HR 8799 b, c and d, and similar planets discovered
in the future. This paper presents the results of numerical experiments
comparing the long-period planet formation efficiency of each possible
mechanism in model A star, G star and M star disks.
  First, a simple core accretion simulation shows that planet cores forming
beyond 35 AU cannot reach critical mass, even under the most favorable
conditions one can construct. Second, a set of N-body simulations demonstrates
that planet-planet scattering does not create stable, wide-orbit systems such
as HR 8799. Finally, a linear stability analysis verifies previous work showing
that global spiral instabilities naturally arise in high-mass disks. We
conclude that massive gas giants on stable orbits with semimajor axes greater
than 35 AU form by gravitational instability in the disk. We recommend that
observers examine the planet detection rate as a function of stellar age,
controlling for planet dimming with time. If planet detection rate is found to
be independent of stellar age, it would confirm our prediction that
gravitational instability is the dominant mode of producing detectable planets
on wide orbits. We also predict that the occurrence ratio of long-period to
short-period gas giants should be highest for M dwarfs due to the inefficiency
of core accretion and the expected small fragment mass in their disks.

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The categories of this paper are: physics
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The categories of this paper are: bayes-an
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The categories of this paper are: acc-phys
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The categories of this paper are: atom-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: alg-geom
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The categories of this paper are: ao-sci
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The categories of this paper are: econ
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The categories of this paper are: gr-qc
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The categories of this paper are: dg-ga
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The categories of this paper are: cond-mat
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The categories of this paper are: plasm-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: supr-con
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The categories of this paper are: adap-org
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The categories of this paper are: comp-gas