Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Planets in the Early Universe   Several planets have recently been discovered around old metal-poor stars,
implying that these planets are also old, formed in the early Universe. The
canonical theory suggests that the conditions for their formation could not
have existed at such early epochs. In this paper we argue that the required
conditions, such as sufficiently high dust-to-gas ratio, could in fact have
existed in the early Universe immediately following the first episode of metal
production in Pop. III stars, both in metal-enhanced and metal-deficient
environments. Metal-rich regions may have existed in multiple isolated pockets
of enriched and weakly-mixed gas close to the massive Pop. III stars.
Observations of quasars at redshifts $z\sim 5$, and gamma-ray bursts at $z\sim
6$, show a very wide spread of metals in absorption from $\rm [X/H]\simeq -3$
to $\simeq -0.5$. This suggests that physical conditions in the metal-abundant
clumps could have been similar to where protoplanets form today. However,
planets could have formed even in low-metallicity environments, where formation
of stars is expected to proceed due to lower opacity at higher densities. In
such cases, the circumstellar accretion disks are expected to rotate faster
than their high-metallicity analogues. This can result in the enhancement of
dust particles at the disk periphery, where they can coagulate and start
forming planetesimals. In conditions with the low initial specific angular
momentum, radiation from the central protostar can act as a trigger to drive
instabilities with masses in the Earth to Jupiter mass range. Discoveries of
planets around old metal-poor stars (e.g. HIP 11952, $\rm [Fe/H]\sim -1.95$)
show that planets did indeed form in the early Universe and this may require
modification of our understanding of the physical processes that produce them.
This work is an attempt to provide a heuristic scenario for their existence.

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The categories of this paper are: physics
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The categories of this paper are: q-alg
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The categories of this paper are: q-fin
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The categories of this paper are: cond-mat
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The categories of this paper are: chem-ph
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The categories of this paper are: funct-an
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The categories of this paper are: solv-int
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The categories of this paper are: math
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The categories of this paper are: cs
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The categories of this paper are: adap-org
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The categories of this paper are: atom-ph
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The categories of this paper are: nucl-ex
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The categories of this paper are: econ
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The categories of this paper are: nlin
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The categories of this paper are: hep-ex
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The categories of this paper are: hep-ph