Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The HARPS search for southern extra-solar planets XVIII. An Earth-mass
  planet in the GJ 581 planetary system   The GJ 581 planetary system was already known to harbour three planets,
including two super-Earths planets which straddle its habitable zone. We report
here the detection of an additional planet -- GJ 581e -- with a minimum mass of
1.9 M_earth. With a period of 3.15 days, it is the innermost planet of the
system and has a ~5% transit probability. We also correct our previous
confusion of the orbital period of GJ 581d (the outermost planet) with a
one-year alias, thanks to an extended time span and many more measurements. The
revised period is 66.8 days, and locates the semi-major axis inside the
habitable zone of the low mass star. The dynamical stability of the 4-planet
system imposes an upper bound on the orbital plane inclination. The planets
cannot be more massive than approximately 1.6 times their minimum mass.

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The categories of this paper are: quant-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: funct-an
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The categories of this paper are: patt-sol
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The categories of this paper are: cs
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The categories of this paper are: hep-ex
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The categories of this paper are: q-fin
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The categories of this paper are: math
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The categories of this paper are: atom-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: chem-ph
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The categories of this paper are: q-bio
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The categories of this paper are: alg-geom
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The categories of this paper are: cond-mat
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The categories of this paper are: math-ph
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The categories of this paper are: adap-org