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	<p><strong>Packed Planetary Systems</strong></p>
	<p>Rory Barnes</p>
	<p><em>Lunar and Planetary Lab</em></p>
	<p><em>University of Arizona</em></p>
	<p><em>1629 E. University Blvd., Tucson, AZ 85721</em></p>
	<p><em>USA</em></p>
	<p>rory@lpl.arizona.edu</p>
	<p>Richard Greenberg</p>
	<p><em>Lunar and Planetary Lab</em></p>
	<p><em>University of Arizona</em></p>
	<p><em>USA</em></p>
	<p>Planetary systems show a wide variety of mass distributions, radial spacing, orbital eccentricities, and resonances. As new discoveries of these systems continue, we seek to identify characteristics that they have in common, as clues to their origins and ultimately to the range of habitability that may be expected. We find that most planetary pairs (including those in our own solar system) seem to lie close to the limits of dynamical stability, i.e. they are dynamically “packed”. This result is based on numerical integration of many hypothetical systems bracketing the parameters of known systems. Results are consistent with earlier analytically derived criteria for stability. Planetary systems are typically so close to the boundary of instability that uncertainty in observationally derived parameters is expected to falsely place some systems in conditions of instability. Indeed, for a reasonable number of observed systems, nominal parameters do yield unstable orbits, which are probably not real. Stability comparisons also facilitate categorizing multiplanet systems as either resonant, packed, separated, or tidally circularized. The orbital packing indicates that additional planets cannot exist between the known planets. The packing of planetary systems constrains models of planet formation, places our solar system in the context of planetary systems in general, and suggests that all life (on planets or their moons) exists near a precarious boundary between regular motion and dynamical ejection into deep space.</p>
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