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Paper: How Extreme Can Planetary Systems Be?
Volume: 398, Extreme Solar Systems
Page: 491
Authors: Lissauer, J.J.; Slartibarfast
Abstract: The first system of extrasolar planets was found in orbit about a pulsar. The next was a giant planet in a 4 day orbit, and following that a more massive giant in an eccentric orbit. These types of planetary systems were not expected prior to being found. What else is out there, in our galaxy? Dynamical stability (for a large number of orbits) is a requirement, but beyond that what restrictions are there? For instance, a nearly coplanar system analogous to our solar system but with planets alternating between prograde and retrograde orbits with increasing distance from star could survive for a long time, but no plausible model of its formation (apart from construction by a technologically advanced civilization) exists. A pure lead planet could survive, but how could such a planet form? These and other speculations are discussed, together with a sobering assessment of our ability to make predictions of exoplanet characteristics.
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