Astronomers have found over a thousand extrasolar planets now.  How does our solar system compare?  Thanks to the Kepler spacecraft, we now have a catalog of 1,235 alien planet candidates after just four months of operation.  Of the 408 that have been found in multiple-planet systems, 170 of these containing two to six planets have been pictured in a “Kepler Orrery” posted by the Harvard-Smithsonian Center for Astrophysics.  The press release says, “most of those look very different than our solar system” (see also 05/21/2011, bullet 2).

The poster is accompanied by an animated version that shows the 170 systems revolving like gears (see also PhysOrg).  Due to selection effects of the transiting method, Kepler has tended to find systems with low inclinations.  These have planets smaller than Neptune, because large gas giants can perturb the orbits of member planets into higher inclinations.  The Kepler team was surprised to find so many multiple-planet systems in their quarry: over 100, when only two or three were expected.  It is still too early, though, to detect earth-mass planets within their stars’ habitable zones.

How do planets form?  The astronomical community has undergone a paradigm revolution in the last decade about planet formation (05/07/2001).  Ever since Laplace, astronomers have assumed that disks of dust and gas will slowly condense into planets (the nebular hypothesis).  The discovery of “hot Jupiters” (gas giants orbiting extremely close to their parent stars) was shocking.  It indicated that planets migrate inward and will quickly be destroyed unless they can form faster than the core accretion model permits….

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