2003 Ub 313: The Incredible Shrinking Planet?, No bigger than Pluto? Options

[Guest_BruceMoomaw_*]

(Thanks to Emily Lakdawalla):

http://sciencenow.sciencemag.org/cgi/content/full/2006/127/1

[edstrick]

The arm-waving idea I've posted here before is that a planetesimal becomes a planet when, during accretion, its gravity field is great enough so that it's accretional capture diameter is significantly, perhaps 2 times, greater than it's physical diameter.

During accretion, at any given time and any given distance from the sun, there is a mean encounter velocity between planetisimals. It's probably bigger near resonances and near accreting giant planets. The mass of an accreting body and the average encounter velocity between it and candidate stuff to be accreted determines the "capture diameter".

Something 1 km across will have a capture diameter of something like 1.0001 km. Something 10,000 km across might have a capture diameter of 25,000 km. That's what lets accreting planets start to grow faster than their smaller competitors and end up eating the competition. That really is a "physics of origins" related distinction between planet and planetesimal