Are planetary systems flat?

Scott Tremaine

Institute for Advanced Study

Laplace argued, correctly, that the small inclinations of planetary
orbits implied that the solar system formed from a flat disk.  The
observational and theoretical evidence on whether extrasolar planetary
systems are flat, however, is still ambiguous. I will discuss
constraints on flatness from the Kepler spacecraft and other sources;
measurements of the Rossiter-McLaughlin effect in transiting planets,
which show that many planetary systems have large stellar obliquities,
and disk and high-eccentricity migration as competing mechanisms for
the formation of hot Jupiters.