  Strong tidal interaction with the central star can circularize the orbits of
close-in planets. With the standard tidal quality factor Q of our solar system,
estimated circularization times for close-in extrasolar planets are typically
shorter than the ages of the host stars. While most extrasolar planets with
orbital radii a < 0.1 AU indeed have circular orbits, some close-in planets
with substantial orbital eccentricities have recently been discovered. This new
class of eccentric close-in planets implies that either their tidal Q factor is
considerably higher, or circularization is prevented by an external
perturbation. Here we constrain the tidal Q factor for transiting extrasolar
planets by comparing their circularization times with accurately determined
stellar ages. Using estimated secular perturbation timescales, we also provide
constraints on the properties of hypothetical second planets exterior to the
known ones.
