spaceJunki - HI! Well, yes, there have been studies done. With varying results considering varying parameters.<br /><br />Remember, not all stars are visible. Brown dwarfs would likely be invisible, for example.<br /><br />There have been studies indicating a 10th planet, a supergiant larger than Jupiter but smaller than a brown dwarf, is even now perturbing orbits in the Oort cloud.<br /><br />Others dispute this as the cause.<br /><br />Some do consider the cycle of perturbations which may include the extinction of the Dinosaurs to be caused by close encounters (not of the third kind, btw). <br /><br />One should also be careful about quoting dating estimates as gospel. One must consider the method and other details leading to the conclusion.