Some hot-jupiters are orbiting in the reverse direction - hence called "retrograde" exoplanets. Therefore, I think, there is a problem in using the famous assumption - that is assuming that the gravitational force provides the centripetal force for the orbital motion planet around the star. This is because all planets of our solar system are orbiting in the forward direction. For more information, one can contact me directly on dvsathe[at]gmail.com or see the second point (Troubling Solar System) in my Letter in CHANGE, May-June 2008 / 5 / http://changemag.org/Archives/... Alternatively, on can see my discussion of Mechanics Education on this site on the page on Prof. Dennis Sciama