<font color="yellow">Reading this thread raises a question that has been nagging me since the discovery of these rapidly orbiting gas giants.<br /><br />I must admit that I remain skeptical that a gas giant could possibly orbit its star in four days. Perhaps we have something wrong. Has anyone offered any alternative explanations for these dips?</font><br /><br />I think we can say, with just about as much certainty as you can say anything in astronomy, that the transiting planets are in fact planet mass objects orbiting their stars with short periods. For these planets we have both light curves and radial velocity curves, together with the type of star inferred from its spectrum, that is enough information to pin down the mass and radius for the transiting object. For two of these transiting planets people have actually seen the secondary eclipse when the planet passes behind its star which further constrains the models on the things. People have also observed atmospheres for these planets during transit. How these things actually formed is certainly much less certain - the prevailing theory is that they did form like Jupiter but then migrated close to the star, there is very strong theoretical reasons to believe that planets should migrate a lot, in fact it's a bit of a problem to figure out how things stop or don't migrate toward the star. But, it's not totally impossible to say that these hot planets formed more like stars do (direct gravitational collapse). But we do know that these are planet mass objects orbiting right next to the star. <div class="Discussion_UserSignature"> </div>