51 Pegasi b is referred to in The Planet Factory as "the impossible planet" because it orbits its star so closely it seems impossible that it was detected it orbits a star much hotter than our Sun it is nearly 150 times more massive than Earth it orbits closer to its star than was thought possible for a gas giant QUESTION 5 The Hill radius for a planetesimal is the point where the planetesimal's gravitation force balances the star's gravitational force the height a mountain can reach on a planet based on the gravitational pull of the core The extent of the reach of the star's gravitational field the defined limit of a solar system beyond which planets can't form.