Fulton gap

The Fulton gap (aka radius gap and radius valley) is an interval of planet radii (1.5 to 2 Earth radii) within which extra-solar planets are less common, which became evident during follow-up study of Kepler Telescope discoveries. Some kind of selection bias was necessarily considered, but physical mechanisms have been proposed to explain it. The term evaporation valley is also used, referring to one model, that the gap is due to atmospheric escape. A model is that the escape that causes the gap results from photoevaporation, presumed to be so rapid for a planet close to its host star that a large atmosphere either remains essentially intact or is essentially gone. Another proposed model of note, termed care-powered mass loss, suggests such escape is due to heat from the planet's core, left over from planet formation. The latter model would take longer, e.g., up toward a gigayear, versus 100 million years for photoevaporation, which provides some means of checking these two models.