File:Puffyplanet.jpg

English: Gas giants with a large radius and very low density are sometimes called "puffy planets" or "hot Saturns", due to their density being similar to Saturn's. Puffy planets orbit close to their stars so that the intense heat from the star combined with internal heating within the planet will help inflate the atmosphere. Six large-radius low-density planets have been detected by the transit method. In order of discovery they are: HAT-P-1b, CoRoT-1b, TrES-4, WASP-12b, WASP-17b, and Kepler-7b. Some hot Jupiters detected by the radial-velocity method may be puffy planets. Most of these planets are around or below Jupiter mass as more massive planets have stronger gravity keeping them at roughly Jupiter's size. Indeed, hot Jupiters with masses below Jupiter, and temperatures above 1800 Kelvin, are so inflated and puffed out that they are all on unstable evolutionary paths which eventually lead to Roche-Lobe overflow and the evaporation and loss of the planet's atmosphere. Even when taking surface heating from the star into account, many transiting hot Jupiters have a larger radius than expected. This could be caused by the interaction between atmospheric winds and the planet's magnetosphere creating an electric current through the planet that heats it up, causing it to expand. The hotter the planet, the greater the atmospheric ionization, and thus the greater the magnitude of the interaction and the larger the electric current, leading to more heating and expansion of the planet. This theory matches the observation that planetary temperature is correlated with inflated planetary radii.