Puffy planet

Hot Jupiters (also called roaster planets,epistellar jovians,pegasids or pegasean planets) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital period (P<10 days). The close proximity to their stars and high surface-atmosphere temperatures resulted in the moniker "hot Jupiters".

Hot Jupiters are the easiest extrasolar planets to detect via the radial-velocity method, because the oscillations they induce in their parent stars' motion are relatively large and rapid compared to those of other known types of planets. One of the best-known hot Jupiters is 51 Pegasi b. Discovered in 1995, it was the first extrasolar planet found orbiting a Sun-like star. 51 Pegasi b has an orbital period of about 4 days.

Though there is diversity among hot Jupiters, they do share some common properties.

There are two general schools of thought regarding the origin of hot Jupiters: formation at a distance followed by inward migration and in-situ formation at the distances at which they're currently observed. The prevalent view is migration.

In the migration hypothesis, a hot Jupiter forms beyond the frost line, from rock, ice, and gases via the core accretion method of planetary formation. The planet then migrates inwards to the star where it eventually forms a stable orbit. The planet may have migrated inward smoothly via type II orbital migration. Or it may have migrated more suddenly due to gravitational scattering onto eccentric orbits during an encounter with another massive planet, followed by the circularization and shrinking of the orbits due to tidal interactions with the star. A hot Jupiter's orbit could also have been altered via the Kozai mechanism, causing an exchange of inclination for eccentricity resulting in a high eccentricity low perihelion orbit, in combination with tidal friction. This requires a massive body -- another planet or a stellar companion -- on a more distant and inclined orbit; approximately 50% of hot Jupiters have distant Jupiter-mass or larger companions, which can leave the hot Jupiter with an orbit inclined relative to the star's rotation.

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Wikipedia