Hot Jupiter

Hot Jupiters are a class of gas giant exoplanets that are physically similar to Jupiter but have very short orbital periods (P < 10 days). They are close to their stars, with high surface-atmosphere temperatures. Hot Jupiters are usually tidally locked.

Hot Jupiters are the easiest extrasolar planets to detect by the radial-velocity method. The oscillations of their parent stars' motion are large and rapid compared to those of other types of planets. These oscillations are caused by the planet orbiting its star in the line of sight of the Earth.

Since it seems that they are common, that raises the question of why the Solar System has its largest planet so far from its star. This led astronomers to think the Earth's Jupiter was first much closer to the Sun, and moved out later to its present orbit.

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.