Hot Jupiter (also called roaster, pegasid or Pegasi planet) is a class of extrasolar planets whose mass is close to or exceeds that of Jupiter (1.9 × 1027 kg), but unlike in our own solar system, where Jupiter orbits at 5 AU, the planets referred to as Hot Jupiters orbit within approximately 0.05 AU of their parent stars. In comparison, a Hot Jupiter is about eight times closer to its star than Mercury is to the Sun.
Hot Jupiters have some common characteristics:
- They have a much greater chance of transiting their star as seen from Earth than planets of the same mass in larger orbits.
- Due to high levels of insolation they are of a lower density than they would otherwise be. This has implications for radius determination, because due to limb darkening of the planet against its background star during a transit, the planet's ingress and egress boundaries are harder to determine.
- They are all thought to have migrated to their present positions because there would not have been enough material so close to the star for a planet of that mass to have formed in situ.
- They all have low eccentricities. This is because their orbits have been circularised, or are being circularised, by the process of libration. This also causes the planet to synchronise its rotation and orbital periods, so it always presents the same face to its parent star.
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 other known types of planets.