A hot Jupiter is defined as any extrasolar planet that is large and gaseous like Jupiter, but is much hotter and orbits much closer to their parent stars than Jupiter does to the Sun. These planets have a mass that is close to or exceeds Jupiter’s (1.9 x 10^27 kg), but unlike Jupiter who has an orbit of 5 AU, hot Jupiter’s orbit within .05 AU of their parent stars. AU stands for astronomical unit, and is defined as a unit of length approximately equal to the mean distance between the Earth and Sun. Hot Jupiter is also referred to as roaster, pegasid or Pegasi planet.

Hot Jupiters have many 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. This means that there is a higher likelihood that, from Earth, we will be able to observe the planet cross in front of its parent star. A transit usually causes dimming of the starlight that hits Earth. A transit requires three bodies to be lined up in a single line, in this case, the Earth, the hot Jupiter and it’s parents star.

Generally, a hot Jupiter is of lower density than normal. This is due to the high levels of insulation, or solar radiation. This affects the capability to effectively determine the radius of the planet. Specifically, due to limb darkening, the gradual decrease in observed brightness of the disk of a star from its center to its edge, the planet’s ingress and egress boundaries are harder to determine.

Also, all hot Jupiters are thought to have migrated to their present positions. Plantetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellites orbital parameters, especially its semi-major axis. Hot Jupiters are thought to have made this migration because there would not have been enough material so close to the star for a planet or that mass to form in situ, or in its original location.

In addition, all hot jupiters have low eccentricities. The orbit’s eccentricity is an important parameter of the orbit that defines its absolute shape. Eccentricity may be interpreted as a measure of how much this shape deviates from a circle. Hot Jupiters have low eccentricities because their orbits have been circularized, or are being circularized, through a process of liberation. Liberation is defined as a very slow oscillation, real or apparent, of a satellite as viewed from the larger celestial body around which it revolves. This also causes the planet to synchronize 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. Radial velocity is the speed with which a star moves toward or away from the sun. It is determined from the red or blue shift in the star’s spectrum.

There is a theory that the parent stars may be swallowing hot Jupiters. University of Washington astronomer Guillermo Gonzalez proposes this theory. Gonzalez has observed that stars that are orbited by hot Jupiters have a higher than average level of metals (elements heavier than helium or hydrogen). These metals are thought to be a result of the planets moving close to the stars and eventually becoming absorbed, or swallowed. If stars were frequently swallowing planets, Gonzalez points out, this too would be a reason not to expect abundant life throughout the universe.

 

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