Hot Jupiter - General Characteristics

General Characteristics

Hot Jupiters have some common characteristics:

• They have similar characteristics to Jupiter (gas giants, usually with masses close to or exceeding that of Jupiter, which is 1.9×1027 kg), however, orbit much more closely to the star and experience a high surface temperature.
• They have a much greater chance of transiting their star as seen from a farther outlying point than planets of the same mass in larger orbits. The most notable of these are HD 209458 b, the first transiting hot Jupiter found, HD 189733 b, which was first mapped in 2007 by the Spitzer Space Telescope, and HAT-P-7b, which was recently observed by the Kepler mission.
• 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's 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.
• Most of these have nearly circular orbits (low eccentricities). This is because their orbits have been circularized, or are being circularized, by the process of libration. This also causes the planet to synchronize its rotation and orbital periods, so it always presents the same face to its parent star — the planet becomes tidally locked to the star.
• They exhibit high-speed winds distributing the heat from the day side to the night side, thus the temperature difference between the two sides is relatively low.

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.

They are thought to form at a distance from the star beyond the frost line, where the planet can form from rock, ice and gases. The planets then migrate inwards to the star where they eventually form a stable orbit. The planets usually move by type 2 migrations, or possibly via interaction with other planets. The migration happens during the solar nebula phase, and will typically stop when the star enters its T-Tauri phase. The strong stellar winds at this time remove most of the remaining nebula.

After their atmospheres and outer layers are stripped away (hydrodynamic escape), their cores may become chthonian planets. Losing of the outermost layers depends on the size and the material of the planet and the distance from the star. In a typical system a gas giant orbiting 0.02 AU around its parent star loses 5–7% of its mass during its lifetime, but orbiting closer than 0.015 AU can mean evaporation of the whole planet except for its core.