Hot Jupiters (also called roaster planets, epistellar jovians, pegasids or pegasean planets) are a class of exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital radii with semi-major axes from 0.015 to 0.5 astronomical units (2.2×106 to 74.8×106 km). The close proximity to their stars and high surface 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.

Formation and evolution

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.

Migration

In the migration hypothesis, hot Jupiters 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 II orbital migration, or possibly via interaction with other planets or a stellar companion. It has been shown that approximately 50% of hot Jupiters have distant Jupiter-mass or larger companions. The migration happens during the solar nebula phase, i.e. when gas is still present. Energetic stellar photons and strong stellar winds at this time remove most of the remaining nebula.

If their atmospheres are stripped away via hydrodynamic escape, their cores may become chthonian planets. The amount of gas removed from the outermost layers depends on the planet size, the envelope gases, the orbital distance from the star, and on the stellar luminosity. 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 a substantially larger fraction of the planet's mass. It should be noted that none of such objects have been found yet and they are still hypothetical.

In situ

It is also theorized that a substantial fraction of hot Jupiters may have formed in-situ via the core accretion method of planetary formation. This theory is particularly attractive because it has measurable consequences including the expectation that hot Jupiters should frequently be accompanied by additional low-mass planets with periods shorter than ~100 days. Traditionally, this mode of conglomeration has been disfavored due to the fact that there may not be enough solid material orbiting close to the star to allow for the in situ assembly of massive cores, which are necessary for the formation of hot Jupiters. Recent surveys, however, have found that the inner regions of planetary systems are not empty, and are frequently occupied by super-Earth type planets. Yet, direct calculations indicate that in situ formation of super-Earths in the close proximity of a solar-mass star require surface densities of solids ≈ 104 g/cm2, or larger.

Source: www.nasa.gov and [ Ссылка ]

CREDIT: National Aeronautics and Space Administration

Support the Channel vie BOOK DEPOSITARY Shopping
Book Depository: Millions of books with free delivery worldwide
[ Ссылка ]

Enjoy, Like and Subscribe:)

свернуть