Hot Neptune

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Possible structure of Gliese 436 b , the first Hot Neptune discovered .

A Hot Neptune is an exoplanet of the ice giant type such as Uranus or Neptune , which, however, in contrast to the mentioned equivalents, moves on a much smaller orbit around its star. As a guideline, the approximate boundary between cold and hot Neptunes is a distance of significantly less than 1 AU to the central star, which means that the normally solid ice components of this planetary class liquefy due to the higher solar radiation or appear as gases in the respective planetary atmosphere. For a long time, such planets were considered hypothetical, but in 2004 the exoplanet Gliese 436 b was discovered and later identified as a safe “Hot Neptune” using the transit method .

Theoretical structure

In essence, such planets will be constructed like the ice giants of our solar system. However, the temperature in the outer atmosphere is correspondingly higher; the lower gas layers, like those of the solar gas planets, will have high pressure and great heat. A rock and ice core with a pressure of over a million bar and a temperature of 7000 to 9000 Kelvin is conceivable. This would be surrounded by a mantle of various compounds, depending on how the planet was formed, more volatile substances such as ammonia , methane or water or heavier compounds (various salts ) (see Formation). The outer layers of such a planet would consist partly of hydrogen and helium , but their share in the atmosphere would probably be smaller, since the proximity to the mother star and the associated high temperatures would lead to a partial volatilization of these light gases despite the high gravity of the gas giant . An "abrasion" of lighter materials by the relatively strong stellar wind at a short distance from the star is probably prevented by a strong magnetic field caused by the mantle.

Emergence

There are two theories about the possibilities of creating a hot neptune. On the one hand, it is assumed that it formed (like a Hot Jupiter according to popular belief ) in the outer areas of the planetary system and then (instead of a stable elliptical orbit) initially moved on a spiral path (for example because of being slowed down by dust) . This brought him closer and closer to his star, until he either moved on a stable path after all or fell into the star.

The other theory is that the planet has already formed close to the stars. In the latter case, the planet would consist of heavier materials, as these, unlike the more volatile, can condense even at high temperatures and are more strongly influenced by gravitational fields, whereas light compounds are driven away by the solar wind before they can be absorbed by a sufficiently massive planet. An atmosphere of sulfur compounds or silicate vapors would be conceivable . If a hot neptune has arisen in the outer areas, however, it contains a higher proportion of volatile substances, which would be spectroscopically detectable if the celestial body were to transit. It is also possible that a Hot Neptune has formed from a Hot Jupiter, which gradually loses its mass if it is too close to the stars. In this case, a hot neptune represents a transition stage between a hot Jupiter and a terrestrial planet .

Possible candidates

The following extrasolar planets could be Hot Neptunes:

See also

literature

  • I. Baraffe, Y. Alibert, G. Chabrier, W. Benz: Birth and fate of hot-Neptune planets . In: Astronomy and Astrophysics . tape 450 , no. 3 , 2006, p. 1221–1229 , doi : 10.1051 / 0004-6361: 20054040 (on the possible formation of gas planets close to the sun).

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