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What It Takes to Destroy a Gas GiantGiant planets migrate slowly towards their stars |
By Gabriel Gache, Science News Editor
6th of December 2007, 07:47 GMT
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The general theory about how rocky planets like our own form is pretty well understood, as gas and dust remnant from the process of star formation collapse and stick together to form a rough rock, in the shape of a ball which collides with other such objects to form a planet.
Nevertheless, the formation process of giant planets such as Jupiter is somehow puzzling. For example, most of the planets found outside our solar system are gas giants due to the fact that they are easier to find, and many of them can be found in the close proximity of their star, which in some cases determines large quantities of material to be blown off the planet. Astronomers believe that such planets could not have formed close to their stars, as they would not exist today due to the loss of material, but rather that most probably they formed in the outer regions of the solar system and slowly migrated inward.
This model implies disastrous consequences for the inner planets of the respective solar system, as gas giants similar to Jupiter usually assimilate smaller planets in their path to the inner regions of the solar system. So scientists asked themselves, what keeps these planets from getting too close to the central star and vaporize?
In order to find out how close a gas giant can get to its star before its atmosphere is blown off and the planet becomes unstable, researchers from University College London, carried a study that compares our solar system's largest planet to gas giant exoplanets.
Observations made over decades on Jupiter suggest that it has a thin, stable atmosphere, and it orbits the Sun at a distance of 5 astronomical units, while gas giant exoplanets orbit their star much closer, like HD209458b that has an orbit about 100 times closer than Jupiter, and an atmosphere greatly expanded into space.
Simulations of a planet the size of Jupiter showed that when it is brought inside the Earth's orbit, at about 0.16 astronomical units, the planet remains mostly in the same configuration, with a stable atmosphere. However, at 0.14 astronomical units Jupiter's atmosphere starts to suddenly expand and become too unstable to ensure that matter is not lost.
Gas giants are cooled by the winds that circle around them, which keeps the atmosphere stable. A second cooling effect takes place when a charged type of hydrogen, called H3 reflects part of the incoming sunlight back into space. Simulations show that when a planet the size of Jupiter is brought closer to the Sun, the levels at which H3 in being produced are boosted, which sustains the cooling mechanism.
However, if the planet crosses the 0.15 astronomical unit mark, the molecular hydrogen becomes unstable and H3 is no longer being produced, stopping the thermo-regulating process which in turn triggers the atmosphere to heat up and become unstable.
This simulation gives us a new model on how gas giants usually form in the outer regions of the solar system, from ice cores that gather large quantities of gas over long periods of time, after which they start moving slowly towards the inner regions of the solar system, closer to the star.
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