Planets can get twice as close to their suns without evaporating

December 6th, 2007 - 3:25 pm ICT by admin  

London, Dec 6 (ANI): A new computer simulation has suggested that giant gas planets can get twice as close to their stars as Mercury is to the Sun without evaporating.
Many gas giants have been found very close to their parent stars a handful even lie less than 6% of Mercury’s distance from the Sun. But it has never been clear just how close planets could get without heating up so much that their atmospheres would start escaping or “evaporating” into space.
The outermost layer of a planet’s atmosphere is where the action is as far as evaporation is concerned. The star blasts this layer with ultraviolet light and X-rays, which heat it up. The atmosphere can stay cool and avoid evaporating if it can radiate enough energy back into space in the form of infrared light.
To see how the balance between these two effects shifted with distance from the star, Tommi Koskinen of University College London, and his team, created a 3D computer simulation of the upper atmosphere of a planet with the mass of Jupiter orbiting a Sun-like star. They found that evaporation did not occur until the planet was within about 40% of Mercury’s distance from the Sun about twice as close as a previous estimate.
The difference comes from taking into account a natural coolant called H3+, an ion made of three hydrogen atoms joined together that is found in the atmosphere of Jupiter and other gas giant planets. H3+ is a very efficient radiator of infrared light, so it helps the upper atmosphere stay cool.
Without the H3+, the upper atmosphere jumps from about 3000deg Celsius to more than 20,000deg C, hot enough for it to start boiling away into space.
“Even so, planets at that distance would lose material only very slowly, allowing them to survive for many billions of years,” Koskinen told New Scientist.
The team said that their results agree with observations of a planet with about 70% the mass of Jupiter called HD 209458b, which orbits its star at about 12% of Mercury’s distance from the Sun, well within the evaporation zone.
Researchers have calculated that even HD 209458b should last for many billions of years at the rate that it is losing matter, with all the known planets around other stars appear to be capable of surviving where they are for billions of years.
“They all look very stable and many of them are very close in to their star,” said Koskinen. “But there’s a lot of uncertainty about these things and we shouldn’t say anything too definite at this point,” he added. (ANI)

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