Miércoles 12 de Diciembre de 2007, Ip nº 220

Planets can survive extreme roasting by their stars
Por David Shiga

Gas giant planets can get twice as close to their stars as Mercury is to the Sun without evaporating, a new computer simulation suggests. The work suggests the 'hot Jupiters' discovered on tight orbits around their stars are in no immediate danger of boiling away into space.

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.

A new study suggests this threshold lies more than twice as close as Mercury's distance to the Sun (or about 0.15 astronomical units, where 1 AU is the distance between the Earth and the Sun). Tommi Koskinen of University College London, UK, led the team that carried out the study.

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, Koskinen's 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.

Efficient radiator

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.

But even the H3+ cannot completely cancel the increasing radiation from the star at closer orbits. Within about 40% of Mercury's distance from the Sun, the upper atmosphere gets hot enough that both H3+ and H2 molecules – made of two hydrogen atoms – break down into individual hydrogen atoms.

Without the H3+, the upper atmosphere jumps from about 3000° Celsius to more than 20,000° C, hot enough for it to start boiling away into space. Even so, planets at that distance would lose material only very slowly, Koskinen says, allowing them to survive for many billions of years.

The team says 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 the researchers calculated. Some scientists say that signs of the planet's atmosphere evaporating can be found in the spectrum of light that filters through it from the parent star, although not everyone agrees.

Remnant cores?

But other researchers have calculated that even HD 209458b should last for many billions of years at the rate that it is losing matter, Koskinen says, adding that 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," he told New Scientist. "But there's a lot of uncertainty about these things and we shouldn't say anything too definite at this point."

Sara Seager of MIT in Cambridge, Massachusetts, US, says that even if hot Jupiters like HD 209458b are not drastically affected by evaporation, smaller ones might be. "This could be an important mechanism for complete atmospheric loss for lower-mass planets [inside the evaporation zone] such as super Earths orbiting Sun-like stars," she told New Scientist.

Exactly how close a giant planet would need to get before losing large amounts of mass to evaporation is still unknown. Even if they lost their atmospheres, the solid cores thought to be present inside them might remain behind.

"Some researchers have speculated that hot Neptune-mass and super-Earth-mass exoplanets orbiting near their host stars are all remnant cores of giant planets!" says Seager. Other researchers, however, disagree, arguing that such planets actually began their lives at their current sizes.

Journal reference: Nature

  05/12/2007. New Scientist Magazine.