Miércoles 21 de Junio de 2006, Ip nº 158

'Dead zones' may save planets from fiery death
Por Maggie McKee

"Dead zones" of very calm gas may prevent planets from falling into their host stars, researchers say. The work may explain why many exoplanets skirt their host stars at extremely close distances and suggests habitable, Earth-like planets may survive at higher rates than expected.

Most of the 180 or so known extrasolar planets are about the size of Jupiter and all lie relatively close to their host stars - within just 5 astronomical units (1 AU is the distance between the Sun and Earth). But the planets probably did not form there because the region cannot have contained enough gas and dust to amass such giant worlds.

So researchers believe the planets formed at distances of 10 to 20 AU, then migrated towards their host stars. This movement resulted from the planets losing energy by gravitational interaction with gas in their natal discs.

"But models of planet migration were unable to stop a migrating planet" from crashing into its star, says Nader Haghighipour of the University of Hawaii, US.

Low turbulence
Now, Soko Matsumura and colleagues at McMaster University in Canada say that planets might be slowed down by "dead zones" of low-viscosity gas that surround the stars out to distances of 13 AU.

The dead zones are "calm" because the disc is so thick near the star that radiation from the star and surrounding stars cannot ionise the gas there. That prevents magnetic fields in the disc from stirring up the gas there, since magnetic fields act only on charged particles. "So the disc is very quiet, and [planets] can't flow very fast through it," says team member Ralph Pudritz.

The team modelled the evolution of planet-forming discs of gas and dust over their full lifetimes of about 10 million years. And they found that massive planets like Jupiter open up a gap in the disc and then drift inward very slowly in the dead zone.

Less massive planets, the size of Earth or Neptune, actually rebound from the dead zone and start moving away from their stars if they form outside the zone. But if they form inside the zone – at distances similar to where those planets are found in our solar system - they also open up a gap in the disc and move very slowly towards their central star.

"Our calculations show a wide variety of effects that arise in helping to stop the migration of planets," Pudritz told New Scientist. He presented the research on Monday at a meeting of the American Astronomical Society in Calgary, Canada.

Habitable zone
Massive planets usually end up in orbits beyond 5 AU - farther than current techniques can detect them, the researchers find. And Earth-like planets are generally prevented from falling into their stars and tend to end up in orbits of 0.1 AU, he says.

That is too close for the Earth-like planets to be habitable, as defined by having a surface on which water is liquid. But Pudritz says Earth-like planets could be stopped while still in habitable zones if the discs evaporated in less than 10 million years.

And he says the fact that larger planets are stopped from migrating inwards can increase the chances of an Earth-like planet surviving in the habitable zone. That is because the giant planets are thought to sometimes scatter Earth-like planets into the star - or out of the planetary system - as they migrate towards their star.

Scott Kenyon of the Smithsonian Astrophysical Observatory in Massachusetts, US, says the research is interesting. "And I suspect there is a range of turbulence" from one disc to another, he told New Scientist.

Planetary systems like our solar system have little or no turbulence, he says, allowing planets to stay far away from their stars. While other systems appear to have high amounts of turbulence, which causes planets to migrate towards their host stars.

  05/06/2006. New Scientist Magazine.