Parent Stars Can Smother Planets

* Pairs of closely orbiting binary stars show telltale debris rings that scientists believe are the smashed remains of planets.
* Tidally locked binary stars seem to be poor places to look for planets in Earth-like orbits.

Stars that are as old as the ones studied have long ago shed their dusty debris rings, which provided the raw materials for the formation of their systems. That implies that something is happening to refresh the debris disk late in the game.

For a planet to be habitable, one parent star seems to be a better bet than two, especially if the stars are physically very close.

So concludes researchers who discovered what they believe to be the rocky remains of planets around three pairs of closely orbiting binary stars.

"It really only takes one rogue planet to have its orbit perturbed and that can wreak havoc on the whole system," Jeremy Drake, with the with Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., told Discovery News.

Drake and colleagues found warm dust disks around sun-like stars that should have long ago dissipated any clouds of debris left over from their formative years.

"The fact that they're old, mature systems must mean that they've made the stuff relatively recently," said Drake.

Planets have been found in systems with two stars, but those stars' orbits are more stable. The type of binary stars targeted in the new study are located only about 2 million miles from their partners -- about 2 percent of the distance between the sun and Earth.

At that range, powerful magnetic fields and strong stellar winds slow the stars' spin, causing them to move even closer together over time. The resulting gravitational disturbances would impact any planets in tow and eventually cause the orbiting bodies to crash into one another.

Astronomers, who used NASA's Spitzer Space Telescope for their study, are hoping to get a more detailed look at the debris rings with another infrared observatory named Herschel. They also plan to develop computer models to better understand the destabilizing effects of the stars on any planets.

The discovery of these late-stage debris rings doesn't bode well for the existence of habitable planets in these systems, adds Drake.

"You could be lucky. You could be not. It is possible that a planetary system in a well evolved state could, over time, be driven to a destructive phase," he said.

NASA's Marc Kuchner, with the Goddard Space Flight Center in Maryland, thinks planets orbiting farther from the parent stars would have a better shot at survival.

"The region near the stars is very unstable, but if you were to go far enough out I'd expect there'd be a large region of stability," Kuchner told Discovery News.

"On these planets, there would be two suns in the sky. At least one sun would be very luminous, highly variable and magnetically active, which might make the surface of the planet a hostile place. But on a planet with oceans, there could be happy sea creatures not too far beneath the protective water," he said.

The research appears in last week's issue of Astrophysical Journal Letters.