Planet-forming disks provide galactic timeline, researchers say

Planet-forming disks provide galactic timeline, researchers say

By RICK CALLAHAN

(September 29, 1999 3:24 p.m. EDT http://www.nandotimes.com) - The same gravitational shoving match that swept dense swarms of comets and asteroids from our solar system long ago is apparently at work around nearby stars, astronomers say.

The findings suggest there's a distinct timeline in the evolution of solar systems - at least in the violent "clean up" stage believed to cap the planet-forming process.

European astronomers scrutinized infrared images of 84 stars of varying sizes and intensity, looking for the planet-forming disks of dust, asteroids and comets that encircle many young stars like Saturn's rings.

They found that most of the stars shed their gritty halos at about 400 million years of age - the age the sun is believed to have done the same thing.

The images from the European Space Agency's Infrared Space Observatory showed that only 15 of the 84 stars had stellar debris disks and nine of those encircled stars under 400 million years of age. Sixty-three of the 69 stars lacking disks were older than 400 million years.

The researchers said that means 90 percent of stars cast off their debris disks between 300 and 400 million years of age.

"We were quite surprised that things separated so neatly," said lead researcher H. J. Habing of the Leiden Observatory, Leiden, the Netherlands.

The findings were reported in Thursday's issue of the journal Nature.

Though the team didn't see any planets orbiting the stars, their work matches the time frame during which our solar system's late-forming outer planets probably thinned out planetary leftovers by catapulting comets and asteroids into deep space, said Dana Backman, a professor of physics and astronomy at Franklin & Marshall College in Lancaster, Pa.

Afterward, our solar system became an orderly place with planets circling the Sun in roughly the same orbits they currently occupy.

Backman said the disk-less stars Habing's team saw likely witnessed a scenario similar to that believed to have followed the formation of Uranus and Neptune. Scientists think the gravitational influence of those two outermost gas planets helped thin out the debris disk by ejecting material into the Kuiper Belt - a collection of small, icy bodies circling the sun beyond Neptune's orbit.

About the same time, Jupiter - and to a lesser degree Saturn - are believed to have hurled icy bodies far beyond the solar system to form a shell-like cloud of comets called the Oort Cloud.

The smaller inner planets - Mars, Mercury - still bear the scars from this chaotic period, when they were pummeled with asteroid and comet impacts.

"It was basically time to get under the table in the solar system because there was all this stuff flying around," Backman said.

Weathering has erased most of the Earth's scars from this period, but they are well-preserved on the Moon, which lacks an atmosphere.

Jonathan Lunine, a professor of planetary sciences at the University of Arizona in Tucson, said a powerful infrared observatory set to launch in late 2001 will likely shed more light on stellar debris disks.

Pluto Express, a spacecraft proposed for launch in 2004, would swoop past Pluto and could determine if the outermost planet really qualifies as a planet, or - as many astronomers suspect - is just another leftover given the boot by Neptune as it tidied up its neighborhood, Lunine said.

"Pluto's probably the biggest brick left over from the construction of the planets," he said.