Wednesday, January 26, 2005

Astronomical Article

SAN DIEGO — An exploding star in our solar system's infancy may have saved Earth from extinction.

Astronomers studying the planet-forming disks of dust that orbit young, distant stars are hoping to solve the mystery of our own solar system's youth. Why is our system so different in form and function from others they can see?

It's a difference that may have saved Earth, because the scientists suspect that Jupiter and Saturn would have collided with the planet — or slung it out of the solar system like a slingshot — if the disk surrounding our young sun hadn't been so damaged.

These "protoplanetary" disks were a hot topic at a recent meeting of the American Astronomical Society. "Something very bad happened to our solar system's disk in its early years," says Steve Desch of Arizona State University in Tempe.

An exploding star, or supernova, likely occurred within a light-year — about 5.9 trillion miles — of our sun in its infancy, he argues. (The closest star to our solar system now, Proxima Centauri, is about 4 light-years away).

First spotted by an infrared satellite in the 1980s, the disks are the swirling leftovers of nebulas, pockets of gas within galaxies that spawn new stars.

Some of this dust compresses into planets, more of which have been discovered orbiting nearby stars during the past decade. A presentation at the meeting about a Hubble Space Telescope survey of 25 nearby stars, all youngsters less than 10 million years old, provides evidence that dust disks congeal into more compact bodies over only a few million years.

Over time, the rest of the dust is blown away by solar wind or other effects. What is left behind is a solar system like our own.

Glances at nearby disks, and some leftover clues, are telling researchers how things began for our sun. And it looks like we may inhabit a solar system that's something of a runt because of the damage from an exploding star.

Only the eruption of a star 25 to 40 times bigger than our sun could have littered our solar system with the radioactive elements seen in meteorite surveys reported by Desch's team at the meeting.

Astronomers have seen just such explosions blasting protoplanetary disks in the Orion Nebula, a star-forming factory 1,500 light-years away. Rather than blowing away the disks, the supernova blasts appear to seed them with metals rocketed out of the heart of the exploding star.

The supernova that blasted our solar system may explain some of its other peculiarities:

• Planets in our solar system follow nearly circular orbits far from the sun. Most planets detected orbiting other, nearby stars follow either highly elongated orbits or circle incredibly close to their stars. Scientists suspect that a stellar explosion could have stopped these developments in our solar system.

• Dust disks seen orbiting nearby stars typically contain much more material, sometimes 100 times more, than our solar system. A Spitzer Space Telescope survey of 26 nearby sun-like stars known to have planets found evidence that six of them have comet belts. But all appear filled with about 100 times more comets than our own.

"There's good evidence the solar system had a stunted formation when the (supernova) injection happened," Desch says. And that may have been very good for Earth.

Many astronomers believe that Jupiter and Saturn formed deep in space, far beyond Pluto's orbit, and spiraled into the solar system. Why they stopped a safe distance from the sun and left Earth undisturbed — unlike the history of many other solar systems seen nearby — is the final mystery that disk studies may help answer.

Scientists also discussed the collision of planets within disks in distant solar systems. "Something big is going on in those disks," says astronomer Scott Fisher of the Gemini Observatory in Hawaii.

For evidence, astronomers can point to a collision between mini-planets within the dust disk circling the star Beta Pictoris, which was announced by Fisher's team. In a deep-freeze orbit 4.65 billion miles from its star, a pair of icy asteroids perhaps 60 miles across collided, leaving behind telltale debris.

Images from the Spitzer Space Telescope also suggest that the disk around the star Vega was probably the site of a collision of planets that were about the size of Pluto — 1,200 miles across. "It seems you can't live very long in a disk without getting crunched," Fisher says.

Besides enjoying the planetary mayhem, scientists are interested in these collisions because of similar smash-ups in our own solar system billions of years ago. The moon most likely formed in a collision between the early Earth and a planet slightly larger than Mars.

"There is a huge race to take the first picture of a planet forming in one of these disks," Fisher says. Closing the loop between dust disks and planets with that observation may answer big questions about our own origins, he says.


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