Rocky Planet Births Are Common, Dead Stars Suggest

Anne Minard in Long Beach, California

The materials needed to create rocky planets are common in the universe, according to new information from the planetary graveyards around dead stars.

Astronomer Michael Jura of the University of California, Los Angeles, found a pattern in the debris around some of the brightest known white dwarfs—the remains of dead sunlike stars.

His study reveals chemical similarities suggesting that, when the white dwarfs were alive, they could have hosted terrestrial worlds akin to Earth, Venus, Mercury, or Mars.

Jura presented his findings during a Monday press briefing at the 213th meeting of the American Astronomical Society in Long Beach, California. The results will appear in a future issue of the Astronomical Journal.

Also during the briefing, a separate team of astronomers announced new data that suggest gas giant planets like Jupiter must form relatively quickly after the birth of a star, within two to five million years.

Skeleton Crew

A sunlike star in its death throes swells into a red giant, devouring its innermost planets and disturbing the orbits of outer planets and asteroids. The star then collapses to become a small but dense white dwarf.

Our sun has the potential to undergo this process in about five billion years.

Sometimes the surviving asteroids get knocked off course, veer too close to a white dwarf, and are ripped apart by its intense gravity, leaving dust and debris.

Using infrared data from the Spitzer Space Telescope, UCLA's Jura found asteroid debris around at least one percent of the thousand brightest known white dwarfs.

Analyzing the chemical composition of this dust allowed Jura to see that eight white dwarf systems have several key traits in common with each other and with our solar system, suggesting they all once had similar planetary bodies.

"If you ground up our asteroids and rocky planets, you would get the same type of dust we are seeing in these star systems," Jura said.

For example, the systems all contain glassy silicates similar to a mineral called olivine that is common in Earth's crust.The systems are also low in carbon, which is similarly lacking in the geologic profile of our sun's rocky planets.

Two of the systems had been known about before, but six were newly analyzed for this study.

"What was once kind of a freak thing is now apparently a systematic pattern," Jura said.

But, he added, the real power of observing the white dwarf systems is still to come.

The asteroid dust around white dwarfs is much finer than dust grains found around living stars, allowing astronomers to see chemical compositions that larger pieces don't yield.

Further study of the white dwarfs' dust grains in other light wavelengths should reveal even more detail about the exact materials in the asteroids.


Growup Plan

In a separate investigation, Thayne Currie of the Harvard-Smithsonian Center for Astrophysics and his team revealed that gas giants like Jupiter must form even quicker than previously thought.

Scientists have known for some time that gas giants form relatively quickly, within ten million years of a parent star's formation.

After that, materials for the formation of such bodies are either decimated or flung out of the system.

By contrast, the solid matter that forms rocky planets hangs around for many millions of years. Earth, for example, needed up to 30 million years to reach its final mass.

Currie and his co-authors used Spitzer to study the five million-year-old star cluster NGC 2362. They discovered that all the stars in that cluster with the same mass as our sun or greater have already lost their planet-forming disks.

This means that gas giants must form within five or even two million years after the birth of a star.

Previous work had found that gas giants are most common around high-mass stars, which means their "formation must be very efficient," Currie said.

Golden Age

Adam Burrows, an astrophysicist from Princeton University, gave an overview of exoplanetary studies, which focuses on planets outside our solar system, at the AAS meeting.

He pointed out that the average age of workers in his field is decreasing—a mark of its wide appeal among incoming astronomers.

"It's a golden age for this subject," he said.

The first confirmed exoplanet was found in 1995, and since then the number of cataloged worlds beyond our solar system has jumped to more than 300.

Since the first find, astronomers have made discoveries about what types of stars are most likely to host planets, have described a variety of possible planetary orbits, and have revealed the atmospheric phenomena and chemical compositions of several exoplanets, Burrows noted.

Exoplanet discoveries should become even more exciting with the planned 2013 launch of NASA's James Webb Space Telescope, he added.

"We're going to be able to look at these same planets and get much more detailed information," he said. "It's going to be epic."