Late developers: New observations reveal that two ancient stars might be undergoing a second wave of planet formation. Both stars appear to be shrouded in planet-birthing disks of gas and dust like the one depicted in this artist's illustration.

Credit: NASA/JPL-Caltech/T. Pyle (SSC)

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NEW YORK: Two ancient stars, long past their supposed planet-forming prime, have been caught giving birth to a new wave of worlds.

"This is a new class of stars, ones that display conditions now ripe for formation of a second generation of planets, long, long after the stars themselves have formed," said astronomer Carl Melis, who reported the findings yesterday at the annual meeting of the American Astronomical Society in Texas.

Youthful features

The stars - BP Piscium in the constellation Pisces, and TYCHO 4114 329 2 in the constellation Ursa Major - both exhibit characteristics that belie their true age.

Like young stars, they glow brightly in the infrared, shoot jets of gas into space, and are surrounded by thick disks of gas and dust. Planets, comets, and asteroids are known to form from these disks in young stars.

"With all these characteristics that match so closely with young stars, we would expect that our two stars would also be young," said Melis, who is based at the University of California in Los Angeles (UCLA).

But as his team gathered more data about the pair, certain things failed to add up. For example, because stars burn lithium as they age, young stars should contain large quantities of lithium. But observations revealed that BP Piscium had seven times less lithium than would be expected for a young star of its mass.

"There is no known way to account for this small amount of lithium if BP Piscium is a young star," Melis said. "Rather, lithium has been heavily processed, as appropriate for old stars."

Mind blowing

The exact ages of the stars are not yet known, but the team estimates they are at least several hundred million - and possibly several billion - years old.

It's possible that the stars already gave birth to planets once before and are now doing it all over again, said study team member Benjamin Zuckerman, also of UCLA.

"If we took a rocket to one of these stars and discovered there were two totally distinct ages for their planets and more minor bodies like asteroids, that would blow scientists' minds away," he said.

The researchers are continuing to study the stars to understand how they are achieving the feat so late in life.

Sara Seager, an astronomer at the Massachusetts Institute of Technology, in Boston, said the results were very interesting. "It just shows that planet formation is ubiquitous, and that we have a chance to find lots of planets, and hopefully some with life someday."