Second Time Around: Some old stars may make new planets

How do you transform an old coot into a virile young whippersnapper? For a puffy middle-aged star, the cosmic version of taking Viagra seems to be ingesting a nearby companion—then burping up the stellar meal. That appears to be the best explanation for how two aging stars, long past their birthing years, have once again become ripe for making planets, an endeavor normally reserved for the youngest stars in the universe.

YOUNG AT HEART. Gas and dust surrounding the middle-aged star BP Piscium (center) suggest it may be undergoing a second wave of planet formation. Jets of gas visible above and faintly below the star are associated with the disk. M. Perrin and J. Graham

At first glance, the two stars in the new study appear young, notes Carl Melis of the University of California, Los Angeles. Both emit a lot of infrared light, a sign that they are surrounded by large, dusty disks that absorb starlight and reradiate it in the infrared. These disks provide the raw material for planet making. Common around newborn stars, the disks usually vanish after several million years.

Melis and his colleagues imaged the disks around the two stars, BP Piscium and TYCHO 4144 329 2, using the large Keck II Telescope atop Hawaii’s Mauna Kea.

Other features, however, indicate that the stars aren’t young. Newborn stars have an allotment of lithium that’s burned up as they age. Low abundances of lithium in both BP Piscium and TYCHO 4144 329 2 show that they’re not newcomers. In addition, spectra of the stars reveal that both have relatively high surface gravity. That also indicates middle age because young stars, larger and still contracting, have lower surface gravity.

Moreover, TYCHO 4144 329 2 orbits a companion that appears to be an ordinary old star, the researchers found. Analyzing the companion, Melis and his colleagues deduced that TYCHO 4144 329 2 must be at least 400 million years old, he reported in Austin, Texas, last week at a meeting of the American Astronomical Society. The age of BP Piscium remains uncertain, but observations suggest it should be classified as a giant star, an aging and bloated body.

The disks around these old stars can be explained only by a kind of stellar rejuvenation in which each star recently swallowed a neighboring body—either another star or a lower-mass object called a brown dwarf, Melis asserts. Material subsequently ejected by the star could form a dusty, planet-making disk hundreds of millions to perhaps billions of years after these stars first formed planets. Melis says the team has evidence that a few other aging stars possess similar disks.

No one has yet examined the dust-cloaked stars for signs that they may harbor a previous generation of planets. But submillimeter observations of BP Piscium show that the star’s dust grains are relatively large—100 micrometers in diameter—an indication that tiny dust particles have already begun to coalesce into larger, rocky bodies. In addition, the thicknesses of the disks suggest that each has been stirred up by a massive object such as a planet or asteroid, says Melis.

“This is a fascinating, brand-new situation,” comments theorist Sara Seager of the Massachusetts Institute of Technology. “If planet formation is indeed occurring around these unusual [older] stars, it gives hope that planet formation is truly ubiquitous.”

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