Two old stars appear to be gearing up for a second generation of planet formation, a phenomenon astronomers say they have never seen before.
"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 formed," said UCLA astronomy graduate student Carl Melis, who reported the findings at a recent meeting of the American Astronomical Society in Austin, Texas.
The stars are BP Piscium in the constellation Pisces and TYCHO 4144 329 2, which resides in the constellation Ursa Major. The exact ages of the stars are unknown, but it is estimated they are at least hundreds of millions or possibly billions of years old, and might have already given birth to planets long ago.
"Most astronomers now believe that most stars are accompanied by first-generation planets of some sort, even if the planets are not massive enough to be picked up by the radial velocity [detection] technique," Melis said.
Second generation of planets
The unusual thing about these stars is that they appear to be giving birth to planets again.
"We currently understand planet formation to occur around stars when they are very young and enshrouded in dusty and gaseous disks, the material necessary to form planetary bodies," Melis told SPACE.com. "This material is completely used up after a couple to ten million years after the star is born and is not replenished during the star's life. As such, we would never expect a star to undergo planet formation late in its life as the necessary conditions are not present."
How they can do this is still unclear, but the stars seem to have kept many of their youthful qualities. For instance, the researchers found orbiting disks of gas and dust extended around the stars, and, in the case of BP Piscium, jets of gas being ejected into space. These gas-and-dust rings provide the fodder for the making of planetesimals, such as comets and asteroids that can merge to form larger bodies, along with planets.
"With all these characteristics that match so closely with young stars, we would expect that our two stars would also be young," Melis said. "As we gathered more data, however, things just did not add up."
The lack of lithium gave away the true stellar ages. Since stars burn lithium as they get older, younger stars should pack large stores of the chemical element. The astronomers found, however, that BP Piscium contained much 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. Other spectral measurements also indicate it is a much older star."
The researchers speculate that the senior stars might be borrowing material from their neighbors to construct new worlds.
"Our team believes that these stars, as they aged and began to expand into giant stars, engulfed very short-period companion stars orbiting around them," Melis said. "Interactions with these companions caused matter to be flung into disks surrounding the two stars."
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