Dear friends Of Space science and astronomy,

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."

Aging stars

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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