Washington, Jan 10 : Astronomers have observed two unusual stars going through a second wave of planet formation, hundreds of millions of years after planets formed around them initially.
Observed by UCLA (University of California, Los Angeles) astronomers, the stars are known as BP Piscium, in the constellation Pisces, and TYCHO 4144 329 2, in the constellation Ursa Major.
Though the astronomers have deduced that TYCHO 4144 329 2 is just 200 light-years from Earth, they do not know its precise age, or BP Piscium's age or distance from Earth.
"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.
According to researchers, these two stars have characteristics of very young stars, including rapid accretion of gas, extended orbiting disks of dust and gas, a large infrared excess emission and, in the case of BP Piscium, jets of gas that are being shot into space.
"With all these characteristics that match so closely with young stars, we would expect that our two stars would also be young," said Melis. "As we gathered more data, however, things just did not add up," he added.
For example, because stars burn lithium as they get older, young stars should have large quantities of lithium. The astronomers found, however, that the spectroscopic signature of lithium in BP Piscium is seven times weaker than 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," said Melis. "Rather, lithium has been heavily processed, as appropriate for old stars. Other spectral measurements also indicate it is a much older star," he added.
As seen from Earth, some 75 percent of BP Piscium's radiant energy is being converted by the dust particles into infrared light, and about 12 percent of TYCHO 4144 329 2's. According to Melis, these are unusually high amounts, which are spectacular in comparison to other stars that are known to be not-young.
The astronomers are continuing to study these stars with a variety of ground-based telescopes and with space-based observatories, including NASA's Hubble Space Telescope and Chandra X-ray Observatory, and they are searching for additional similar stars.