By Paul Rincon
Science reporter, BBC News, Austin
Two old stars may be undergoing a second episode of planet formation, long after their initial window of opportunity.
BP Piscium is an old star with youthful features
Astronomers believe the stars once had orbiting companions, but that these were engulfed when the stars expanded.
This caused matter to be ejected from the stars, forming a disc of dust and gas from which planets could form anew.
Details were presented at the 211th American Astronomical Society meeting in Austin, Texas.
Jets of gas
The two stars described in the latest study - known as BP Piscium and Tycho 4144 329 2 respectively - possess many signatures characteristic of stellar youngsters.
These include the active, rapid accretion of gas, an extended orbiting disc of dust and gas, excess emission in the infrared spectrum and - in the case of BP Piscium - jets of gas being fired off into space.
After star formation, the dust and gas left over gradually forms planetesimals, like comets and asteroids, as well as planets. So astronomers should see dusty discs around very young stars.
"If it walks like a duck and talks like a duck, we should expect that it is a duck," explained co-author Carl Melis, a graduate student at the University of California, Los Angeles.
"But as we got more data on these stars, a lot of things just didn't add up."
Young stars should have large amounts of the metal lithium, because stars burn this element as they get older. However, the amount of lithium in BP Piscium was seven times lower than one would expect for a star of its temperature.
The researchers compared BP Piscium with statistical models for stars of different ages.
They found it was a bad fit for a young star, but the data were in perfect agreement with the scenario for an old star.
In addition, there was no molecular cloud nearby that could have given rise to the star if it was truly young.
Tycho 4144 329 2 has a companion, allowing astronomers to ascertain a distance for the star and that the age of the system was at least 400 million years old, and probably much older.
But the question of why the old stars should be undergoing a "second youth" is harder to explain.
"The only reasonable conclusion we can come to as far as the origin of the dust and gas around these stars goes is that they used to have a very short orbital period companion nearby, and that as the stars expanded... this companion was engulfed, that caused the stars to eject matter," said Mr Melis.
"But we don't have any evidence for any planets forming in these systems as yet."
Sara Seager, a professor of astronomy at Massachusetts Institute of Technology (MIT), said the work was "really interesting".
She added: "It shows that planet formation is ubiquitous. It gives us the hope to find lots of planets and hopefully some with life someday."