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Astronomers find oldest-known solar system

Early universe An ancient Sun-like star surrounded by orbiting planets dating back to the dawn of our galaxy has been discovered by astronomers.

They say the discovery indicates that planets have been forming almost from the time the Universe began.

The star, known as Kepler-444, is 11.2 billion years old, and began forming at a time when the Universe was less than a fifth of its current age, they report today in The Astrophysical Journal.

"We've found the oldest known system of terrestrial planets," says one of the study's authors, Dr Daniel Huber of the University of Sydney.

The five Earth-size planets orbiting the star range in size between that of Mercury and Venus.

"It is extraordinary that such an ancient system of terrestrial-sized planets formed when the universe was just starting out," says Huber.

"We actually know that [exoplanets] are very abundant, especially Earth-size planets and a bit larger than that, but in this particular case we now know such planets with such sizes have existed for most of the history of our Universe."

The international team of scientists, led by Dr Tiago Campante of the University of Birmingham, discovered the system after poring over four years of data from NASA's Kepler spacecraft.

Kepler-444 is a very bright star located 117 light-years away from Earth in the direction of the constellation Lyra.

"This particular star is actually one of the brightest stars that Kepler has observed in its entire mission," says Huber.

To determine the size of the star the astronomers listened to the sound waves travelling through it -- a technique known as asteroseismology. The sound waves affect the star's temperature, creating changes in brightness that offer clues to the star's diameter, mass and age.

The planets were detected by observing the light from Kepler-444 -- which would dim when a planet travelled across the star.

Huber says the five planets are likely to be rocky.

"At these small sizes you can't really make a planet that has a substantial gaseous atmosphere," he explains.

But, with orbital periods of less than 10 days, they are unlikely to be habitable.

"While the size makes them very special, the orbital periods are much too short for liquid water to exist," he says.