The parent star of the ancient system is Kepler-444, a Sun-like star formed 11.2 billion years ago, when the universe was less than 20 percent its current age, according to the team's paper published in the US journal Astrophysical Journal.

This is the oldest known system of terrestrial-sized planets in our Galaxy -- two and a half times older than our system, which is only a youthful 4.5 billion years old."We thus show that Earth-size planets have formed throughout most of the universe's 13.8-billion-year history, leaving open the possibility for the existence of ancient life in the galaxy," the team said.

The scientific collaboration was led by the University of Birmingham and contributed to by researchers from Denmark, the US, Australia, Portugal, Germany and Italy. The team determined Kepler-444's diameter and age using a technique that observes miniscule changes in the star's brightness. The study showed that Kepler-444 is 25 percent smaller than our sun and is 117 light years from Earth. The star's five known planets are smaller than Earth, with sizes varying between Mercury and Venus.

The planets are so close to their star that they complete their orbits in fewer than 10 days and at that distance they're all much hotter than Mercury and aren't habitable, the team said.

That discovery may help astronomers learn even more about early planet formation in the galaxy."By the time the Earth formed, the planets in this system were already older than our planet is today," Tiago Campante of the University of Birmingham, who led the study, said in a statement."This discovery may now help to pinpoint the beginning of what we might call the 'era of planet formation'," he added.

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