The possibility of finding a rocky planet where water-based life may exist took a giant leap forward after five Earth-sized planets were identified around a distant star in the Milky Way galaxy.
Though all five planets are too close to their sun and too hot to harbour life, scientists said the fact that they were formed many billions of years ago when the galaxy was still young suggests that rocky, Earth-sized planets could be more ubiquitous than previously thought.
The planets in the distant solar system range in sizes between Mercury and Venus and were discovered by analysing data gathered by Nasa's Kepler space telescope.
Kepler has so far identified more than 4000 planetary "candidates", of which 1013 have been confirmed as true planets.
The star around which the five exoplanets orbit is known as Kepler-444, located 117 light years away in the constellation Cygnus and Lyra.
The star, which is 25 per cent smaller than the Sun, formed 11.2 billion years ago, less than 20 per cent of the age of the galaxy and long before the formation of the Sun.
This means that this ancient solar system is the oldest known group of terrestrial-sized planets in the Milky Way, about two-and-a-half times older than Earth, scientists said in a study published in the Astrophysical Journal.
"There are far-reaching implications for this discovery. We now know that Earth-sized planets have formed throughout most of the Universe's 13.8 billion-year history, which could provide scope of the existence of ancient life," said Tiago Campante of the University of Birmingham.
"By the time the Earth formed, the planets in this system were already older than our planet today. This discovery may now help to pinpoint the beginning of what we might call the era of planet formation."
The scientists made their discovery by monitoring the tiny fluctuations in light from the Kepler-444 star as each planet passed in between the star and the telescope.
The size and age of the planets were estimated from analysing the resonating sounds trapped within the star, in a technique known as asteroseismology.
Professor Bill Chaplin of Birmingham University said finding terrestrial-sized planets in such an ancient solar system demonstrates that planets similar in size to the Earth were formed many billions of years before the birth of our Solar System.
"It's a very old star so we are looking back a long way into the history of the galaxy. It tells us that small planets were formed in this early epoch of the galaxy," Professor Chaplin said.
"It demonstrates that small rocky planets were being formed in the early universe, which would have been true for many, many other stars in the galaxy," he said.
"It raises the potentially interesting possibility that if rocky planets formed so early, then some would be in the habitable zone around their stars which opens the possibility of early rocky planets where life would have developed," he added.
All five planets have orbits around their parent star of less than 10 days, which is equivalent to less than a tenth of the distance between the Earth and the Sun, and within the orbit of Mercury, where surface temperatures can reach 420C.
The Kepler telescope has found several distant solar systems, including a system with five planets, called Kepler-62, which included two "super-Earth"-sized planets in the habitable zone where liquid water could exist.
Astrobiologists believe that liquid water and a rocky surface are probably both necessary for the origin and evolution of life, which is why Kepler's efforts are concentrating on Earth-sized rocky planets orbiting within the habitable zone where it is not too hot nor too cold for water to exist in liquid form.