THE discovery of five Earth-sized planets in the Milky Way is believed to be the strongest sign yet that there are aliens out there.
The planets were identified around a distant star in the galaxy and the find raises the possibility of eventually finding another rocky planet where water-based life may exist, scientists say.
While all five of the planets are too close to the sun and therefore too hot for life, the fact they were formed many billions of years ago is leading experts to think Earth-type planets could be more abundant than previously thought.
Nasa's Kepler space telescope discovered the planets, which adds to more than 4,000 potential planets it has found, with 1,013 now confirmed as true planets.
The star the planets orbit around is thought to be 11.2 billion years old, making it the oldest system with Earth-sized planets ever discovered.
Speaking to News.com-au, Daniel Huber from the University of Sydney's School of Physics said the human race had "never seen anything like this".
"One of the big quests over the past few years has been to find out whether planets similar to Earth exist around stars," said Dr Huber.
"Kepler has found a lot of these stars … but what makes this very special is that it's very, very old.
"It all goes back to curiosity about where we came from. The holy grail is discovering finally whether life exists on other planets, whether we are alone, whether other planets like Earth exist, how far away from stars they are, when they started forming."
He said the discovery was similar to our solar system, in that it contained planets orbiting around a Sun-like star.
The discovery suggests that there could be other systems in the galaxy that are more favourable to supporting life.
"We know that planets exist that are that old. We can assume that there might be stars similarly old and they might have planets that are further away that might be habitable," said Dr Huber.
He added that the next goal was to find an Earth-like planet that had a similar year-long orbit around a Sun-like star, meaning it could support human life.