Scientists work out how many planets like Earth are in the universe, marking breakthrough in search for alien life

Discoveries could help decide which world we examine for life

Scientists have worked out just how many planets like our own might be waiting out in the universe.

The new study gives the best estimate yet of how many Earth-like planets are orbiting around Sun-like stars.

The discovery will help guide astronomers as they search those planets for signs of alien life, by trying to understand more about the planets.

Nasa's Kepler Space Telescope helped find that there are thousands of planets waiting outside of our solar system, orbiting around their own suns. Over the nearly ten years it scanner the sky, it was watching out for transit events – the slight dips in light that happen when a planet moves in front of a star, which can be used to understand those planets' size and characteristics.

But scientists want to know how many of those alien worlds are like ours, sitting close enough to their star that they get enough light to provide energy for life.

"Kepler discovered planets with a wide variety of sizes, compositions and orbits," said Eric B. Ford, professor of astronomy and astrophysics at Penn State and one of the leaders of the research team.

"We want to use those discoveries to improve our understanding of planet formation and to plan future missions to search for planets that might be habitable. However, simply counting exoplanets of a given size or orbital distance is misleading, since it's much harder to find small planets far from their star than to find large planets close to their star."

To try and understand how many of those planets might be out there waiting, scientists came up with a new method to help estimate how many planets might have formed. They developed a model that allowed them to create new, imaginary universes of stars and planets – and then watch to see how many of those would have been picked up by the Kepler Space Telescope, and how many would have been missed.

"We used the final catalogue of planets identified by Kepler and improved star properties from the European Space Agency's Gaia spacecraft to build our simulations," said Danley Hsu, a graduate student at Penn State and the first author of the paper.

"By comparing the results to the planets catalogued by Kepler, we characterised the rate of planets per star and how that depends on planet size and orbital distance. Our novel approach allowed the team to account for several effects that have not been included in previous studies."

Scientists can now use the findings to comb through the rest of the universe looking for planets that look like our own, with the hope of launching a major mission to study them. How ambitious that mission needs to be will depend partly on just how many planets there are waiting to be found, and so the discovery will help decide the scale of that search.

The researchers found that there are probably planets like our own – between three-quarters and one-and-a-half times as big as our planet, and similar length years – waiting to be found around roughly one in four stars.

But they also worked out how accurate that estimate could be. The potential uncertainty means that missions should plan to find such worlds as often as every two stars, they suggest.