Scientists have discovered 18 Earth-sized planets beyond the solar system, including one of the smallest known so far and another that could offer conditions friendly to life.
The exoplanets are so small that previous surveys had overlooked them, said researchers at the Max Planck Institute for Solar System Research (MPS) in Germany.
The study, published in the journal Astronomy & Astrophysics, re-analysed a part of the data from NASA's Kepler Space Telescope with a new and more sensitive method that they developed.
The team estimates that the new method has the potential of finding more than 100 additional exoplanets in the Kepler mission's entire data set.
Somewhat more than 4,000 planets orbiting stars outside our solar system are known so far.
Of these so-called exoplanets, about 96 per cent are significantly larger than our Earth, most of them more comparable with the dimensions of the gas giants Neptune or Jupiter.
This percentage likely does not reflect the real conditions in space, however, since small planets are much harder to track down than big ones.
Moreover, small worlds are fascinating targets in the search for Earth-like, potentially habitable planets outside the solar system.
The 18 newly discovered worlds fall into the category of Earth-sized planets.
The smallest of them is only 69 per cent of the size of the Earth; the largest is barely more than twice the Earth's radius.
Common search algorithms were not sensitive enough, researchers said.
In their search for distant worlds, scientists often use the so-called transit method to look for stars with periodically recurring drops in brightness.
If a star happens to have a planet whose orbital plane is aligned with the line of sight from Earth, the planet occults a small fraction of the stellar light as it passes in front of the star once per orbit.
Small planets, however, present scientists with immense challenges.
Their effect on the stellar brightness is so small that it is extremely hard to distinguish from the natural brightness fluctuations of the star and from the noise that necessarily comes with any kind of observation.
The team, including Rene Heller from MPS, has now been able to show that the sensitivity of the transit method can be significantly improved, if a more realistic light curve is assumed in the search algorithm.
The new algorithm does not search for abrupt drops in brightness like previous standard algorithms, but for the characteristic, gradual dimming and recovery.
This makes the new transit search algorithm much more sensitive to small planets the size of the Earth.
"Our new algorithm helps to draw a more realistic picture of the exoplanet population in space," said Michael Hippke of Sonneberg Observatory.
"This method constitutes a significant step forward, especially in the search for Earth-like planets," Hippke said.
"In most of the planetary systems that we studied, the new planets are the smallest," said Kai Rodenbeck of the University of Gottingen in Germany.
Most of the new planets orbit their star closer than their previously known planetary companions.
The surfaces of these new planets therefore likely have temperatures well in excess of 100 degrees Celsius; some even have temperatures of up to 1,000 degrees Celsius.
Only one of the bodies is an exception: it likely orbits its red dwarf star within the so-called habitable zone.
At this favourable distance from its star, this planet may offer conditions under which liquid water could occur on its surface -- one of the basic prerequisites for life as we know it on Earth.