This chart compares the smallest known alien planets discovered by the Kepler space telescope to Mars and Earth. Credit: NASA/JPL-Caltech.
COPENHAGEN, Denmark, June 13 (UPI) -- Small, rocky planets similar to Earth may be more common in our Milky Way and other galaxies than previously thought, European astronomers say.
While large numbers of Jupiter-like exoplanets have been found around stars with high concentrations of what astronomers term "metals," elements heavier than hydrogen and helium, smaller terrestrial planets don't need metal-rich stars to form, they said.
"Small planets could be widespread in our galaxy, because they do not require a high content of heavy elements to form," study lead author Lars Buchhave of the University of Copenhagen in Denmark told SPACE.com.
The researchers analyzed observations by NASA's Kepler planet-seeking space telescope of 226 planet candidates circling 152 stars.
More than three-quarters of them are smaller than Neptune, and some of them are as small as the earth, they said.
A study of the stars' spectra showed small, rocky worlds are associated with stars with a much broader range of metal content than those with giant planets.
"Naively, one might think that the more material you have in the [protoplanetary] disk, the more likely you are to form [small] planets," Buchhave said.
"What we see, though, is that small planets form around stars with a wide range in heavy element content, while the close-in Jupiter-type planets seem to predominantly form around stars with a higher metal content."
The findings suggest Earth-size worlds may not be rare inhabitants of the Milky Way, which increases the possibility of life on other worlds, Buchhave said.
"Since small planets could be widespread in our galaxy, the chances of life developing could be higher, simply because there could be more terrestrial-sized planets where life could evolve."