Washington: Astronomers have found two planets smaller than Neptune orbiting Sun-like stars 3,000 light-years from Earth, in the star cluster NGC 6811.
The discovery shows that planets can develop even in crowded clusters jam-packed with stars.
"Old clusters represent a stellar environment much different than the birthplace of the Sun and other planet-hosting field stars," lead author Soren Meibom of the Harvard-Smithsonian Center for Astrophysics (CfA) said.
"And we thought maybe planets couldn`t easily form and survive in the stressful environments of dense clusters, in part because for a long time we couldn`t find them," Meibom said.
The two new alien worlds appeared in data from NASA`s Kepler spacecraft.
Kepler hunts for planets that transit, or cross in front of, their host stars.
During a transit, the star dims by an amount that depends on the size of the planet, allowing the size to be determined.
Kepler-66b and Kepler-67b are both less than three times the size of Earth, or about three-fourths the size of Neptune (mini- Neptunes).
Kepler -66b and -67b are the smallest planets to be found in a star cluster, and the first cluster planets seen to transit their host stars, which enables the measurement of their sizes.
The findings are published in the journal Nature.