Two super-Earths have for the first time, been found in the hostile environment of an open star cluster.
The discovery reported in the journal Nature, proves that small planets can survive where stars are tightly packed together.
Stellar clusters represent a very difficult environment for planetary formation, according to the study’s lead author, Dr Soren Meibom of the Harvard-Smithsonian Center for Astrophysics.
The two newly detected planets, named Kepler 66b and Kepler 67b, formed in a place where stars are up to a hundred times more tightly packed than in our stellar neighbourhood.
Both planets would have formed in disks of gas and dust by the thousands of stars jostling for space in the cluster.
“We were very sceptical that planets could survive,” says Meibom.
Both worlds were found by NASA’s planet hunting Kepler Space Telescope during a survey of 377 stars in the NGC 6811 cluster, located 3000 light-years from Earth.
Kepler searches for planets which transit, or cross in front of, their host stars. The transit blocks out some of the star’s light as seen by Kepler.
The duration of the dip in light tells scientists how big the planet is, and the interval between consecutive dips tells how far away from the star the planet is orbiting.
Kepler 66b takes just over 17 days to orbit a star about the same size as our Sun, while Kepler 67b takes a little over 15 days to orbit a star slightly smaller than our Sun.
“The stars are too faint for us to determine the masses of the planets directly from measurements, so we can only go by our experience and theoretical models to try to estimate the mass and the structure of the planets,” says Meibom.
“They’re in the super-Earth or mini-Neptune category, so we think they might have a significant amount of volatile icy gases in their atmospheres. They would not have a solid surface.”
According to Meibom, the cluster of stars is about one billion years old, indicating planets can survive at least that long in such a chaotic and hostile environment.
“Clusters are groups of stars that are born together at the same time in the same place,” says Meibom. “This cluster would have initially contained up to ten thousand stars in a very small amount of space.”
More than 850 planets has so far been discovered beyond our solar system. Four of these are giant Jupiter-sized planets in star clusters. Kepler-66b and Kepler-67b are the smallest cluster planets to be found, and the first seen to transit their host stars.