Two Tiny Planets Survived Birth in Harsh, Stressful Star Cluster NGC 6811

When a star is first born it begins its life in a cluster, forming out of the raw material that collects together in a group. Some of these stellar groups are benign, quickly falling apart. Others, though, form in dense swarms that survive for billions of years as stellar clusters, exuding strong radiation and harsh stellar winds that scour interstellar space. Now, astronomers have located two alien worlds among these harsh conditions, revealing a bit more about planet formation.

About 3,000 light-years from Earth, two planets orbit sun-like stars in the star cluster NGC 6811. Smaller than Neptune, these planets are located in a cluster that's jam-packed with stars. At first glance, this area would seem hostile to planets; stellar winds strip planet-forming materials from nearby stars. Yet it seems that against all odds these small planets have survived.

The astronomers first found these two planets in data from NASA's Kepler spacecraft. Named Kepler-66b and Kepler-67b, the planets are both less than three times the size of Earth--about three-fourths the size of Neptune. They're the smallest to be found in a star cluster and the very first cluster planets seen to transit their host stars.

"Old clusters represent a stellar environment much different than the birthplace of the sun and other planet-hosting field stars," said Soren Meibom of the Harvard-Smithsonian Center for Astrophysics in a news release. "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."

The detection of these small planets implies that the frequency and properties of planets in open clusters are consistent with those of planets around field stars. Field stars are stars that are not within a cluster or association.

"These planets are cosmic extremophiles," said Meibom. "Finding them shows that small planets can form and survive for at least a billion years, even in a chaotic and hostile environment."

The research shows that astronomers still have quite a lot to learn about our universe. The new planets prove that it's possible for worlds to form even under extreme conditions, which means the range and type of planets varies far more widely than thought.

The findings are published in the journal Nature.