The environment of a star cluster might be harsh, but the movement of a couple of planets transiting across the face of two members of NGC 6811 some 3,000 light years away hasn’t gone unnoticed by data returned by NASA’s Kepler spacecraft, adding more fuel to the fire that planets can indeed exist in these extreme conditions.
NGC 6811 is an open cluster in the constellation Cygnus and features approximately 70 Sun-like stars of roughly the same age, loosely bound to each other by a mutual gravitational attraction. These rich collections of stars, which survive for billions of years, are thought to be beds of high-energy radiation and unforgiving stellar winds that rip the very building blocks of planetary formation from the clutches of nearby stars.
However, to the surprise of astronomers like Soren Meibom at the Harvard-Smithsonian Center for Astrophysics (CfA) who led the study, some worlds can actually withstand the hot and star-cramped conditions.
“Our results show that planets can form and survive in the extreme stellar environment of rich open star clusters,” Meibom tells All About Space. “Our future studies of clusters with even higher stellar number densities will show whether there is a threshold to how dense and hostile an environment can become and still form planets.”
In their transit across their parent stars the two worlds, dubbed Kepler-66b and Kepler-67b, which rest around 15 million kilometres from their host stars (around one-tenth the Earth-Sun distance), have blocked out enough starlight for their sizes to be determined. The gaseous duo are less than three times the size of Earth and, due to their size and composition, are so-called mini-Neptunes. “The planets are thought to have gaseous envelopes,” says Meibom. “We base that on their sizes and models of planetary structure, so they likely do not have a rocky surface and oceans on or in which life could evolve.”
Since the majority of stars in an open cluster are of the same age, Meibom and his team have not only been able to date the cluster, but also the planets as well. “The planets formed at the same time the stars formed,” he says. “The cluster is about a billion years old.”
And it seems that this pint-sized Neptune variety of planet is common in these somewhat violent conditions. “Other types of planets can be found in clusters, however, the mini-Neptunes are the most common size planet and, since we have observed only a relatively small number of cluster stars, it is not expected that we would have found any after looking at only 377 stars,” explains Meibom.
While some information has been yielded about the planets, there are some characteristics of these alien worlds that remain intelligent guesses. “[We] are limited by the fact that we cannot measure their mass,” admits Meibom. “We can only try to constrain their masses using models of planetary structure relating size to mass. In the future we might build telescopes and instruments to measure their masses. The two planets probably weigh less than 20 Earth masses.”
If we were to somehow grab a view into space from either one of the duo, we would get a night sky different to our own. “When the cluster formed it contained at least 6,000 stars, many of them 20-30 times as massive as our Sun and hundreds of thousands times brighter,” explains Meibom, whose discovery has brought the number of star cluster planets to four out of more than 850 known distant worlds beyond the confines of our Solar System. “The average distance between stars would have been at least 50 times smaller than the distance to the Sun’s closest neighbour, making all stars more than 2,000 times brighter than their counterparts in the solar neighborhood. In other words, the night sky would be peppered with very bright stars, not like the Sun seen from Earth, but very bright stars.”
You can follow Gemma on Twitter @Gemma_Lavender
Images courtesy of Michael Bachofner (top) and Roberto Mura (bottom)