The discovery of two mini-Neptunes around Sun-like stars in a distinctly un-Sun-like environment reveals that small planets can live in more crowded neighborhoods than we thought.Most stars form in clusters. Those stars, and the planets they host, grow up surrounded by tens or even thousands of stellar siblings born from the same gas cloud. In the jostling for space, it’s a wonder any planets survive. The question is, how dense is too dense?
Answering that question has been difficult: until now, astronomers had detected only four planets inside clusters, compared to more than 850 planets found orbiting isolated stars in the galactic field. But Soren Meibom (Harvard-Smithsonian Center for Astrophysics) and a team of astronomers recently reported in Nature two mini-Neptunes inside a dense cluster, showing that even stars in the densest clusters can form planets.
The two planets, Kepler 66-b and Kepler 67-b, are slightly smaller than Neptune, each almost three times as wide as Earth. They closely circle Sun-like stars, whipping around every 17.8 and 15.2 days, respectively. But their environment is anything but Sun-like. They live in NGC 6811, a billion-year-old open cluster that was once jam-packed with more than 6,000 stars.
The Sun also grew up in an open cluster. But the Sun’s birthplace was 100 times less dense than NGC 6811, and the surrounding stars went their separate ways over the course of several million years, maybe even before the stars and planets had finished forming. NGC 6811, on the other hand, survived to the present-day, even though it lost some members to gravitational interactions or supernova explosions.
In the jam-packed environs of an open cluster, countless dangers await a forming planet. A gravitational bump from a passing star could disrupt the protoplanetary disk or nearby supernovae explosions might obliterate a newly formed planet.NGC 6811 is one of four old and dense open clusters studied in the Kepler Cluster Survey before the planet-hunting spacecraft’s demise.
The glaring brilliance of the most massive stars in these clusters has long since burnt out, so Kepler was able to closely watch 377 fainter, longer-lived stars. So far, two of these stars turn out to host transiting planets.
Two planets around 377 stars — Meibom and his colleagues wondered, was that rate normal? Or did the tightly-packed cluster stifle planet formation? If all the stars in NGC 6811 were field stars, Meibom’s team calculated that they would expect to find between 2 and 6 transiting planets and between 0.7 and 3.7 mini-Neptunes. (Of course, they’re not going to find partial planets, but these figures are based on a statistical study.) The two transiting mini-Neptunes Meibom’s team discovered fall squarely in the middle of the expected range.
They calculated that between 0.1 and 2.3 Earth-sized planets and between 0 and 1.2 giant planets would be detectable — so the fact that the team didn’t find any Earth-sized or giant planets is also expected.
“Whatever the process is that makes planets out of gas clouds, it works in the very dense regions of space where hundreds of stars are formed,” explains Kenneth Janes (Boston University).
The existence of these two mini-Neptunes shows that planets can form just as well in neighborhoods far more crowded and violent than the calm environment of our Sun’s youth. Meibom plans to continue planet-hunting in even denser clusters until he finds the “crowdedness” threshold beyond which planets can’t form.