Planets can form and survive in insanely dense star clusters

Could you imagine a view of the night sky that looks like this? It’s a star cluster — a region of space in which as many as 10,000 stars are jam-packed to within a few light years of each other. Now, evidence shows that even these areas can give rise to planets, implying that someone out there might actually have this view.

Top image: The globular star cluster Omega Centurai. This image shows a panoramic view of ~100,000 stars lying in the centre of the cluster. Credit: NASA, ESA and the Hubble SM4 ERO Team.

Virtually all stars are born in open clusters, including our own. Nearly 95% of these clusters fall apart after a few hundred million years owing to low stellar densities. But some clusters are so rich and dense that they last for literally billions of years.

Needless to say, these environments are far removed from our own. Cosmologists have speculated that it may be next to impossible for planets to form owing to intense radiation and harsh stellar winds that strip planet-forming materials from nearby stars.

Planets can form and survive in insanely dense star clusters

Artistic impression of NGC6811. Credit: Michael Bachofner.

But a new paper from researcher Soren Meibom of the Harvard-Smithsonian Center for Astrophysics shows that these assumptions may likely be wrong. By analyzing data provided by the Kepler space telescope, and by using the transit method of detecting extrasolar objects, Meibom and his team found two planets just slightly smaller than Neptune about 3,000 light-years from Earth in the open star cluster NGC6811.

The two new planets, Kepler-66b and -67b, join an exclusive club of star cluster planets. Of the 850 exoplanets discovered thus far, only four have been found in clusters. That’s only 0.47%! But this is what’s referred to as an observation selection effect. It’s not that these planets are rare — it’s just that they’re exceptionally hard to detect.

And indeed, considering the number of stars observed in this particular cluster, the discovery of these planets suggests that small planets can form and survive in a dense cluster environment. Moreover, the frequency and properties of planets in open clusters are consistent with those of planets around stars not located in a cluster (i.e. “field stars”).

So that view of Omega Centurai may not be so rare after all.

Read the entire study at Nature: “The same frequency of planets inside and outside open clusters of stars.”