Planets in star clusters

In the star cluster NGC 6811, astronomers have found two planets smaller than Neptune orbiting sun-like stars. The research published in Nature may help show that planetary systems are much more common than thought. (Michael Bachofner / Harvard-Smithsonian Center for Astrophysics / June 26, 2013)

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Astronomers peering at stellar data from the Kepler space telescope have discovered a surprising find: two planets smaller than Neptune that appear to have formed even in the harsh, unwelcoming environment of a star cluster. The discovery, published in the journal Nature, may show that planetary systems are more robust – and far more common – than previously thought.

Such a planetary find has long been thought extremely unlikely because star clusters are thought to be very hostile places for a planet to survive. Only four planets have ever been found, and they were all Jupiter-sized or larger, William Welsh, an astronomer at San Diego State University who was not involved in the study, wrote in a commentary.

But scientists studying Kepler data from the open star cluster called NGC 6811, which sits thousands of light-years away in the constellation Cygnus, discovered two planets just two and three times the size of Earth. That’s after sampling just 377 stars in clusters, which is pretty good odds in such an environment, Welsh said.

Stars can grow in clusters – they begin to condense out of giant clouds of gas and dust in space, forming groups of bright young stars surrounded by the gassy molecular haze that birthed them. Dusty debris begins to form a disc around the star — just as it did around our sun, eventually coalescing to form the eight planets we know today.

But scientists have thought that a star living in such a dense knot of others stars would have trouble holding onto its delicate protoplanetary discs. After all, the gravity from neighboring stars in the cluster would wreak havoc on the ring of matter, possibly ripping it apart; and stellar winds and ultraviolet radiation from other hot, young stars nearby would blow much of the dusty debris away. As if that weren’t enough, much of a rising star’s protoplanetary entourage could get wiped out by supernovae, or the explosions of dying stars, in the cluster.

A star’s ring of dust – and the planets it could birth – would surely only survive if it were out of harm’s way, far enough from other stars to be affected by its siblings, scientists thought.

More than 95% of such clusters tend to dissipate in about 100 million years, the study authors pointed out – just a fraction of a typical star’s lifetime – blown out in part by the same forces that would have removed clustered stars’ planetary debris.

But now researchers have found two planets in a once-dense cluster, catching the dips in starlight from Kepler data. Kepler 66b and 67b are just 2.8 and 2.9 times the Earth’s size, and they have very short orbital periods, circling their stars in about 18 and 16 days, respectively.
What’s more, NGC 6811 is small but old, about a billion years old – meaning it must have been massive enough early in its history for its gravity to keep it together, preventing the cluster from dissipating entirely. This once-massive, dense cluster, which the authors say may have contained at least 6,000 stars, was probably particularly unfavorable for a young planet to survive in.

Perhaps planetary systems around stars of all kinds are far more common than thought, scientists say.

“Planet formation is robust — nature likes to create planetary systems, and many survive the birthing process,” Welsh wrote.