Unlikely Planets Found in Violent Star Cluster

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Artist's impression of exoplanets orbiting stars in a dense cluster.
Michael Bachofner

When it comes to forming planets, Mother Nature isn’t very picky. Despite horrific conditions inside densely packed open clusters, stars apparently have no problem forming and hanging on to an orbital brood.

That’s the conclusion from a new study that used data collected by NASA’s now-dormant Kepler space telescope to hunt for planets in a one-billion-year old open cluster called NGC 6811, a collection of about 70 stars located about 3,400 light years away in the constellation Cygnus.

Astronomers found two small Neptune-like planets orbiting sun-like stars in the cluster, which is believed to have once been far more populated than how it appears today.

The planets, named Kepler-66b and Kepler-67b, are not the first discovered in an open cluster (as opposed to free-flying so-called “field stars” which are easier to scan for planets) but they are the smallest.

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To find two such worlds by Kepler, which sees only the fraction of planets that pass directly in front of their parent stars, relative to the telescope’s point of view, means that they should be as common as planets found orbiting field stars, said astronomer Soren Meibom with Harvard University.

The planets are too close to their host stars for liquid water, a condition believed to be necessary for life, and they are unlikely to even be solid bodies. Nevertheless, the study expands the domain where small planets can exist.

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So far, the only place where planets haven’t been found yet is in globular clusters, an environment even more extreme that open clusters like NCG 6811.

“There is a threshold for when you cannot form planets or you make very different planets,” Meibom told Discovery News.

Astronomers believe most, if not all, stars started off in clusters, which then collapsed over time. Some member stars gradually drifted off, becoming field stars, taking their planets along with them.

What was unknown is if the most commonly found planets in the galaxy, the super-Earths and mini-Neptunes (none of which ironically appear in our own solar system) could survive all the gravitational elbowing, radiation bombardment, supernova explosions and other conditions resulting from so many stars being jammed into a relatively small slice of celestial real estate.

“It’s a very different birth environment than our own sun and planets,” Meibom said. “There was a very high probability that fragile planetary systems would fall apart.”

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The study likely means that the estimate of 50 billion planets in the Milky Way galaxy, a number derived from Kepler data, is low since it does not include stars in clusters, added astronomer William Welsh, with San Diego State University.

“It looks like planets are just as common in clusters as everywhere else,” Welsh told Discovery News. “It is a little surprising because the cluster environment is harsh.”

Clusters represent a small fraction of the approximately 160,000 target stars that were watched by Kepler from 2009 until a positioning wheel failure suspended operations last month. Trouble-shooting and a potential attempt to resurrect the observatory are under way.

The telescope is designed to look for Earth-sized planets positioned the right distance from their parent stars for liquid surface water.

Meibom’s next project is too look for planets in two even-more densely packed open clusters.

The research appears in this week’s Nature.