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Galactic miracle babies? Smallish planets survived birth in stellar maelstrom.

Astronomers say the Kepler mission found two mini-Neptune planets orbiting stars in a stellar cluster that would have been a most inhospitable environment at the time they were born.

By Staff writer / June 26, 2013

In the star cluster NGC 6811, astronomers have found two planets smaller than Neptune orbiting Sun-like stars.

Michael Bachofner

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In a cosmic episode of "Survivor," astronomers say they have found two mini-Neptunes, each orbiting its own star in a stellar cluster that would have been a very rough neighborhood when the planets were born.

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The discovery addresses a longstanding question: "What is the effect of the stellar environment on the process of planet formation?" writes astronomer Soren Meibom, who led the team announcing the find, in an e-mail.

The find suggests that planet formation is a more robust, insistent process than previously thought. Planets appear to form at about the same rate in dense, open clusters as they do in far more benign ones, writes Dr. Meibom, a researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. The team is publishing a formal report of its results in Thursday's issue of the journal Nature.

Four other planets have been found previously orbiting stars in clusters, but they have been Jupiter's size or larger. These two new planets represent the smallest yet found in a once-dense cluster.

These are not the kind of planets that would set an astrobiologist to tingling with delight. Each planet is about three times the size of Earth. Each orbits a 1-billion-year-old, sun-like star every 16.8 days for one planet and 15.7 days for the other. These planets would be baking.

Even so, they represent the galaxy's miracle babies.

They appeared in data gathered by NASA's ailing Kepler mission. Kepler is a craft designed to orbit the sun at Earth's distance and stare at one patch of sky continuously, taking in views of some 170,000 stars. The craft detects the slight wink a planet imparts to starlight as it transits in front of its host star. The goal is to develop a planetary census, with a particular eye to estimating the number of Earth-mass planets orbiting sun-like stars at earth-like distances.

The two new planets are the first to be found orbiting stars in a cluster in Kepler's data.

The stars, Kepler 66 and 67, appear in an open cluster dubbed NGC6811, some 3,600 light-years away in the constellation Cygnus. The cluster contains at least 450 stars. The stars are loosely bound by their collective gravity and so disperse over time, hence the moniker "open." [Editor's note: The original version of this story incorrectly identified how many stars the cluster contains.]

Nearly all stars form in open clusters as they condense out of common clouds of gas and dust, researchers say. Most of these open clusters are relatively sparsely populated – perhaps forming fewer than 100 stars for each cubic parsec of space – a cube roughly 3 light-years on a side. Even that is overpopulation by the standard's of today's sun. Its closest neighbor is Proxima Centauri, about 4 light-years away.

These less-dense clusters, such as the one that gave birth to the sun, are relatively peaceful planetary nurseries and tend to disperse quickly.

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