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Planet Hunters Find Circumbinary Planet in 4-Star System
PH1 system illustration
In this artist's rendition, PH1 is depicted with the two eclipsing stars it orbits. Off in the distance is a second pair of stars bound to the planetary system. Credit: Haven Giguere/Yale

The Planet Hunter volunteers, Kian Jek of San Francisco, CA, and Robert Gagliano of Cottonwood, AZ, found PH1 using the transit method, studying faint dips in starlight as a planet passes in front of (transits) its parent stars.

Gagliano is "absolutely ecstatic" about the finding. "It's a great honor to be a Planet Hunter, citizen scientist, and work hand in hand with professional astronomers, making a real contribution to science," he said.

Jek expressed amazement. "It still continues to astonish me how we can detect, let alone glean so much information, about another planet thousands of light-years away just by studying the light from its parent star," he said in a statement.

PH1 is a Neptune-size planet, with radius about 6.2 times that of Earth's with a 138 day orbit. The two parent stars have masses about 1.5 and 0.41 times that of the Sun and circle each other once every 20 days. Two other stars orbit PH1's twin suns at a distance of about 1,000 astronomical units.

This strange, four-star planetary system is the first confirmed world discovered as part of the Yale University-led Planet Hunters project, in which armchair astronomers work with professional scientists to find evidence of new worlds in the bountiful data collected by NASA's Kepler space telescope.

"Planet Hunters is a symbiotic project, pairing the discovery power of the people with follow-up by a team of astronomers," said Debra Fischer, a professor of astronomy at Yale and planet expert who helped launch Planet Hunters in 2010, in a statement. "This unique system might have been entirely missed if not for the sharp eyes of the public."

"I celebrate this discovery as a milestone for the Planet Hunters team: discovering their first exoplanet lurking in the Kepler data. I celebrate this discovery for the wow-factor of a planet in a four-star system," said Natalie Batalha, Kepler scientist at NASA Ames Research Center, Moffett Field, Calif. "Most importantly, I celebrate this discovery as the fruit of exemplary human cooperation-- cooperation between scientists and citizens who give of themselves for the love of stars, knowledge, and exploration."
Artist's illustration shows PH1, circumbinary planet, transiting the larger of the two eclipsing stars it orbits. Two more stars orbit the circumbinary system, making a total of 4 stars in the system. Credit: Haven Giguere/Yale

Since its discovery by Planet Hunters, PH1 has been confirmed by professional astronomers who presented their work Oct. 15 at the annual meeting of the Division for Planetary Sciences of the American Astronomical Society in Reno, Nev.

The researchers estimate PH1's temperature to be too hot to be in the habitable zone. "Although PH1 is a gas giant planet, even if there is a possibility of rocky moons orbiting the body, their surfaces would be too hot for liquid water to exist," researcher Meg Schwamb of Yale University and colleagues write in a draft of their research article -

Until now, there have been discovered only six planets orbiting two parent stars, called circumbinary planets, and none of these have stellar companions orbiting them.

...Schwamb said in a statement. "The discovery of these systems is forcing us to go back to the drawing board to understand how such planets can assemble and evolve in these dynamically challenging environments."

For more information on the Planet Hunters project, visit:

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