SkyandTelescope.com March 2014 21as our fi rst confi rmed planet discovery. Planet Hunters
volunteers performed their own investigations to look for
additional transiting circumbinary planets with our Talk
discussion tool. Such planets have proved challenging for
automated routines to identify. The planet transits can
be easily washed out by the much deeper stellar eclipses,
drops in light caused by the two stars passing in front
and behind each other. Unlike planets around a single
star, the shape and repeat timing of the transits also vary
signifi cantly due to the changing velocities and positions
of the binary’s two stars.Jackpot!
After careful scrutiny, volunteers Robert Gagliano of Cot-
tonwood, Arizona, and Kian Jek from San Francisco, Cali-
fornia, spotted two transit-like features in the light curve
of the known eclipsing binary KIC 4862625. From the
NASA public archive, they downloaded the light curve for
the next three months of Kepler observations, which had
not yet been uploaded onto the Planet Hunters site. They
found a third transit close to the time they predicted, with
a depth similar to the previous two, and then notifi ed the
Planet Hunters science team. Analysis of the Kepler light
curve, combined with follow-up observations with the
10-meter Keck telescopes and deep optical imaging from
the 0.9-meter SARA telescope on Kitt Peak, confi rmed the
planetary nature of the transiting body. This is only the
seventh confi rmed circumbinary planet, and as Planet
Hunter’s fi rst confi rmed planet, it’s been named PH1 b.CIRCUMBINARY
PLANET
Far left: In this illus-
tration, we see a ren-
dition of PH1 b — the
fi rst planet discovered
by Planet Hunters’
citizen scientists and
the fi rst planet known
to exist in a quadru-
ple-star system. It
orbits the two stars to
the planet’s immedi-
ate left. In the far
upper left, we see the
other two stars in the
system.CITIZEN
SCIENTISTS
Planet Hunters
volunteers Robert
Gagliano and Kian
Jek discovered PH1 b,
which turned out to
have major scientifi c
importance.PH1 b is a 6.2-Earth-radii giant (between Uranus and
Saturn in size) that resides beyond the 20-day orbit of an
eclipsing pair of 1.5- and 0.4-solar-mass stars. The planet
transits across the larger star every 138 days. Further
observations, including adaptive-optics imaging, revealed
another pair of stars about 1,000 astronomical units from
the planet. This second binary is almost certainly orbiting
the previously known binary, making PH1 the fi rst known
quadruple-star system to host a planet. PH1 b’s properties
will help shed light on how planets form in such dynami-
cally extreme environments.
PH2 b, the project’s second confi rmed planet, is a
10.1-Earth-radii world orbiting every 282.5 days around
Sun-like star KIC 12735740. The planet was discovered
during a volunteer-organized search of the Talk tool for
new planet candidates. PH2 b’s discovery was truly an
international eff ort. Rafał Herszkowicz from Poland was
the fi rst person to fl ag a transit in the main Planet Hunt-
ers website. In February 2012, Mike Chopin in the U.K.
was the second to fl ag a transit and the fi rst to post his
fi nding on the discussion tool. Hans Martin Schwenge-
ler in Switzerland then went on to look at the rest of the
publicly released Kepler data months later and spotted
additional transits, making PH2 b a likely planet candi-
date. The discovery was then passed onward to the Planet
Hunters science team, who subsequently confi rmed PH2
b’s planetary nature.
The Jupiter-size gas giant resides in the star’s habit-
able zone, the goldilocks region where it is predicted toHAVEN GIGUERE / YALE UNIVERSITYROBERT GAGLIANOKIAN JEKPlanet Hunters2.indd 21 12/24/13 11:45 AM