20 March 2014 sky & telescopeEasy Amateur Science560,000 Eyes Search for Planets
One of the Zooniverse’s most successful projects is Planet
Hunters (planethunters.org), led by Debra Fischer (Yale
University). Launched in late 2010, Planet Hunters enlists
the public’s help in the search for exoplanets.
NASA’s Kepler spacecraft spent the past four years
staring at the same patch of sky in Cygnus and Lyra,
monitoring the brightness of nearly 160,000 stars nearly
continuously for the signatures of transiting exoplanets.
During a transit, a planet dims a tiny fraction of its host
star’s light, with the amount enabling astronomers to
estimate the planet’s diameter. Jupiter-size worlds orbit-
ing a Sun-like star produce whopping transit signals with
depths of about 1%. Unlike ground-based telescopes,
Kepler could detect the less-than-0.01% transit depth pro-
duced by Earth-size planets orbiting Sun-like stars with
the sensitivity equivalent to the task of spotting a fruit fl y
passing in front of a distant stationary car’s headlight.
Kepler has truly revolutionized the fi eld, discovering
more than 3,500 planet candidates (last month’s issue,
page 14). It has found Earth-size planets, rocky plan-
ets, the fi rst small planets residing in a star’s habitable
zone, and the fi rst confi rmed transiting planets orbiting
two stars (circumbinary planets). Despite these impres-
sive “fi rsts,” Kepler is primarily a statistical mission to
measure the frequencies of planets around Sun-like stars.
With Kepler, we’ve learned that planets are abundant
in our galaxy. Kepler has also revealed a treasure trove
of exotic systems, whose architectures and frequencies
provide a window into the formation and evolution of
planetary systems (August 2013 issue, page 18).
The Kepler science team uses automated detection
algorithms to search the Kepler light curves for repeat-
ing transit-like features. Planet Hunters complementsthe machines by utilizing the human brain’s innate
ability to pick out outliers to identify planet transits that
the computers potentially missed. More than 280,000
volunteers worldwide have helped to visually inspect
the publicly released Kepler light curves for the signa-
tures of transiting exoplanets, drawing boxes around
potential transits spotted in the web interface. Five to 10
people independently review the same 30-day segment
of a random Kepler star’s light curve. To identify new
planet candidates, these multiple responses are assessed
and combined. Our volunteers also identify new planet
candidates with a companion to the main classifi cation
interface known as Talk (talk.planethunters.org). With
Talk, volunteers can actively discuss the light curves
served on the Planet Hunters site with other members of
the community and the science team.
Launching Planet Hunters was a bit of gamble. My
fellow members of the Planet Hunters team and I didn’t
know if people would come and review graphs of Kepler
light curves, which are not as beautiful as Galaxy Zoo’s
stunning galaxy images. We wondered if the project
would fi nd anything new. The Kepler automated algo-
rithms had a head start and already searched the same
observations for planets. But the gamble paid off big
time! The response has been overwhelming, with more
than 20 million classifi cations made to date. The project
has discovered two confi rmed planets and more than 30
unique planet candidates, with potentially many more to
come. Our volunteers have also discovered new RR Lyrae
variable stars, dwarf novae, and eclipsing binary stars.
With so many eyes inspecting the light curves with
Planet Hunters, there were bound to be surprises, suchPolish citizen
scientist Rafał
Herszkowicz
was one of the
fi rst to notice
a transit sig-
nal from the
planet PH2 b.Left: Planet Hunters’s Talk sec-
tion enables citizen scientists to
converse with one another and
with the Planet Hunters science
team about light curves and
other topics.COMMUNITY ENGAGEMENTPLANET
CODISCOVERERRAFAŁ HERSZKOWICZPlanet Hunters2.indd 20 12/24/13 11:45 AM