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A family portrait of the PH1 planetary system: The newly

discovered planet is depicted in this artist’s rendition

transiting the larger of the two eclipsing stars it orbits. Off

in the distance, well beyond the planet orbit, resides a

second pair of stars bound to the planetary system.

©Haven Giguere/Yale

Searching for Planets from Your Sofa

An extrasolar planet or exoplanet is a planet orbiting a star outside of our own Solar System.

When an exoplanet passes or transits in front of its parent star, a small portion of the star's

light is blocked out. The star momentarily dims, signaling the existence of another solar

system outside our own with a distant world. This dimming of starlight lasts for a few hours

or more and repeats once per orbit of the planet. NASA's Kepler mission has spent the past

four years staring at a single patch of sky, simultaneously monitoring the same ~160,000 stars

for these signatures of transiting exoplanets.

A Jupiter-sized planet produces a large 1% drop in light as it transits across a Sun-like star.

Rocky planets generate a much smaller dimming, with the Earth only producing a 0.01% drop

in our Sun's light! From the ground it is difficult to detect the small transit depths from rocky

planets, but above the atmosphere with nearly uninterrupted observing Kepler is uniquely

providing a census of both gas giant and terrestrial planets. Understanding the abundances of

different types of planets is crucial to understanding planet formation and providing context

for our Solar System. Kepler is capable of detecting small rocky planets, enabling the

measurement of the frequency of potentially Earth-sized planets in the habitable zone, the

Goldilocks region around a star where it is not too hot and not too cold for liquid water to

exist on the surface of an orbiting rocky world.

The Kepler team developed

automated computer algorithms

to search the Kepler light

curves, the time series of

brightness measurements, for

the repeated signal of exoplanet

transits. To date over 3,000

planet candidates have been

discovered, but the Kepler light

curves are complex, many

exhibiting short-lived

brightness variations that are

often difficult to characterize.

Despite the impressive success

of the computers, the star itself

may have natural variability,

this sometimes makes it

difficult for the automated

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The light curve of a transiting planet with a 9 Earth radii (Jupiter-sized) planet on

a ~9 day orbit about its parent star. ©Planet Hunters/Zooniverse

routines to find transits. The human brain excels at pattern recognition and easily recognizes

transits that sophisticated automated routines may miss.

It is impossible for a single person to review the 4 years of observations for each of Kepler's

~160,000 stars, but with the Internet we can gather the help of hundreds of thousands of

people to hunt for exoplanets. Planet Hunters (http://www.planethunters.org) uses the World

Wide Web to enlist the general public to identify transits in the pubic Kepler light curves.

Visitors to the Planet Hunters' website are asked to draw boxes to mark the locations of visible

planet transits. Anyone can help. No training required, all you need is a web browser. To date,

280,000 volunteers worldwide have participated, contributing over 20 million classifications.

Planet Hunters is a novel and complementary technique to the Kepler Team’s automated

detection algorithms.

Planet Hunters is completing an independent assessment of the exoplanet population and

identifying planet candidates that have been missed by the automated routines. In the past 3

years, Planet Hunters has discovered PH1 b - one of only 7 known transiting circumbinary

planets (where the planet orbits both stars in a stellar binary) and the first confirmed planet

residing in a four star system, PH2 b - a confirmed Jupiter-sized planet in the habitable zone

of a Sun-like star, and over 40 additional planet candidates previously missed by traditional

techniques.

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We have just scratched the surface of the Kepler observations. A tiny fraction of the Kepler

data to date has been searched by Planet Hunters for additional planets that the automated

routines may have missed. There are so many light curves that have yet to be seen by human

eyes on Planet Hunters. We need your help! Join in the search for exoplanets today at

http://www.planethunters.org

(Author/Megan Schwamb)

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