Kepler mission discovers first planet orbiting two stars
By Darren Quick
September 19, 2011
In news that conjures up visions of Luke Skywalker looking wistfully at the twin sunset of Tatooine accompanied by a stirring John Williams score, NASA's Kepler mission has detected the first planet orbiting two stars. The circumbinary planet, dubbed Kepler-16b, is some 200 light-years from Earth and, though gaseous and not thought to harbor life, its discovery broadens the opportunities for life in our galaxy according to Kepler principal investigator William Boruckias, because most of the Milky Way's stars are part of binary systems.
Although scientists have theorized for decades that circumbinary planets were possible and previous research had hinted at their existence, the discovery of Kepler 16b is the first time the theory could be confirmed. The newly discovered planet was detected in the Kepler-16 system - a pair of orbiting stars that eclipse each other from our vantage point here on Earth. When the smaller star partially blocks the larger star, a primary eclipse occurs, and a secondary eclipse occurs when the smaller star is occulted, or completely blocked, by the larger star.
Observations from the Kepler space telescope showed that the brightness of the system dipped even when the stars weren't eclipsing one another, which hinted at a third body. The additional dimming events, called the tertiary and quaternary eclipses, reappeared at irregular intervals, indicating that the stars were in different positions in their orbit each time the third body passed. This showed that the third body was circling both stars in a wide circumbinary orbit.
Using the changes in their eclipse times, the scientists were able to measure the gravitational pull exerted on the stars by the planet, which provided a good indicator of the mass of the third body. Since only a very slight gravitational pull was detected, the scientists say the third body must only have a small mass. They say Kepler-16b is an inhospitable, cold world about the size of Saturn that is thought to be made up of a roughly 50/50 split of rock and gas.
Every 229 days Kepler-16b orbits around both stars, which are both smaller than our sun. One is 69 percent the mass of the sun and the other is only 20 percent. Although this is similar to Venus' 225-day orbit, Kepler-16b lies outside its system's habitable zone (where liquid water could exist on the surface), because the stars are cooler than our sun.
The discovery of Kepler-16b is detailed in a new study published in the journal Science.
Here's NASA's video overview of the discovery: