Kepler Discovery Confirms First Planet Orbiting Two Stars

The existence of a world with a double sunset, as portrayed in the film Star Wars more than 30 years ago, is now scientific fact. NASA’s Kepler mission has made the first unambiguous detection of a circumbinary planet — a planet orbiting two stars — 200 light-years from Earth, in the constellation Cygnus.

Astronomers used the Kepler Space Telescope to detected the planet, known as Kepler-16b, by observing transits, where the brightness of a parent star dims from the planet crossing in front of it. This milestone discovery confirms a theory that scientists have had for decades but could not prove until now.

When the smaller star partially blocks the larger star, a primary eclipse occurs, and a secondary eclipse occurs when the smaller star is occulted by the larger star. Astronomers further observed that the brightness of the system dipped even when the stars were not eclipsing one another, hinting at a third body.

The additional dimming in brightness events, called the tertiary and quaternary eclipses, reappeared at irregular intervals of time, indicating the stars were in different positions in their orbit each time the third body passed. This showed the third body was circling, not just one, but both stars, in a wide circumbinary orbit.

Kepler-16b is an inhospitable, cold world about the size of Saturn and thought to be made up of about half rock and half gas. The parent stars are smaller than our sun. One is 69% the mass of the sun and the other only 20%. Kepler-16b orbits around both stars every 229 days, similar to Venus’ 225-day orbit, but lies outside the system’s habitable zone, where liquid water could exist on the surface, because the stars are cooler than our sun.