NASA’s Kepler telescope has discovered the existence of a world with a double sunset – as famously depicted in Star Wars more than 30 years ago.
However, unlike Star Wars’ Tatooine, the circumbinary planet – which is located more than 200 light-years from Earth – is cold, gaseous and not thought to harbor life.
Nevertheless, the discovery of Kepler-16b demonstrates the diversity of planets in our galaxy. Indeed, previous research hinted at the existence of circumbinary planets, but clear confirmation proved somewhat elusive until the telescope confirmed 16b by observing transits, where the brightness of a parent star dims from the planet crossing in front of it.
“This discovery confirms a new class of planetary systems that could harbor life,” Kepler principal investigator William Borucki explained.
“Given that most stars in our galaxy are part of a binary system, this means the opportunities for life are much broader than if planets form only around single stars. This milestone discovery confirms a theory that scientists have had for decades but could not prove until now.”
According to Laurance Doyle of the SETI Institute in Mountain View, scientists detected the new planet in the Kepler-16 system – a pair of orbiting stars that eclipse each other from our vantage point 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. 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, known as 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 indicated the third body was circling, not just one, but both stars, in a wide circumbinary orbit.
The gravitational tug on the stars, measured by changes in their eclipse times, was a good indicator of the mass of the third body. Only a very slight gravitational pull was detected, one that only could be caused by a small mass.
“Most of what we know about the sizes of stars comes from such eclipsing binary systems, and most of what we know about the size of planets comes from transits,” said Doyle. ”Kepler-16 combines the best of both worlds, with stellar eclipses and planetary transits in one system.”
Unfortunately, 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, with one pegged at 69% percent the mass of the sun and the other at 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, as the stars are cooler than our sun.