Planet Discovered in Binary Star System

Planet Discovered in Binary Star System

Planet discovered orbiting two stars in a circumbinary system.

Article by Hudson Lofchie

Photo: Star Wars IV A New Hope

Published in the California Aggie ( on October 5th, 2011


What would our sky look like if two suns rose every morning and two suns set every evening?  Star Wars fans should recall the iconic scene at the end of episode IV “A New Hope” where Luke Skywalker gazes at two setting suns on the planet Tatooine.  It is a rare occurrence that scientists discover an astronomical phenomenon that already existed in science fiction, but researchers from UC Santa Barbara using NASA’s Kepler space telescope have done just that.The planet, romantically nicknamed “Tatooine”, is officially known as Kepler 16 (AB)-b and is about 200 light years from earth.  It is quite large, about the same size and mass as Saturn and is the first planet to be discovered orbiting a binary star system.

The two stars are quite different from each other, and from our own sun.  The larger star is orange, and is 69 percent the mass of our sun.  The smaller star is red, and is only 20 percent the mass of our sun.  The suns orbit each other every 41 days, and the planet orbits the stars every 229 days.

The sunset seen from the planet’s surface would be far better than the one depicted in Star Wars.

“The relative position of the two stars will change every day over a 41 day period,” said Avi Shporer, an astrophysicist at UCSB and part of the NASA team that discovered the system.  “It would be a very beautiful sunset to look at.”

The Kepler mission is based on planet-hunting.  It looks at a patch of space that is 1/400th of sky visible from earth and measuring changes in brightness of the stars it is watching.

“Kepler is examining 160,000 stars every 30 minutes,” said Matthew Richter, a UC Davis research physicist who studies star-forming planetary systems.  “[Kepler] takes thousands of pictures and looks for repetitive periodic changes in a star’s brightness.”

These periodic changes are indicative of a planet passing in front of the star.  However, Kepler can only detect if a planet is there, not what the planet is composed of or the planet’s atmospheric content.

“A problem with Kepler is that it has to be in the exact same plane as the planet relative to the star,” said Robert Becker, a UC Davis physicist studying extragalactic astrophysics and radio astronomy.  “For every planet that [Kepler] detects, there are hundred that it does not.”

This means that to be detectable, a planet must pass directly between a star and the Kepler satellite.

K16 is an important discovery because astronomers have predicted the existence of a system like it, but have never found one until now.  The system also challenges some of our known models of how orbits work.  It was believed that in a two-star system, a planet would have to orbit at seven times the distance that the stars orbit each other.  Surprisingly, K16’s orbit is only about half that distance.

However, astronomers do not believe that K16 was originally formed in its current orbit.

“The planet probably formed fairly far out in the circumbinary disk and then migrated inward,” Richter said.  “it is very unlikely that a planetary capture event occurred in this instance.”

Although Tatooine was capable of supporting life in a binary system, it is very unlikely that K16 would have that ability.  It lays far outside the stars’ habitable zone and has a surface temperature between negative 150 and negative 95 degrees Fahrenheit, far too cold to support liquid water–a necessity for life as we know it to exist.

It is hard to imagine what life would be like if it developed in the presence of two suns instead of one.  We wouldn’t want to see more scifi-turned-reality in the form of planet MC-117 from the movie “Pitch Black.”


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