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## NASA's Kepler Discovers Multiple Planets Orbiting a Pair of Stars121

DevotedSkeptic writes "Kepler has continued its stellar (pun intended) discovery spree, this time locating multiple planets orbiting a binary star system. This is especially interesting because it proves that more than one planet can form under the stresses of a binary star system. The system is known as a circumbinary planetary system, a mechanism where a planet orbits two stars. Prior to this discovery, having multiple planets in a circumbinary system was unproven. Named Kepler-47, the system consists of a pair of orbiting stars that eclipse each other every 7.5 days. One star is similar in size to our Sol, however it only provides approximately 84% of Sol's light, the other is smaller, measuring one third of the size of Sol and emits less than 1% of Sol's light. Kepler-47b is the closer planet to its two suns, orbiting in 50 Earth days. Kepler-47c is further out and orbits every 303 days, within the Goldilocks zone. 'Unlike our sun, many stars are part of multiple-star systems where two or more stars orbit one another. The question always has been — do they have planets and planetary systems? This Kepler discovery proves that they do,' said William Borucki, Kepler mission principal investigator at NASA's Ames Research Center in Moffett Field, Calif. 'In our search for habitable planets, we have found more opportunities for life to exist.'"
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## NASA's Kepler Discovers Multiple Planets Orbiting a Pair of Stars

• #### Re:7.5 days? (Score:5, Informative)

on Wednesday August 29, 2012 @10:58AM (#41167939)
Using Kepler's 3rd Law, a^3 = p^2, with a = average orbital separation in AU (Earth to Sun distance), and p the orbital period in years: a = (7/365)^(2/3) = 0.07 AU. 1 solar radius is about 0.0046 AU. Go to the original paper here: http://www.sciencemag.org/content/early/2012/08/27/science.1228380.abstract [sciencemag.org] and you see the larger star is about the size of our Sun, the smaller star 1/3 the size. 0.07 AU/0.0046(AU per radius) = 15.2 Solar radius separation between the stars. So, close but not close to overlapping.
• #### Re:Velocity? (Score:5, Informative)

on Wednesday August 29, 2012 @11:05AM (#41168037)
Looks like I'm fielding the astronomy numbers today. Ok, look at my response to the 7.5 days question. The stars are separated by 0.07 AU (distance Earth to Sun in our system). The center of gravity's closer to the more massive star, so let's say the center of mass is 1/4(0.07 AU) from the larger star. Assume a circular orbit (not a bad assumption). Then, v = 1/4*2PiR/(t) = 2*Pi*(1/4*0.07AU*1.15x10^11 m/AU)/4*(7 days*24*60*60) = 390,000 m/s = 3.9*10^5 m/s, a tiny fraction of c, which is 3*10^8 m/s. Fast, not that fast. 0.001 c
• #### Re:Do they have a figure 8 orbit? (Score:3, Informative)

on Wednesday August 29, 2012 @11:16AM (#41168173)

Not possible given the orbital periods. Both planets orbit outside the orbits of the twin stars. (That's the definition of circumbinary, also.)

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