Detect the Transit of a Circumbinary Planet This June 28th!

by Laurance R. Doyle

The transit of Venus June 5, 2012 was not the only transit you may be able to see this month.  On June 28th (from India eastward to Japan) there will be another planetary transit. But this one is special as it will be the transit of the first such circumbinary planet (CBP) discovered, Kepler-16b. This primary transit of the planet—where the planet crosses the brightest star of the binary system—should be detectable from the Northeastern Hemisphere as a 1.7% drop in brightness. With a good sky, this should be  detectable even with small telescopes since the star system is so bright, about 11.7th magnitude. (This is about 190 times fainter than the naked eye can see.) So if you have a telescope, a CCD camera, and a clear sky somewhere in the Northeastern Hemisphere, you may be able to detect your first circumbinary planet!

The planet is the first transiting circumbinary planet ever detected, Kepler-16b (see Doyle et al., 2011, Science 333, 1602-1606), a Saturn-sized planet in a circular 229- day orbit around an orange (K2-dwarf) and red (M4.5-dwarf) eclipsing double star system. These two stars orbit (and also eclipse) each other every 41 days, resulting in a brightness drop of 13% when the primary, or larger star, is eclipsed, and a drop of about 1.6% when the smaller, secondary (red) star is eclipsed. On June 20th, the planet Kepler-16b will transit the smaller star but this secondary transit eventwill only produce an 0.1% drop in brightness – extremely difficult to detect from the ground. You would need a large telescope to even attempt this secondary transit. Below is a Table of the times (in universal time, UT) of these events, along with the dates in the Gregorian Calendar and times in along with the uncertainties in these times (in minutes).

Primary Transits (depth = 1.7%)
Julian Day                          Year: Month: Day: Hr: Min:Sec (UT)
2456107.2336 ± 0.0027   2012: June: 28: 17:36:23 ± 3.9 min.
2456328.7260 ± 0.0112   2013: February: 05: 05:25:26 ± 16.1 min.
2456558.9913 ± 0.0091   2013: September: 23: 11:47:28 ± 13.1 min.

Secondary Transits (depth = 0.1%)
Julian Day                           Year: Month: Day: Hour: Minute: Second
2456098.9150 ± 0.0060   2012: June: 20: 09:57:36 ± 8.6 min.
2456337.0155 ± 0.0155   2013: February: 13: 12:22:19 ± 22.3 min. 
2456550.7122 ± 0.0102   2013: September: 15: 05:05:34 ± 14.7 min.

Even though the planet’s orbit is aligned with that of the eclipsing binary system within 0.5 degrees, the system rotates and the planet is so far away from the stars that these transits will disappear for a time (the nodes will not align). Thus the primary transits of Kepler-16b will cease in early 2018 for 24 years, reappearing again sometime in 2042, (and the secondary transits of Kepler-16b will cease in May 2014 for 35 years, reappearing again sometime in 2049). You will find the star system at celestial coordinates (2000.0) Right Ascension = 19h 16m 18.17s and Declination = +51° 45’ 26.8’’. A finding chart is included here also. Please let me know (ldoyle@seti.org) if you are indeed able to detect the primary transit of Kepler-16b this June 28th as it will of significant interest in refining the orbits in this first detected transiting circumbinary planetary system. Clear skies! 

[image below: Kepler-16 Field of View]