A circumbinary planet is a one which orbits around a pair of normal stars. Kepler-16b became the first known circumbinary planet when it was discovered in 2011. Since then, two more circumbinary planets have been discovered and they are Kepler-34b and Kepler-35b. All three circumbinary planets known to date were found by NASA’s Kepler space telescope which detects planets around other stars by searching for tiny dips in a star’s brightness when a planet happens to cross in front of its parent star. Both Kepler-34b and Kepler-35b are low-density gas-giant planets and their orbits are closely aligned with that of their parent stars such that each planet is observed to transit both its parent stars. The planets Kepler-34b and Kepler-35b were identified using 671 days of data from the Kepler space telescope.
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Figure 1: An artist’s conception of Kepler-34b: a gas-giant planet that orbits a pair of Sun-like stars. Credit: David A. Aguilar (CfA)
Kepler-34b orbits a pair of Sun-like stars every 289 days where both the Sun-like stars orbit around each other every 27.8 days. For Kepler-34b, its discovery was determined from 3 detected transit events. The first and second transits are of the primary star (Kepler-34A), while the third is of the secondary star (Kepler-34B).
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Figure 2: An artist’s rendition of Kepler-35b circumbinary planet. Credit: Lior Taylor
Kepler-35b orbits a pair of stars every 131 days. Each star in the Kepler-35 system is somewhat smaller than the Sun and both stars orbit around each other every 20.7 days. For Kepler-35b, its discovery was determined from 4 detected transit events. The first, second and fourth transits are of the primary star (Kepler-35A), while the third is of the secondary star (Kepler-35B).
Circumbinary planet system parameters for Kepler-34b, Kepler-35b and Kepler-16b:
| Kepler-34b | Kepler-35b | Kepler-16b | |
| Planet’s Properties: | |||
| Mass of planet (Earth = 1) | 69.9 | 40.4 | 106 |
| Mass of planet (Jupiter = 1) | 0.220 | 0.127 | 0.333 |
| Radius of planet (Jupiter = 1) | 0.764 | 0.728 | 0.754 |
| Mean density of planet (kg/m3) | 0.613 | 0.410 | 0.964 |
| Planet’s surface gravity (m/s2) | 9.36 | 5.96 | 14.5 |
| Planet’s surface gravity (Earth = 1) | 0.954 | 0.608 | 1.48 |
| Properties of Planet’s Orbit: | |||
| Orbital period (days) | 289 | 131 | 229 |
| Semi-major axis (AU) | 1.09 | 0.603 | 0.705 |
| Orbital eccentricity | 0.182 | 0.042 | 0.007 |
| Properties of the Stars: | |||
| Mass of primary (Sun = 1) | 1.05 | 0.888 | 0.690 |
| Radius of primary (Sun = 1) | 1.16 | 1.03 | 0.649 |
| Mass of secondary (Sun = 1) | 1.02 | 0.809 | 0.203 |
| Radius of secondary (Sun = 1) | 1.09 | 0.786 | 0.226 |
| Properties of Stars’ Orbit: | |||
| Orbital period (days) | 27.8 | 20.7 | 41.1 |
| Semi-major axis (AU) | 0.229 | 0.176 | 0.224 |
| Orbital eccentricity | 0.521 | 0.142 | 0.159 |
For Kepler-34b and Kepler-35b, the average insolation received by each planet is 2.4 and 3.6 times the Earth’s insolation respectively. Furthermore, the maximum-to-minimum insolation ratios for Kepler-34b and Kepler-35b are 250 percent and 160 percent respectively. Such highly variable and multi-periodic fluctuations in insolation are unique to circumbinary planets, and are expected to lead to complex climate cycles and interesting atmospheric dynamics. Together with Kepler-16b, the discovery of Kepler-34b and Kepler-35b establishes circumbinary planets as a new class of planets. A unique feature of these 3 circumbinary planets is that the orbital plane of each planet is remarkably coplanar with the orbital plane of its central binary stars. Based on a conservative estimate, millions of nearly coplanar circumbinary planets are expected to exist in this galaxy alone.
Reference: Welsh, et al. “Transiting circumbinary planets Kepler-34 b and Kepler-35 b”, Nature 481, 475–479 (26 January 2012)