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.
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).
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:
|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|
|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|
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)