Monday, March 5, 2012

Worlds with two Suns

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).

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)