Kepler-39b is a gas-giant planet in orbit around an F-type
star. It is 18 times Jupiter’s mass, 1.22 times Jupiter’s radius and it
transits its host star every 21.09 days. A study by Wei Zhu et al. (2014) using
data from NASA’s Kepler space telescope found that Kepler-39b has an oblateness
of 0.22 ± 0.11. In fact, this is the first tentative detection of oblateness
for a planet outside the Solar System. When an oblate planet transits its host
star, the transit light curve will exhibit small differences from that of a
purely spherical planet.
In the Solar System, the gas-giant planets Jupiter and
Saturn are oblate in shape due to their rapid rotations. The oblateness of an
object is expressed as the ratio of its equatorial-polar radius difference to its
equatorial radius. The equatorial radius is larger than the polar radius by 7
percent for Jupiter and by 10 percent for Saturn. As such, Jupiter’s oblateness
is 0.07 and Saturn’s oblateness is 0.1. With an oblateness of 0.22 ± 0.11,
Kepler-39b is substantially more oblate than any planet in the Solar System.
The large oblateness of Kepler-39b is most likely
rotationally induced. With that, its rotation period is estimated to be 1.6 ± 0.4
hours. For comparison, the rotation periods of Jupiter and Saturn are 9.9 and
10.6 hours, respectively. Although the rotation of Kepler-39b is remarkably fast,
it is lower than its estimated break-up rotation period of ~0.9 hours. In
addition to its large oblateness, Kepler-39b is also inflated in size. Its close
proximity to its host star and its estimated equilibrium temperature of around
900 K is insufficient to account for its inflated size.
Reference:
Wei Zhu et al. (2014), “Constraining the Oblateness of
Kepler Planets”, arXiv:1410.0361 [astro-ph.EP]