Constraining the Oblateness of Kepler Planets
We use Kepler short cadence light curves to constrain the oblateness of planet candidates in the Kepler sample. The transits of rapidly rotating planets that are deformed in shape will lead to distortions in the ingress and egress of their light curves. We report the first tentative detection of an oblate planet outside of the solar system, measuring an oblateness of 0.22±0.11 for the 18 M J mass brown dwarf Kepler 39b (KOI-423.01). We also provide constraints on the oblateness of the planets (candidates) HAT-P-7b, KOI-686.01, and KOI-197.01 to be < 0.067, < 0.251, and < 0.186, respectively. Using the Q'-values from Jupiter and Saturn, we expect tidal synchronization for the spins of HAT-P-7b, KOI-686.01 and KOI-197.01, and for their rotational oblateness signatures to be undetectable in the current data. The potentially large oblateness of KOI-423.01 (Kepler 39b) suggests that the Q'-value of the brown dwarf needs to be two orders of magnitude larger than that of the solar system gas giants to avoid being tidally spun-down.
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Interesting if confirmed, particularly the apparent consistency with beta Pic b. Perhaps old brown dwarfs are often oblate (they are expected to spin up as they age, unlike stars)?
KOI 423.01 (Kepler 39b) shows a potentially large oblateness (f ⊥ = 0.22+0.11−0.11). However, we find that the oblateness signal is only mildly self-consistent over multiple epochs, suggesting that the detection may not be robust.
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