We present constraints on the shape of Kepler-51d, which is a superpuff with a mass ∼6 M⊕ and a radius ∼9 R⊕, based on detailed modeling of the transit light curve from James Webb Space Telescope (JWST) NIRSpec. The projected shape of this extremely low-density planet is consistent with being spherical, and a projected oblateness f⊥ > 0.2 can be excluded regardless of the spin obliquity angles. If this is taken as the limit on the true shape of the planet, Kepler-51d is rotating at ≲50% of its breakup spin rate, or its rotation period is ≳33 hr. In the more plausible situation that the planetary spin is aligned with its orbital direction to within 30°, then its oblateness is <0.08, which corresponds to a dimensionless spin rate ≲30% of the breakup rotation and a dimensional rotation period ≳53 hr. This seems to contradict the theoretical expectation that planets with such low masses may be spinning near breakup. We point out the usefulness of the stellar mean density and the orbital eccentricity in constraining the shape of the transiting planet, so planets with well-characterized host and orbital parameters are preferred in the detection of planetary oblateness with the JWST transit method.

中文翻译：

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一颗极低密度的系外行星旋转缓慢


我们提出了对开普勒-51d 形状的约束，开普勒-51d 是一个质量 ∼6 M⊕半径 ∼9 R⊕ 的超级泡喷，基于詹姆斯韦伯太空望远镜 （JWST） NIRSpec 的凌日光曲线的详细建模。这颗极低密度行星的投影形状与球形一致，无论自旋倾斜角如何，都可以排除投影的扁度 f⊥ > 0.2。如果将此视为行星真实形状的极限，则开普勒-51d 以 ≲50% 的解散自旋速率旋转，或者其自转周期约为 33 小时。在行星自旋与其轨道方向对齐到 30° 以内更合理的情况下，其扁圆度为 <0.08，这对应于无量纲自旋速率 ≲分解旋转的 30% 和维度旋转周期 ≳53 小时。这似乎与理论上的预期相矛盾，即质量如此低的行星可能在接近解体时自转。我们指出了恒星平均密度和轨道偏心率在约束凌日行星形状方面的作用，因此在使用 JWST 凌日方法检测行星扁度时，优选具有良好表征的宿主和轨道参数的行星。