Relationship between oblateness and rotation period for Kepler-51d with different assumed interior structures parameterized by the rotational response coefficient Λ₂. The upper x-axis shows the rotation period relative to the breakup period of 17 hr. To conform with the upper limit f  < 0.15, Kepler-51d requires P_rot  ≳ 40 hr, corresponding to a rotation speed ≲42% of the breakup speed.