The missing number in the series 9 81 ? 6561 59049

The given series is 9 81 ? 6561 59049

It follows a pattern that

9

81 = 9 x 9

729 = 9 x 9 x 9

6561 = 9 x 9 x 9 x 9

59049 = 9 x 9 x 9 x 9 x 9

Hence, the missing number in the series is 729.