What is the next number in the series?
Series: $9,27,81,243,\_\_\_\_\_$

Let's determine the pattern or common ratio between consecutive terms in the given series:
* $9\times 3=27$
* $27\times 3=81$
* $81\times 3=243$

Alternatively, the terms can be expressed as powers of $3$:

* $9=3^2$
* $27=3^3$
* $81=3^4$
* $243=3^5$

Following this geometric progression pattern, the next term must be multiplied by $3$ (or equal to $3^6$):

$$\text{Next Number}=243\times 3=729$$

Thus, the next number is $729$.