To estimate the mass of water on Earth, we start with the total mass of the Earth:
\[ \text{Mass of Earth} = 5.9726 \times 10^{24} \text{ kg} \]
We know that the mass of the water is given as \( 5 \times 10^{-4} \) percent of the Earth's total mass. First, we need to convert this percentage into a decimal for calculations:
\[ 5 \times 10^{-4} \text{ percent} = \frac{5 \times 10^{-4}}{100} = 5 \times 10^{-6} \]
Next, we calculate the mass of the water by multiplying the Earth's mass by this decimal fraction:
\[ \text{Mass of water} = \text{Mass of Earth} \times \left(5 \times 10^{-6}\right) \]
Substituting the mass of the Earth into the equation:
\[ \text{Mass of water} = (5.9726 \times 10^{24}) \times (5 \times 10^{-6}) \]
Now, we perform the multiplication step-by-step. First, multiply the coefficients:
\[ 5.9726 \times 5 = 29.863 \]
Next, multiply the powers of ten:
\[ 10^{24} \times 10^{-6} = 10^{24 - 6} = 10^{18} \]
Now, combining these results, we find:
\[ \text{Mass of water} = 29.863 \times 10^{18} \text{ kg} \]
We can convert this into scientific notation by writing it as:
\[ 2.9863 \times 10^{19} \text{ kg} \quad (\text{moving the decimal one place to the left increases the exponent by 1}) \]
Thus, the estimated mass of the planet's water is:
\[ \boxed{2.9863 \times 10^{19} \text{ kg}} \]
This calculation includes all of the estimates and steps used to arrive at the final answer.