It is defined by the equation

$ES{I}_j=pro{d}_i=1^n(1-|\frac{x_{i,j}-x_{i,o+}}{x_{i,j}+x_{i,o+}}|{)}^{\frac{w_i}{n}}$

where the parameters $x_{i,j}$ are described, and their terrestrial values, $x_{i,o+}$ and weights $w_i$ given in the table below. The symbol $$ is a cumulative product, so in the same way that $$ means to sum up all the elements in a sequence, $$ means that we take the product of all elements in a sequence. $n$ is the number of parameters we are using to compare similarity to Earth by$,$ so for our purposes, it will always be $4.$

The radius, density, and escape velocities are taken relative to the terrestrial $($Earth$)$ values, given in the table below. You can verify for yourself that plugging in the four Earth values to the equation above will yield an ESI of $1,$ indicating strong similarity to Earth.

$\backslash$table$[[i,$Parameter,Earth value, $x_{i,o+},$Weight, $w_i$