rather simple, simply count how many zeros are there in each and that'll be the exponent of the base 10.
[tex] \bf 1\stackrel{\leftarrow \qquad \textit{23 zeros}\qquad \to }{00,000,000,000,000,000,000,000}\implies \stackrel{Mars}{10^{23}~Kgs}
\\\\\\
1\stackrel{\leftarrow \qquad \textit{26 zeros}\qquad \to }{00,000,000,000,000,000,000,000,000}\implies \stackrel{Neptune}{10^{26}~Kgs} [/tex]
