According to kinetic theory of gases, average kinetic energy of gas molecules is
$
K=\frac{3}{2} R T
$
i.e. $\quad K \propto T$
We know that, kinetic energy may not be negative. Its minimum value may be equal to zero.
Hence, minimum possible temperature on the earth is equal to $0 \mathrm{~K}$ at which molecular motion starts ceasing.
$\therefore$ Minimum possible temperature
$
=0 \mathrm{~K}=(0-273)^{\circ} \mathrm{C}=-273^{\circ} \mathrm{C}
$