Since the force acting is central, that is, its directed towards the centre of the planet at every instant, the angular moment of the particle about the centre of the planet will be conserved.
that is
mVsinθ*5R=mvR
or v=5Vsinθ ... (i)
Since gravitation field is a conservative field,
the energy will be conserved
or
-GMm/5R +1/2 mV2=-GMm/R + 1/2mv2
or GMm*(4/5R)=1/2mV2(25sin2θ-1) ...using (i)
or \theta =\sin^{-1} \left[\frac{1}{5}\sqrt{\left(\frac{8GM}{5V^{2}R}+1 \right)} \right]