Question

sound cannot travel in vacuum then how do they record the sound of planets

Answer 1

Basically, sound travels through compressions and rarefactions of the medium. It is, typically, a longitudinal wave requiring a material medium.The LIGO project is aimed at detecting gravitational waves which travel through the space-time fabric, the literal fabric sort of thing you see in photos that explain Einstein's theory of gravity. The expansion and contraction of this fabric itself is read by the LASERs in LIGO.Clearly, what is being studied is not a sound but the gravitational waves themselves.What can be said, though is that the behavior of media is somewhat similar. This may be considered as oversimplification of the LIGO project.