When you see planet earth from space you see clouds ,sea and gases and you cannot see people...same counts for many planets in our galaxy... Invisible is visible,visible is invisible.... I think depends on chemistry on each planet can give some sort of life but it can remain invisible for us... Life is energy ....that is what Nikola Tesla was talk about ,Morihei Ueshiba ,Masaru Emoto...and many other famous people... We have animals on our planets and still we are not fascinating about life on our earth....i think we have answer on earth ...we need to start find true in ourselves so we can understand why is sun hot....or why earth is spinning...
We are still not developed spiritualy that we can talk about alien life...We are blind because of many interaction and necessary things on earth ....