Sentence:  When we ask what evidence does in fact exist of extraterrestrial sojourns on our planet, we can start with what would surely be the best evidence of all: an actual <WSD>visitor</WSD>, or group of <WSD>visitors</WSD>, visible to crowds of people and ready for photo opportunities, television interviews, handshakes, polite conversation, and dancing. 
Meaning of the word marked with <WSD> tags:  An extraterrestrial being on Earth for any reason.