last question first
the diameter of the sun is measured using trigonometric measurements. using similar math, the distance can be computed and verified, the distance also measuring the apogee.
some of the first astronomers did not believe that the earth orbited the sun as they could not observe parallax in the stars from one position to another. that was only possible with advancements in measuring and optics to get precise images.
as for life here, it is theorized (and there is good evidence) that there was life on mars but died off some time back. it is also believe that there is bacteria there now.
as for life on the distance stars, yes it is highly likely.
Alpha-century is the closest star, at about 4.5 light years. Bernards star is just beyond that being the second closest star.
the give students in the 1940s, an idea of the cosmic distances involved in astronomy, a text book stated, if the period at the end of this sentence was the earth, the next closest star would be 12 miles away.
we are finding more and more stars with plants.
in the 70's when i was an astronomy student is spain, the prof explained that the chance of planets being out there was increasing due to more sophistocated imaging. they were looking for deviation in star path caused by planetary bodies that was less than the thickness of a human hair.
times they do change, now they find planets routinely out there.