about 3 to 1
`` I take it then that there is another . '' `` Yes . There is a super-dense class , still smaller , poorer in hydrogen , than the inner planets of the solar system . The ratio of occurrence of hydrogen-ammonia planets and these super-dense water-oxygen worlds of theirs over the entire Galaxy -- and remember that they have actually conducted a survey of significant sample volumes of the Galaxy which we , without interstellar travel , can not do -- is about 3 to 1 . This leaves them seven million super-dense worlds for exploration and colonization . '' The Industrialist looked at the blue sky and the green-covered trees among which they were making their way . He said , `` And worlds like ours ? ''