_INTRODUCTORY_ 1 Remarkable popular interest in questions concerning other worlds and their inhabitants--Theories of interplanetary communication--The plurality of worlds in literature--Romances of foreign planets--Scientific interest in the subject--Opposing views based on telescopic and spectroscopic revelations--Changes of opinion--Desirability of a popular presentation of the latest facts--The natural tendency to regard other planets as habitable--Some of the conditions and limitations of the problem--The solar system viewed from outer space--The resemblances and contrasts of its various planets--Three planetary groups recognized--The family character of the solar system.