Exo-solar planet definition: must orbit around a star. Free floating objects not in orbit around a star are not extra-solar planets: true mass below limiting mass. Lowest mass is can be = limit of mass is our planet. The mass must be over threshold at which it becomes round in shape. Upper limit: 13x the mass of jupiter: cannot have nuclear fusion. Stars have nuclear fusion occurring in their cores - h he. Brown dwarf (fit other requirements) burn deuterium (isotope of hydrogen with 2 neutrons) - so they cannot be extra-solar planets either. Difficult to find planets around stars because stars are big and bright, and planets are often small and dim. Most of the planets found to date lie within ~300 light years from our sun. We"ve only looked at a very small fraction of our galaxy. There are a few thousand confirmed earth analogs. Few thousand likely - need more research.