Choi, Charles Q. (March 21, 2011) New Estimate for Alien Earths: 2 Billion in Our Galaxy Alone | Alien Planets, Extraterrestrial Life & Extrasolar Planets | Exoplanets & Kepler Space Telescope.
Truy cập 14 tháng 2 năm 2015.  ^ Choi, Charles Q. (2011-03-21) New Estimate for Alien Earths: 2 Billion in Our Galaxy Alone | Alien Planets, Extraterrestrial Life & Extrasolar Planets | Exoplanets & Kepler Space Telescope.