Free Astronomy Magazine September-October 2015

PLANETOLOGY Sun-Earth system (apart from the planet’s possible habitability or not). An unquestion- ably objective difference, but theoretically irrelevant for the purposes of establishing a planet's environmental conditions. Deep down, though, it feeds that form of anthropocentrism still embed- ded in the human race, accord- ing to which, unless a star is identical to the Sun, it cannot host a planet identical to ours. If with Kepler-186f we were just a step away from a new Earth, let us now see if with the discovery of the “older cousin” that last step has been made. The new planet, called Kepler- 452b, orbits in the habitable zone of KIC 8311864 (or Kepler- 452), a solar-type star of magni- tude 13.4, laying about 1,400 light-years from Earth, in the constellation Cygnus. Kepler-452b is the only known planet of that star, and it transits on its disk T he planets ap- parently more similar to Earth discovered by Ke- pler space tele- scope in the last five years. Stars and planets are shown in the re- spective scales. [NASA Ames/W. Stenzel] Below, the twelve exo- planets that, among the 1,030 discovered by Kepler and con- firmed to date, orbit within hab- itable zones and are less than twice the size of Earth (included for comparison). The planets’ sizes are magnified by 25 times com- pared to those of the stars. [NASA Ames/JPL-Cal- tech/R. Hurt] every 384.84 days. The first transit was re- corded by Kepler space telescope in 2009, the debut year of its original science mis- sion, which lasted from 13 May 2009 to 11 May 2013. In these four years the instru-