Free Astronomy Magazine September-October 2015

PLANETOLOGY within the habitable zone, both in the short (annual cycle) and very long-term (billions of years). A star’s habitable zone (which by definition is that range of distances from the star that could permit liquid water to pool on a rocky surface) is a very flex- ible concept that can be interpreted according to an “optimistic” version, which substantially takes into account also relative- ly short liveability periods of a planet within it, or ac- cording to a “conservative” version, which considers only distances from the star suitable for ensuring the survival of possible forms of life for a very long time. As re- gards to Kepler-452b, with the varying of the adopted stellar, orbital and planetary parameters, the chances that the planet is steadily in the habitable zone go from a maximum of more than 96% to a mini- mum of 28%. A range a bit too broad to provide any certainty. D iagram show- ing all planets discovered by Ke- pler (confirmed or yet to be con- firmed) that are least dissimilar to Earth in terms of size, inclusion in the habitable zone, flux of ener- gy received from the own star, and temperature of the latter. Kepler- 452b seems to be the most similar, but only in terms of the type of star and energy flux. The graph on the left shows that all signals recorded by Ke- pler were exam- ined and that al- most 4,700 plan- et candidates have emerged. [NASA Ames/ W. Stenzel and SETI Institute/ J. Coughlin] Let us try at this stage to summarize the key points and see how much Kepler-452b resembles Earth: it has a diameter greater by 63%, a mass from 3 to 7 times greater, a gravity that is at least double, a definitely thicker atmosphere, and thus higher tem- perature and ground pressure, always as- suming that there is an atmosphere and a ground surface. This in addition with the fact that the planet moves along an orbit which for calcula- tion ease is con- sidered circular, but whose true shape is un- known; that it stations in the habitable zone by an uncertain time and that it will continue to do so for an un- certain time. It should be not- ed in this regard that orbiting in a habitable zone