The Kepler spacecraft has found dozens of “potentially habitable” planets around other stars, but this week announced one that some news sources are calling “Earth’s twin.”  The announcement provides an opportunity to study where empirical science ends and speculation begins.

The NASA press release announced only that it had found its first planet in the habitable zone of a sun-like star.  This implies that water could be liquid on its surface, provided other requirements are met.  A diagram shows the planet’s orbital radius compared with that of the sun’s rocky planets.  The Kepler team realizes that much remains unknown about the new planet, dubbed Kepler-22b, except that its radius is 2.4 times that of Earth.  “Scientists don’t yet know if Kepler-22b has a predominantly rocky, gaseous or liquid composition, but its discovery is a step closer to finding Earth-like planets,” the article said.  Very little, therefore, is empirically known to justify the accompanying artwork of a watery world.

The scientific press took this as a cue to push the “Earth is not unique” angle.  NASA’s Astrobiology Magazine was somewhat restrained with the L-word life, noting just that water is a prerequisite.  Reporter Mike Wall at, however, used the word life three times, as in: “The discovery brings scientists one step closer to finding a planet like our own — one which could conceivably harbor life, scientists said.” The BBC News came just short of calling Kepler-22b Earth’s twin sister: “It is the closest confirmed planet yet to one like ours – an ‘Earth 2.0’.”

Last month, the BBC News presented a way of ranking which planets are most “liveable,” with Earth at a rank of 1.0.  The next nearest was Titan (0.64), then Mars (0.59), Europa (0.49), and an exoplanet named Gliese 581g (0.45).  It’s not known where the new planet Kepler-22b would rank, but the criteria included surface properties, the presence of a liquid solvent, and whether the world has an atmosphere and a magnetic field.

In addition, the life history of a planet must be taken into account.  Stephen Battersby in New Scientist wrote about “Earth’s wild ride: Our voyage through the Milky Way.”  Assuming that Earth is as old as scientists believe, the Earth would have orbited the Milky Way every 200 million years in its assumed 4.5 billion year lifetime, sending it into a veritable shooting gallery of other objects and exposing it many times to the lethal radiation of supernovae….

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