To estimate the number of alien worlds, Dr. Cassan and his colleagues studied 100 million stars between 3, 000 and 25, 000 light-years from Earth with gravitational microlensing.

WSJ: Galaxy Hosts 100 Billion Planets, in New Estimate

To compare alien planets, Dr. Mendez at the Planetary Habitability Laboratory and his colleagues are creating an Earth Similarity Index that ranks worlds by their resemblance to our own planet, on a scale from zero to one, based on their mass, radius, density and surface temperature.

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On Wednesday, Dr. Welsh and his colleagues announced that they have confirmed the existence of two more alien planets in distinctive double-star solar systems in the constellation Cygnus.

WSJ: Galaxy Hosts 100 Billion Planets, in New Estimate