"Planets are the rule rather than the exception, " said lead astronomer Arnaud Cassan at the Institute of Astrophysics in Paris.

WSJ: Galaxy Hosts 100 Billion Planets, in New Estimate

Confirmation requires different methods than those used in Dr. Cassan's estimates.

WSJ: Galaxy Hosts 100 Billion Planets, in New Estimate

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

"One can point at almost any random star and say there are planets orbiting that star, " said astronomer Uffe Grae Jorgensen, of the University of Copenhagen in Denmark, a member of Dr. Cassan's team.

WSJ: Galaxy Hosts 100 Billion Planets, in New Estimate