100 billion planets a conservative estimate for our galaxy

Planets around other stars are much more common than previously thought, according to scientists. The  Wall Street Journal reports:

Astronomers said Wednesday that each of the 100 billion stars in the Milky Way probably has at least one companion planet, on average, adding credence to the notion that planets are as common in the cosmos as grains of sand on the beach.

They used gravitational microlensing on millions of potential host stars. So far, only a handful of planets have been detected using this method.

By their calculations, most of the Milky Way's stars—100 billion is the most conservative estimate—have one or more planets, the researchers reported in Nature Wednesday. None of the planets detected so far appear suitable for conventional carbon-based life as known on Earth.

Almost two-thirds of the stars likely host a planet measuring about five times Earth's mass, and half of them harbor a planet about the mass of Neptune, which is 17 times the mass of Earth. About one-fifth of them are home to a gas giant like Jupiter or a still more massive planet.

"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.