Cast Adrift, Billions of Planets, All Alone

Astronomers said Wednesday that space was littered with hundreds of billions of planets that had been ejected from the planetary systems that gave them birth and either were going their own lonely ways or were only distantly bound to stars at least 10 times as far away as the Sun is from the Earth.

There are two Jupiter-mass planets floating around for each of the 200 billion stars in the Milky Way galaxy, according to measurements and calculations by an international group of astronomers led by Takahiro Sumi, of Osaka University in Japan, and reported in the journal Nature. “It’s a bit of a surprise,” said David Bennett, a Notre Dame astronomer who was part of the team. Before this research, it was thought that only about 10 or 20 percent of stars harbored Jupiter-mass planets. Now it seems as if the planets outnumber the stars.