The finding underscores a fundamental shift in scientific understanding of planetary systems in the cosmos. Our own solar system, considered unique not so long ago, turns out to be just one among billions.
Until April 1994, there was no other known solar system, but the discoveries have slowly mounted since then: The Kepler space telescope, designed for planet-hunting, now finds them routinely.
"Planets are the rule rather than the exception," said lead astronomer Arnaud Cassan at the Institute of Astrophysics in Paris. He led an international team of 42 scientists who spent six years surveying millions of stars at the heart of the Milky Way, in the most comprehensive effort yet to gauge the prevalence of planets in the galaxy.
To estimate the number of other worlds, Dr. Cassan and his colleagues studied 100 million stars between 3,000 and 25,000 light-years from Earth with gravitational microlensing. The technique uses distant light amplified by the gravity of a massive star or planet to create an astronomical magnifying lens. Then they combined their findings with earlier surveys, which used other detection techniques, to create a statistical sample of stars and the planets that orbit them, which they say is representative of the galaxy.
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.
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