The Kepler Space Mission's search for habitable planets in a tiny window representing 1/400th of the Milky is proving Arthur C Clark, author of Space Odyssey 2001 right: "The idea that we are the only intelligent creatures in a cosmos of a hundred billion galaxies is so preposterous that there are very few astronomers today who would take it seriously," Clarke wrote. " It is safest to assume therefore, that they are out there and to consider the manner in which this may impinge upon human society."
"This statistical study tells us that planets around stars are the rule, rather than the exception," said study lead author Arnaud Cassan of the Paris Institute of Astrophysics. "From now on, we should see our galaxy populated not only with billions of bright stars, but imagine them surrounded by as many hidden extrasolar worlds."
To date, astronomers have discovered more than 700 planets beyond our own solar system, with 2,300 additional candidates found by NASA's Kepler space telescope awaiting confirmation.
In the new study, the researchers looked at data gathered by a variety of Earth-based telescopes, which scanned millions of stars from 2002 to 2007 for microlensing events, then closely analyzed about 40 of these events and discovered that three betrayed the presence of an alien planet around a star. One of these planets is a bit more massive than Jupiter, one is comparable to Neptune and the third is a so-called "super-Earth" with a mass about five times that of our home planet --an impressive yield considering how perfectly aligned multiple bodies must be to yield an explanet detection via microlensing.
Further, according to the researchers' calculations, every planet in the Milky Way harbors an average of 1.6 planets in the 0.5-10 AU range, which in our solar system corresponds roughly to the swath of space between Venus and Saturn.
Since astronomers estimate that our galaxy contains about 100 billion stars, that works out to at least 160 billion alien planets. Cassan and his team report their results in the Jan. 12 issue of the journal Nature. The true number of alien worlds may be quite a bit larger than 160 billion. Some planets hug their host stars more closely than 0.5 AU, after all, and others are more far-flung than 10 AU. And a great many likely have no host star at all.
"We used to think that the Earth might be unique in our galaxy," study co-author Daniel Kubas, also of the Paris Institute of Astrophysics, said in a statement. "But now it seems that there are literally billions of planets with masses similar to Earth orbiting stars in the Milky Way."