Billions of Earth-like planets in Milky Way, study finds

Approximately one in five sun-like stars in the galaxy are surrounded by planets that could meet some of the basic prerequisites for life, according to a new analysis of telescope data released Monday.

The study, released in the journal Proceedings of the National Academy of Science, is based on a three-year examination of data from NASA’s Kepler space telescope and suggests that at least 8.8 billion planets that fit Earth’s size and temperature requirements for life exist in the Milky Way galaxy alone.

As for what it says about the odds that there is life somewhere out there, it means "just in our Milky Way galaxy alone, that's 8.8 billion throws of the biological dice," study co-author Geoff Marcy, a longtime planet hunter from the University of California at Berkeley, told The Associated Press.

"Planets seem to be the rule rather than exception," study leader Erik Petigura, an astronomy graduate student at the University of California at Berkeley, said during a conference call with reporters on Monday.

The next step, scientists say, is to look for atmospheres on these planets with powerful space telescopes that have yet to be launched. That would yield further clues to whether any of these planets do, in fact, harbor life.

The findings also raise a glaring question, Marcy said: If we aren't alone, why is "there a deafening silence in our Milky Way galaxy from advanced civilizations?"

In the Milky Way, about 1 in 5 stars that are like our sun in size, color and age have planets that are roughly Earth's size and are in the habitable zone where life-crucial water can be liquid, according to intricate calculations based on four years of observations from NASA's now-crippled Kepler telescope.

The Kepler telescope peered at 42,000 stars, examining just a tiny slice of our galaxy to see how many planets like Earth are out there. Scientists then extrapolated that figure to the rest of the galaxy, which has hundreds of billions of stars.

For the first time, scientists calculated — not estimated — what percent of stars that are just like our sun have planets similar to Earth. The answer, they found, was 22 percent, with a margin of error of plus or minus 8 percentage points.

Kepler scientist Natalie Batalha said there is still more data to pore over before this can be considered a final figure.

There are about 200 billion stars in our galaxy, with 40 billion of them like our sun, Marcy said. One of his co-authors put the number of sun-like stars closer to 50 billion, meaning there would be at least 11 billion planets like ours.

Based on the 1-in-5 estimate, the closest Earth-size planet that is in the habitable temperature zone and circles a sun-like star is probably within 70 trillion miles of Earth, Marcy said.

And the 8.8 billion Earth-size planets figure is only a start. That's because scientists were looking only at sun-like stars, which are not the most common stars. An earlier study found that 15 percent of the more common red dwarf stars have Earth-size planets that are close enough to be in the not-too-hot, not-too-cold "Goldilocks Zone."

Put those together and that's probably 40 billion right-size, right-place planets, Marcy said. And that's just our galaxy. There are billions of other galaxies.

Scientists at a Kepler science conference Monday said they have found 833 new candidate planets with the space telescope, bringing the total of planets they've spotted to 3,538, but most aren't candidates for life.

Kepler has identified only 10 planets that are about Earth's size circling sun-like stars in the habitable zone, including one called Kepler 69-c.

Al Jazeera and wire services