Study: Milky Way holds billions of Earth-like planets



WASHINGTON — Space is vast, but it might not be so lonely after all.

A study has found that the Milky Way is teeming with billions of planets that are about the size of Earth, orbit stars just like our sun, and exist in a habitable zone.

Astronomers using NASA data have calculated for the first time that in our galaxy alone, there are at least 8.8 billion stars with Earth-size planets in the habitable temperature zone.

The study was published Monday in the journal Proceedings of the National Academy of Science. For perspective, that’s more Earth-like planets than there are people on Earth.

What are the odds that there is life somewhere out there? “Just in our Milky Way galaxy alone, that’s 8.8 billion throws of the biological dice,” said study co-author Geoff Marcy, a longtime planet hunter from the University of California at Berkeley.

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 harbor life.

The findings also raise another 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 one in five 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 water can be liquid, according to calculations based on four years of observations from NASA’s now-crippled Kepler telescope.

If people on Earth could only travel in deep space, “you’d probably see a lot of traffic jams,” Bill Borucki, NASA’s chief Kepler scientist, said jokingly Monday.

The Kepler telescope examined 42,000 stars, 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 percentage of stars that are like our sun have planets similar to Earth: 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 that 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 sunlike stars closer to 50 billion, so there could be at least 11 billion planets like ours.

The closest Earth-size planet in the habitable temperature zone that circles a sunlike 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 sunlike 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-in enough to be in the not-too-hot, not-too-cold 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 conference Monday said they have found 833 new candidate planets with the space telescope, bringing the total of such planets spotted to 3,538, but most aren’t candidates for life.

Kepler has identified 10 planets that are about Earth’s size circling sunlike stars and are in the habitable zone, including one called Kepler 69-c.