By SETH BORENSTEIN, The Associated Press

WASHINGTON -- WASHINGTON (AP) — Space is vast, but it may not be so lonely after all: A study finds 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 the Goldilocks zone — not too hot and not too cold for life.

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.

kepler62f

Kepler-62f with 62e as Morning Star
The artist's concept depicts NASA's Kepler mission's smallest habitable zone planet. Seen in the foreground is Kepler-62f, a super-Earth-size planet in the habitable zone of a star smaller and cooler than the sun, located about 1,200 light-years from Earth in the constellation Lyra.
Image credit: NASA Ames/JPL-Caltech/Tim Pyle

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.

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," said study co-author Geoff Marcy, a longtime planet hunter from the University of California at Berkeley.

kepler62e

Kepler-62e
The artist's concept depicts Kepler-62e, a super-Earth-size planet in the habitable zone of a star smaller and cooler than the sun, located about 1,200 light-years from Earth in the constellation Lyra.
Image credit: NASA Ames/JPL-Caltech

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

kepler62 system

Kepler-62 and the Solar System
The diagram compares the planets of the inner solar system to Kepler-62, a five-planet system about 1,200 light-years from Earth in the constellation Lyra. The five planets of Kepler-62 orbit a star classified as a K2 dwarf, measuring just two thirds the size of the sun and only one fifth as bright. At seven billion years old, the star is somewhat older than the sun.
Image credit: NASA Ames/JPL-Caltech

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.

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, joked Monday.

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.

kepler69 system

Kepler-69 and the Solar System
The diagram compares the planets of the inner solar system to Kepler-69, a two-planet system about 2,700 light-years from Earth in the constellation Cygnus. The two planets of Kepler-69 orbit a star that belongs to the same class as our sun, called G-type.
Image credit: NASA Ames/JPL-Caltech

For the first time, scientists calculated — not estimated — what percent of stars that are just 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 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-in 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 and are in the habitable zone, including one called Kepler 69-c.

Because there are probably hundreds of planets missed for every one found, the study did intricate extrapolations to come up with the 22 percent figure — a calculation that outside scientists say is fair.

"Everything they've done looks legitimate," said MIT astronomer Sara Seager.

___

Seth Borenstein can be followed at http://twitter.com/borenbears

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  • NASA's Kepler Mission Discovers Planet

    In this handout illustration made available on December 5, 2011 by NASA, the Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star is digitally illustrated. For the first time NASA's Kepler mission has confirmed a planet to orbit in a star's habitable zone; the region around a star, where liquid water, a requirement for life on Earth, could persist. The planet is 2.4 times the size of Earth, making it the smallest yet found to orbit in the middle of the habit. Clouds could exist in this earth's atmosphere, as the artist's interpretive illustration depicts. (Photo Illustration by Ames/JPL-Caltech/NASA via Getty Images)

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    Picture released 04 October 2006 by the European Space Agency shows an artist's impression of a Jupiter-sized planet passing in front of its parent star. Such events are called transits. When the planet transits the star, the star's apparent brightness drops by a few percent for a short period. Through this technique, astronomers can use the Hubble Space Telescope to search for planets across the galaxy by measuring periodic changes in a star's luminosity. The first class of exoplanets found by this technique are the so-called 'hot Jupiters,' which are so close to their stars they complete an orbit within days, or even hours. A seam of stars at the centre of the Milky Way has shown astronomers that an entirely new class of planets closely orbiting distant suns is waiting to be explored, according to a paper published 04 October 2006. An international team of astronomers, using a camera aboard NASA's Hubble telescope, delved into a zone of the Milky Way known as the 'galactic bulge', thus called because it is rich in stars and in the gas and dust which go to make up stars and planets. The finding opens up a new area of investigation for space scientists probing extrasolar planets - planets that orbit stars other than our own. AFP PHOTO NASA/ESA/K. SAHU (STScI) AND THE SWEEPS SCIENCE TEAM

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    Picture released 04 October 2006 by the European Space Agency shows an artist's impression of a unique type of exoplanet discovered with the Hubble Space Telescope. This image presents a purely speculative view of what such a 'hot Jupiter' (word dedicated to planets so close to their stars with such short orbital periods) might look like. A seam of stars at the centre of the Milky Way has shown astronomers that an entirely new class of planets closely orbiting distant suns is waiting to be explored, according to a paper published 04 October 2006. An international team of astronomers, using a camera aboard NASA's Hubble telescope, delved into a zone of the Milky Way known as the 'galactic bulge', thus called because it is rich in stars and in the gas and dust which go to make up stars and planets. The finding opens up a new area of investigation for space scientists probing extrasolar planets - planets that orbit stars other than our own. AFP PHOTO NASA/ESA/K. SAHU (STScI) AND THE SWEEPS SCIENCE TEAM

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    From the Spitzer Science Center. While astronomers have identified over 500 planets around other stars, they're all too small and distant to fill even a single pixel in our most powerful telescopes. That's why science must rely on art to help us imagine these strange new worlds. From Spitzer Space Telescope. Even without pictures of these exoplanets, astronomers have learned many things that can be illustrated in artwork. For instance, measurements of the temperatures of many "Hot Jupiters," massive worlds orbiting very close to their stars, hint that their atmospheres may be as dark as soot, glowing only from their own heat. While "Hot Jupiters" would be relatively dark in visible light, compared to their stars, their brightness is proportionally much greater in the infrared. Illustrating this dramatic contrast change helps explain why the infrared eye of NASA's Spitzer Space Telescope plays a key role in studying exoplanets. As our understanding evolves, so must the artwork. Astronomers found a blazing hot spot on the exoplanet Upsilon Andromedae b that at first, appeared to face towards its star. More data has revealed that the hottest area is actually strangely rotated almost 90 degrees away, near the day/night terminator. WASP 12b is as hot as the filament in a light bulb, and would be blazing bright to our eyes. Most interestingly, if it proves to have a strongly elliptical orbit, as first thought, calculations show it would be shedding some of its outer atmosphere <b>...</b>