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.
The study was published Monday in the journal Proceedings of the National Academy of Science.
This
artist’s rendition provided by NASA shows Kepler-69c, a
super-Earth-size planet in the habitable zone of a star like our sun,
located about 2,700 light-years from Earth in the constellation Cygnus.
AP Photo/NASA/Ames/JPL-Caltech)
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.
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?”
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.
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.
