MOUNTAIN VIEW, Calif. — Somewhere in all of this,
there must be a planet where the volcanoes spout
chocolate.
Astronomers reported Monday that there could be
as many as 40 billion habitable Earth-size planets in
the galaxy, based on a new analysis of data from
NASA’s Kepler spacecraft.
One of every five sun-like stars in the galaxy has a
planet the size of Earth circling it in the Goldilocks
zone — not too hot, not too cold — where surface
temperatures should be compatible with liquid
water, according to a herculean three-year
calculation based on data from the Kepler spacecraft
by Erik Petigura, a graduate student at the
University of California, Berkeley.
Mr. Petigura’s analysis represents a major step
toward the main goal of the Kepler mission, which
was to measure what fraction of sun-like stars in the
galaxy have Earth-size planets. Sometimes called
eta-Earth, it is an important factor in the so-called
Drake equation used to estimate the number of
intelligent civilizations in the universe. Mr.
Petigura’s paper, published Monday in the journal
Proceedings of the National Academy of Science,
puts another smiley face on a cosmos that has
gotten increasingly friendly and fecund-looking over
the last 20 years.
“It seems that the universe produces plentiful real
estate for life that somehow resembles life on Earth,”
Mr. Petigura said.
Over the last two decades, astronomers have logged
more than 1,000 planets around other stars, so-
called exoplanets, and Kepler, in its four years of life
before being derailed by a mechanical pointing
malfunction last May, has compiled a list of some
3,500 more candidates. The new result could steer
plans in the next few years and decades to find a
twin of the Earth — Earth 2.0, in the argot — that is
close enough to here to study.
The nearest such planet might be only 12 light-years
away. “Such a star would be visible to the naked
eye,” Mr. Petigura said.
His result builds on a report earlier this year by
David Charbonneau and Courtney Dressing of the
Harvard-Smithsonian Center for Astrophysics, who
found that about 15 percent of the smaller and more
numerous stars known as red dwarfs have Earth-like
planets in their habitable zones. Using slightly less
conservative assumptions, Ravi Kopparapu from
Pennsylvania State University found that half of all
red dwarfs have such planets. Astronomers estimate
that there are at least 200 billion stars of all types in
the Milky Way galaxy, room for the imagination, and
— who knows — perhaps for a few microbes or more
complicated creatures to roam.
Geoffrey Marcy of the University of California,
Berkeley, who supervised Mr. Petigura’s research
and was a co-author of the paper along with Andrew
Howard of the University of Hawaii, said: “This is the
most important work I’ve ever been involved with.
This is it. Are there inhabitable Earths out there?”
“I’m feeling a little tingly,” he said.
At a news conference Friday discussing the results,
astronomers erupted in praise of the Kepler mission
and its team. Natalie Batalha, a Kepler leader from
the NASA Ames Research Center, described the
project and its members as “the best of humanity
rising to the occasion.”
According to Mr. Petigura’s new calculation, the
fraction of stars with Earth-like planets is 22
percent, plus or minus 8 percent, depending on
exactly how you define the habitable zone.
There are several caveats. Although these planets
are Earth-size, nobody knows what their masses are
and thus whether they are rocky like the Earth, or
balls of ice or gas, let alone whether anything can, or
does — or ever will — live on them.
There is reason to believe, from recent observations
of other worlds, however, that at least some Earth-
size planets, if not all of them, are indeed rocky. Last
week, two groups of astronomers announced that an
Earth-size planet named Kepler 78b that orbits its
sun in 8.5 hours has the same density as the Earth,
though it is too hot to support life.
“Nature,” as Mr. Petigura put it, “knows how to
make rocky Earth-size planets.”
Also, the number is more uncertain than it might
have been because Kepler’s pointing system failed
before it could complete its prime survey. As a
result, Mr. Petigura and his colleagues had to
extrapolate from planets slightly larger than Earth
and with slightly smaller, tighter orbits. For the
purposes of his analysis “Earth-size” was anything
from one to two times the diameter of the Earth, and
Earth-like orbits were between 400 and 200 days.
Dr. Batalha said, “We don’t yet have any planet
candidates that are exact analogues of the Earth in
terms of size, orbit or star type.”
Dr. Charbonneau said that raised “the terrifying
question that haunts us exoplaneteers: Did the
Kepler mission get enough data?”
Though Kepler itself is sidelined while astronomers
devise a new program it can accomplish with less
flexible pointing ability, it has sent back so much
data that there is still a whole year’s worth of results
left to analyze, Dr. Batalha said, and more
improvements to make to the data already obtained.
“Scientists,” she said, “are going to work on Kepler
data for decades.” She said it would be about three
years before they would be able to arrive at a viable
rate for the occurrence of habitable Earths.
Kepler was launched in 2009 to perform a kind of
cosmic census, monitoring the brightness of 150,000
far-off stars in the Cygnus and Lyra constellations,
looking for dips in brightness when planets pass in
front of them.
Dr. Petigura and his colleagues restricted
themselves to a subset of some 42,000 brighter and
well-behaved stars. They found 603 planets, of
which 10 were between one Earth and two Earths in
diameter, and circled in what Mr. Petigura defined
as the habitable zone, where they would receive
between a quarter of the light the Earth gets, and
four times as much. In the solar system, that zone
would spread from inside the orbit of Venus to just
outside the orbit of Mars.
Meanwhile, in an innovation borrowed from other
data-intensive fields like particle physics, Mr.
Petigura designed a computer pipeline so that he
could inject fake planets into the data — 40,000 in
all — and see how efficiently his program could
detect planets of different sizes and orbits.
“It was a ton of work,” he recalled, explaining that
he had to try out tens of billions of different periods
for each star in order to find planets. “Fortunately,
computers are cheap today.”
Sara Seager, an exoplanet astronomer at the
Massachusetts Institute of Technology who was not
involved in the work, said the pipeline testing had
made the results believable. “I would say that small
planets are everywhere and very common — no
matter how you slice and dice the data. But Kepler is
dead and we have no way to get any further data. So
we’ll have to be satisfied with this as the final word,
for now.”
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