• Habitable alien planets similar to
Earth may not be that rare in the
universe, a new study suggests
ABOUT one in five sun-like stars
observed by NASA’s planet-hunting
Kepler spacecraft has an Earth-size
planet in the so-called habitable
zone, where liquid water — and,
potentially life — could exist,
according to the new study.
If these results apply elsewhere in
the galaxy, the nearest such planet
could be just 12 light-years away.
“Human beings have been looking
at the stars for thousands of years,”
said study researcher, Erik Petigura,
a graduate student at the University
of California, Berkeley (UC Berkeley).
“How many of those stars have
planets that are in some way like
Earth? We’re very excited today to
start to answer that question,”
Petigura told SPACE.com.
The findings, detailed in the
journal Proceedings of the National
Academy of Sciences and in a video
describing the frequency of Earth-
like planets, say nothing about
whether these planets actually
support life — only that they meet
some of the known criteria for
habitability.
Petigura also presented the
results in a briefing at the second
Kepler Science Conference at NASA
Research Park in Moffett Field,
California, in which the Kepler team
also announced the discovery of
hundreds of new exoplanets,
including many in the habitable
zone.
“I think it’s by far the most
trustable estimate available, but I
don’t think it’s final,” said Francois
Fressin, an astronomer at the
Harvard-Smithsonian Center for
Astrophysics, who was not involved
with the study.
Petigura and his colleagues
painstakingly developed software to
sift through Kepler’s mammoth data
set. The spacecraft’s field of view
includes about 150,000 stars, but
most of these fluctuate in brightness
too much for a planet to be
detectable. The team examined
42,000 of the “quietest” stars,
finding 603 planet candidates
around these stars, 10 of which were
Earth-size and lay in the habitable
zone.
The team defined Earth-size
planets as ones having a radius one
to two times that of Earth. Planets
were considered to be in the
habitable zone if they received
about as much light as the Earth
does from the sun (within a factor of
four).
They used the Keck I telescope in
Hawaii to take spectra of the stars,
in order to pin down the radii of the
planets.
But this wasn’t the end of the
story. Just as taking a census
requires some statistical corrections
for the people the survey misses,
the researchers had to make
corrections for planets Kepler
missed.
The transit method of finding
planets, by definition, only detects
planets orbiting in the same plane
of view as their host star, which
includes just a fraction of the total
number of planets. Study researcher,
Geoff Marcy of UC Berkeley compared
planetary orbits to papers fluttering
through the air. Very few are going
to be edge-on, he said.
Secondly, the analysis misses
some planets simply because the
tiny amount of starlight they block
makes them tricky to detect. To
correct for this, the researchers
inserted “fake planets” into the data
so they could see how many their
software would miss.
The analysis was a “Herculean
task,” Marcy said.
After making these corrections,
the researchers had their result:
About 22 per cent of sunlike stars
observed by Kepler have Earth-size,
potentially habitable planets.
The researchers were quick to
point out that the fact that these
planets are Earth-size and lie in the
habitable zone does not mean they
could support life. The planets might
have scorching-hot atmospheres, or
no atmospheres at all, they said.
Even if the planets have all the
basic ingredients for life, scientists
don’t know the probability that life
would ever get started.
The definition of Earth-size
planets in this study was pretty
broad, Fressin said. For instance, a
planet that has a radius twice the
size of Earth’s might not even be
rocky, he said.
Kepler mission scientist Natalie
Batalha, an astronomer at NASA’s
Ames Research Center, who was not
involved with the study, agrees it’s a
generous definition. Rocky planets
with a radius about 1 to 1.5 times
the size of Earth’s have been found,
but the fraction of larger planets
that are rocky is probably much
lower, Batalha told SPACE.com. Still,
it’s a fair start, she said.
“Kepler’s prime objective was to
understand the prevalence of
habitable planets in the galaxy,”
Batalha said at a news conference.
“This is the first time a team has
offered such a number for stars like
the sun.”
The researchers had to
extrapolate the number of planets
with orbits longer than 200 days,
because these haven’t been
detected in the Kepler data.
“Ideally, we won’t rely on
extrapolations,” Batalha said. “But
as a first cut, this is a valid thing to
do.”
Last week, Marcy and his
colleagues reported the discovery of
the alien planet Kepler-78b, a rocky
world nearly the same size and
density as the Earth. But Kepler-78b
hugs its star at a distance far too
close and hot to be habitable, with
surface temperatures of about 3,680
degrees Fahrenheit (2,027 degrees
Celsius).
Kepler went out of commission in
May, after the loss of a wheel used
for pointing the spacecraft.
Nevertheless, scientists will mine
Kepler data for decades to look for
potentially habitable planets.
“Maybe with future instruments,
we could actually image these
planets,” Petigura said.
