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April 17, 2014
from
EarthSky Website
The first Earth-sized
exoplanet
orbiting within the habitable
zone of another star
has been confirmed.
The new planet, dubbed
Kepler-186f,
was discovered using NASA's
Kepler telescope.
The artist's concept depicts Kepler-186f, the first
validated Earth-size planet orbiting a distant star
in the habitable zone - a range of distances from a
star where liquid water might pool on the surface of
an orbiting planet.
The discovery of Kepler-186f confirms that
Earth-size planets exist in the habitable zone of
other stars and signals a significant step closer to
finding a world similar to Earth.
Art Credit: Danielle Futselaar.
An international team of researchers
have announced the discovery of a new rocky planet that could
potentially have liquid water on its surface.
The new planet, dubbed
Kepler-186f, was discovered using NASA's
Kepler telescope, launched in March
2009 to search for habitable zone, Earth-sized planets in our corner
of the Milky Way Galaxy.
A habitable zone planet orbits its star
at a distance where any water on the planet's surface is likely to
stay liquid.
Since liquid water is critical to life
on Earth, many astronomers believe the search for
extraterrestrial life should focus
on planets where liquid water occurs.
"Some people call these habitable
planets, which of course we have no idea if they are," said
Kane, an assistant professor of physics and astronomy.
"We simply know that they are in the
habitable zone, and that is the best place to start looking for
habitable planets."
Kepler-186f is the fifth and outermost
planet discovered orbiting around the dwarf star Kepler-186.
The planets were discovered by the
transit method, which detects potential planets as their orbits
cross in front of their star and cause a very tiny but periodic
dimming of the star's brightness.
Kepler-186 and the Solar System: The diagram
compares the planets of the inner solar system to
Kepler-186, a five-planet system about 500
light-years from Earth in the constellation Cygnus.
The five planets of Kepler-186 orbit a star
classified as a M1 dwarf, measuring half the size
and mass of the sun. The Kepler-186 system is home
to Kepler-186f, the first validated Earth-size
planet orbiting a distant star in the habitable zone
- a range of distances from a star where liquid
water might pool on the surface of an orbiting
planet.
Kepler-186f is less than ten percent larger than
Earth in size, but its mass and composition are not
known. Kepler-186f orbits its star once every
130-days and receives one-third the heat energy that
Earth does from the sun, placing it near the outer
edge of the habitable zone.
Image credit:
NASA Ames/SETI Institute/JPL-CalTech
After the astronomers were able to confirm that Kepler-186f was a
planet, they used the transit information to calculate the planet's
size.
Kepler-186f is slightly bigger than
Earth, measuring about 1.1 Earth radii (an Earth radius is the
distance from the Earth's center to its surface). The researchers
don't know yet what the mass of the planet might be, but they can
make an estimate based on other planets of similar radii, Kane
noted.
Having the mass and radius of a planet allows the astronomers to
calculate other features such as a planet's average density,
"and once you know the average
density of a planet, then you can start to say whether it's
rocky or not," Kane explained.
"What we've learned, just over the past few years, is that there
is a definite transition which occurs around about 1.5 Earth
radii," he continued.
"What happens there is that for
radii between 1.5 and 2 Earth radii, the planet becomes massive
enough that it starts to accumulate a very thick hydrogen and
helium atmosphere, so it starts to resemble the gas giants of
our solar system rather than anything else that we see as
terrestrial."
The planet's size influences the
strength of its gravitational pull, and its ability to pull in
abundant gases like hydrogen and helium.
At Kepler-186f's size, there is a small
chance that it could have gathered up a thick hydrogen and helium
envelope,
"so there's a very excellent chance
that it does have a rocky surface like the Earth," Kane said.
Rocky planets like Earth, Mars and Venus
gained their atmospheres as volcanic gasses like carbon dioxide and
water vapor were released from the planets' interiors.
Habitable zone planets like Earth orbit
at a distance from a star where water vapor can stay liquid on the
surface.
Planets like Venus that orbit a little closer to the sun
lose their liquid water and are cloaked mostly in carbon dioxide.
Planets like Mars that orbit further out from the sun than Earth
have their liquid water locked up as ice.
Kepler-186f appears to be orbiting at the outer edge of the
habitable zone around its star, which could mean that any liquid
surface water would be in danger of freezing, Kane said.
"However, it is also slightly larger
than the Earth, and so the hope would be that this would result
in a thicker atmosphere that would provide extra insulation" and
make the surface warm enough to keep water liquid.
Although Kepler-186f shows exciting
signs of being Earth-like, Stephen Kane points out that its differences are
also fascinating.
"We're always trying to look for
Earth analogs, and that is an Earth-like planet in the habitable
zone around a star very much the same as our sun," said Kane,
who is the chair of Kepler's Habitable Zone Working Group.
"This situation is a little bit
different, because the star is quite different from our sun."
Kepler-186 is an
M-dwarf star, much smaller and
cooler than the sun.
These stars are numerous in our galaxy,
and have some features that make them promising places to look for
life.
"For example, small stars live a lot
longer than larger stars," Kane explained, "and so that means
there is a much longer period of time for biological evolution
and biochemical reactions on the surface to take place."
On the other hand, small stars tend to
be more active than stars the size of our sun, sending out more
solar flares and potentially more radiation toward a planet's
surface.
"The diversity of these exoplanets
is one of the most exciting things about the field," Kane said.
"We're trying to understand how
common our solar system is, and the more diversity we see, the
more it helps us to understand what the answer to that question
really is."
Kepler-186f
- Close to Earth Size, in
the Habitable Zone -
by Paul Gilster
April 17, 2014
from
Centauri-Dreams Website
We have another 'habitable
zone' planet to talk about today, one not much bigger
than the Earth, but it's probably also time to renew the caveat that
using the word 'habitable' carries with it no guarantees.
The working definition of habitable zone
right now is that orbital distance within which liquid water might
exist on the surface of a planet. Whether it actually does is just
one of the questions. A second is whether or not we're in fact
dealing with a rocky terrestrial world.
So Centauri Dreams approaches the announcement of
Kepler-186f with guarded enthusiasm for an exoplanet that looks
interesting indeed. Five planets circle this star, an M-dwarf a
great deal smaller and cooler than the Sun.
Discovered by the Kepler space
observatory, the planet presents us with transit information telling
us that it is about 1.1 Earth radii, although we don't yet know what
the mass of this world is, and hence can't make a definitive call on
whether or not it is rocky.
But Stephen Kane (San Francisco
State), one of the researchers involved in today's announcement,
thinks we have reason to think that it is:
"What we've learned, just over the
past few years, is that there is a definite transition which
occurs around about 1.5 Earth radii," said Kane.
"What happens there is that for
radii between 1.5 and 2 Earth radii, the planet becomes massive
enough that it starts to accumulate a very thick hydrogen and
helium atmosphere, so it starts to resemble the gas giants of
our solar system rather than anything else that we see as
terrestrial."
Kepler-186f is thus well below the value
where we would expect it to accumulate a thick hydrogen and helium
envelope, causing Kane to add,
"there's a very excellent chance
that it does have a rocky surface like the Earth."
If that's the case, then we have a
planet on the outer edge of its star's habitable zone, though one
that may have a somewhat thicker atmosphere than Earth's because of
its somewhat larger size.
Perhaps the surface can avoid freezing.
In any case, this is what lead author
Elisa Quintana (NASA Ames) calls,
"the first definitive Earth-sized
planet found in the habitable zone around another star."
The work (An
Earth-Sized Planet in the Habitable Zone of a Cool Star)
appeared today in Science.
Image: The artist's concept depicts Kepler-186f, the
first validated Earth-size planet orbiting a distant
star in the habitable zone - a range of distances
from a star where liquid water might pool on the
surface of an orbiting planet.
The discovery of Kepler-186f confirms that
Earth-size planets exist in the habitable zone of
other stars and signals a significant step closer to
finding a world similar to Earth.
Credit:
NASA Ames/SETI Institute/JPL-Caltech.
The discovery team used so-called
'speckle imaging' in obtaining its high resolution observations from
the eight-meter Gemini North telescope on Mauna Kea as well as
adaptive optics observations from the ten-meter Keck II telescope to
rule out extraneous sources that could account for the Kepler data,
concluding that the signal has to be that of a transiting planet.
The speckle data allowed direct imaging
of the system to within 400 million miles, confirming there were no
other stellar-sized objects orbiting within this distance from the
star.
"The Keck and Gemini data are two
key pieces of this puzzle," adds Quintana. "Without these
complementary observations we wouldn't have been able to confirm
this Earth-sized planet."
The new planet orbits its star once
every 130 days, receiving about a third of the heat energy that
Earth does from the Sun.
The four inner planets,
-
Kepler-186b
-
Kepler-186c
-
Kepler-186d
-
Kepler-186e,
...are all too hot for life as we know
it, with periods of 3, 7, 13 and 22 days.
Image: Kepler-186 and the Solar System: The
diagram compares the planets of the inner solar
system to Kepler-186, a five-planet system about
500 light-years from Earth in the constellation
Cygnus.
The five planets of Kepler-186 orbit a star
classified as a M1 dwarf, measuring half the
size and mass of the sun.
The Kepler-186 system is home to Kepler-186f,
the first validated Earth-size planet orbiting a
distant star in the habitable zone - a range of
distances from a star where liquid water might
pool on the surface of an orbiting planet.
Credit:
NASA Ames/SETI Institute/JPL-Caltech.
The objection to Kepler-186f as a home
for life rests on the dangers of orbiting an M-dwarf, a class of
star prone to flare activity.
Move a planet close enough to the star
to be in its habitable zone and the assumption is that it's also
tidally locked, presenting the same side to the star throughout its
orbit, with all the complications that brings to climate models.
Neither of these factors are complete
show-stoppers - some climate studies show that temperature extremes
can be mitigated by winds or ocean currents - and in the case of
Kepler-186f, we do have a world on the habitable zone's outer edge,
perhaps far enough out not to suffer tidal lock.
So it's an interesting place, this new world, about 490 light years
away in the
constellation Cygnus and thus
tantalizingly out of reach for atmospheric analysis even with
instrumentation planned for the near future.
The
James Webb Space Telescope itself
won't be able to help us with that task.
But it's pleasing to note that
Kepler-186f has been studied over a frequency range of 1 to 10 GHz
looking for emissions, though none has so far been found. Getting a
detectable signal here from this star would require a transmitter
between 10 and 20 times as powerful as the planetary radar system at
Arecibo.
SETI keeps coming up empty, but good for
us if, in addition to our other studies, we keep our ears open for a
long-shot detection.
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