For the first time, an Earth-sized planet has been found in the habitable
zone of its star. This discovery not only proves the existence of worlds
that might be similar to our own, but also will undoubtedly shape future
investigations of exoplanets that could have terrestrial surface
environments.
The new-found body, orbiting the red dwarf star Kepler-186 and
designated Kepler-186f, is the fifth — and outermost — world to be
discovered in this system.
“This is the first definitive Earth-sized planet found in the habitable zone
around another star,” said Elisa Quintana of the SETI Institute at NASA
Ames Research Center in Mountain View, California. “Finding such
planets is a primary goal of the Kepler space telescope. The star is a
main sequence M dwarf, a very common type. More than 70 percent of
the hundreds of billions of stars in our galaxy are M dwarfs.”
Of the nearly 1,800 confirmed exoplanets found in the past two decades,
approximately 20 orbit their host star in the habitable zone — a range of
orbital distances at which surface water on a planet with an atmosphere
would neither freeze nor boil. However, all of these previously discovered
worlds are larger than Earth, and consequently their true nature — rocky
or gaseous — is unknown. On the basis of the observed dimming of
starlight from Kepler-186, the scientists estimate that this newly
discovered planet is roughly the same size as Earth.
“Theoretical models of how planets form suggest that those with
diameters less than 1.5 times that of Earth are unlikely to be swathed in
atmospheres of hydrogen and helium, the fate that’s befallen the gas
giants of our own solar system,” said Thomas Barclay, a staff scientist
for the Kepler mission affiliated with both NASA and the Bay Area
Environmental Research Institute. “Consequently, Kepler-186f is likely a
rocky world, and in that sense, similar to Venus, Earth, and Mars.”
Traditionally, planets orbiting red dwarf stars were considered to be poor
candidates for life. The objection was that star-hugging planets in the
habitable zone would become tidally locked and suffer a synchronous or
pseudo-synchronous rotation that could make climate on these planets
untenable. However, more recent modeling studies suggest that such
worlds are not necessarily inhospitable because atmospheric winds or
ocean currents could even out extreme temperature variations. In
addition, Kepler-186f is far enough away from its host star that it is
unlikely to be locked. This greater distance also reduces the danger to
any potential life-forms posed by stellar flares, which are more common
for dwarf stars.
Since 2012, the SETI Institute's Allen Telescope Array has been
observing Kepler candidate exoplanets looking for signals that would
indicate extraterrestrial intelligence. A search for emissions from
Kepler-186f has been made over the very wide frequency range of 1 to
10 GHz, but none have so far been found. These observations will be
repeated. Note that a detectable signal would require a transmitter
approximately 10 to 20 times more powerful than the planetary radar
system at Arecibo in Puerto Rico.
According to Quintana, at 490 light-years, Kepler-186f may be too dim
for follow-up surveys to probe its atmosphere, even with next-generation
telescopes: “However, our research tells us that we should be able to find
planets around bright stars that will be ideal targets to observe with
James Webb.” NASA’s Webb space-based telescope, now under
construction, will be able to directly image planets around nearby dwarf
stars and use spectral analysis to characterize their atmospheres.
Finding Kepler-186f is a first, but “it’s not a record we wish to keep,”
Quintana said. “We want to find more of these.”


https://www.youtube.com/watch?v=RlidbLyDnPs&feature=youtube_gdata_player

Nice progress. The next hurdle is how to distinguish between a run away greenhouse oven like venus, or a dry desolate low pressure frozen waste land like mars from our lovely blue planet earth since all three lie in the habitable zone.