Astronomers reveal Kepler-186f, the latest planet in a habitable
A planet similar in size to Earth with surface temperatures suitable
for water to exist in liquid form has been discovered orbiting a distant
star in what is the strongest candidate yet for a habitable world
outside the Solar System.
An artist's impression of Kepler-186f which may have conditions
suitable to life
Scientists said that the planet, called Kepler-186f, is the smallest
"exoplanet" so far discovered and is highly likely to be a rocky planet
like Earth, with its own atmosphere and solid surface where liquid water
and possibly life can exist.
Astronomers have now confirmed the existence of nearly 1,000 planets
beyond the Solar System using highly sophisticated techniques based on
measuring minute changes in the intensity of starlight as objects pass
in front of their own sun. Hundreds of other planetary candidates are
However, most of the confirmed exoplanets are gaseous giant planets
like Jupiter with no solid surfaces, potentially toxic atmospheres, and
that are either too hot or too cold for liquid water and hence life.
Kepler-186f, meanwhile, fits the description of a rocky planet with a
benign atmosphere that lies within the water-friendly "Goldilocks zone"
which is neither too hot, nor too cold for life. It comes the closest
yet to being "Earth 2.0", astronomers said.
"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 Centre in Moffett Field, California, and
lead author of the study published in Science.
Finding Earth-like habitable planets outside the Solar System was the
main purpose of the Kepler space telescope and the discovery of such a
planet around a red dwarf star, Kepler 186, suggests there are many more
to be found given that this type of star comprises about 70 percent of
the hundreds of billions of stars in the Milky Way galaxy, Dr Quintana
The size of the planet, estimated to be 1.1 times the radius of
Earth, suggests that it is likely to have a rocky surface rather than
being a large gas giant planet, although this can only be confirmed
after scientists have calculated its mass and hence density.
The red dwarf star Kepler 186 is 500 light years away and the
planet's orbit is similar to that of Mercury's which is too hot for
water. However, because red dwarfs are smaller and cooler than the Sun
the planet lies just on the outer edge of its sun's habitable zone where
liquid water can exist - although if water does exist some of it is
likely to be frozen for at least part of the time.
"We're always trying to look for Earth analogues, and that is an
Earth-like planet in the habitable zone around a star very much the same
as our Sun," said Stephen Kane, an astronomer at San Francisco State
University who chairs the habitable zone working group of the Kepler
mission. "Some people call these habitable planets, which of course we
have no idea if they are. We simply know that they are in the habitable
zone, and that is the best place to start looking for habitable
planets," Prof Kane said.
Planets similar to Venus orbit a little too close to the Sun and so
are in danger of losing their water and being cloaked in carbon dioxide,
whereas planets like Mars orbit a little too far away, which means any
surface water is locked away as ice.
Eventhough the orbit of Kepler 186f takes it on the extreme cold side
of its habitable zone, there is a possibility that it slightly larger
size compared to Earth means that it has pulled in a thicker atmosphere,
making it warmer than it otherwise might be due to an enhanced
greenhouse effect, Prof Kane said.
The planet is the fifth to be discovered orbiting the same red dwarf
star and Dr Quintana and her colleagues detected its faint signature
when analysing the minute fluctuations in the brightness of the distant
star as the planet passed between the star and Earth. Calculations
showed that its orbit of 130 days takes it within the star's vital
habitable zone. "We are not saying that there's water on the surface.
All we know is that the surface has the right temperature that water
could exist there in a liquid state," Steve Howell, Kepler's project
scientist at NASA Ames, told Science.
- The Independent