Figure 2.
Kepler-186f is within
ten percent the size of
Earth, 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
on the outer edge of the
habitable zone (shaded
region in green). The
system is also home to
four inner planets, all
measuring less than 1.5
times the size of Earth,
and orbiting a cooler
and less massive star
than our Sun. Because
Kepler-186 is cooler and
dimmer, the habitable
zone is located closer
in. All five planets in
this system have orbital
distances to their star
less than Mercury’s
distance from the Sun.
Credit: NASA/Ames/JPLCaltech/T. Pyle
Follow-up observations with the 8-meter
Gemini North and 10-meter Keck II telescopes provided high-quality observations
that backed up the spacecraft’s discovery,
making Kepler-186f the first Earth-sized exoplanet with the potential to support life.
In Transit
The Kepler spacecraft discovered Kepler-186f
by observing transits (where a planet passes
in front of its host star) on several occasions.
During a transit, the total amount of light we
see from the star is diminished due to the
Kepler-186f: A Planet with Oceans?
The host star, Kepler-186, is an M-type star, an M dwarf, or a red dwarf. It lies in the direction of
the constellation Cygnus, about 500 light-years away. The star is relatively dim with a luminosity of just about 5 percent that emitted by our Sun.
The habitable zone (HZ) around this lower-luminosity star is located much closer in compared
to the habitable zone of a star like our Sun. The intensity and spectrum of the star’s radiation
determines the boundaries of the HZ. Kepler-186f receives about a third of the insolation (intensity of stellar radiation) as that received by Earth from the Sun and, with an orbital period of 130
days, it resides in the HZ throughout its orbit.
Since Kepler can only measure a planet’s size, we don’t know the mass of Kepler-186f and therefore we cannot say anything about its composition. Theoretical models have shown that planets as small as Kepler-186f are highly unlikely to be dominated by a gas envelope like Neptune,
and more likely is composed of some combination of rock, iron and perhaps water or ice, material that also composes the Earth (and that we refer to as “rocky”). However, if Kepler-186f is
rocky like the Earth, and has an Earth-like atmosphere, then any H2O at its