Figure 4.
An Earth-sized planet
crossing in front of
a Sun-like star (left)
and an M dwarf like
Kepler-186 (right).
The amount of
starlight blocked by
an Earth-sized planet
in the habitable zone
is proportionately
greater for an M dwarf
than a Sun-like star,
creating a larger dip
in the transit light
curve (bottom) and
therefore making
them easier to detect.
Credit: Wendy Stenzel
close that it dilutes the signal, or if the binary
stars barely eclipse each other, then these are
other scenarios that could cause a small dip
in the light curve and be misinterpreted as a
planetary eclipse.
Kepler-186 was observed using the Differential Speckle Survey Instrument (DSSI) on
Gemini North and using the NIRC2 camera on
the Keck-II telescope. Gemini and Keck each
imaged Kepler-186 to different degrees of
M Dwarfs: Prime Targets in the Search for
Habitable Worlds
M dwarfs (stars with 0.1-0.5 times the mass of the Sun) are excellent targets in the search for
habitable worlds. Planets in the habitable zones of M stars are easier to detect than planets
in the habitable zones of Sun-like stars due to their shorter orbital periods and frequency of
transits detected. The proportion of starlight that they block is also greater (see Figure 4) so the
transit depths are deeper. M-dwarfs are also very abundant, comprising about three quarters
of all main sequence stars in our galaxy. They also evolve very slowly in luminosity, thus their
habitable zones remain stable for billions of years. Furthermore, planets around M dwarf hosts
may (ultimately) be imaged more easily due to higher contrast between the planet and the star.
M Dwarfs have long been thought to be unsuitable hosts for habitable planets due to the proximity of planets in the habitable zone and their vulnerability to the stellar environment. M stars
are known to be highly active early in their life, often producing giant and frequent flares which
could scorch planets nearby. They also gravitationally interact with the planets, causing tides
that heat the planet and cause their rotations to be ‘tidally locked,’ which means one side always faces the star and the other side faces the cold open space, much like our moon is tidally
locked with the Earth. Fortunately, Kepler-186f orbits a star that is on the larger end of the M
dwarf mass range and is at a large enough distance where it could very well have escaped all of
these complications to habitability. Regardless, there have been many recent studies that have
shown ways around each of these challenges, and there isn’t any one factor that precludes M
dwarf planets from being habitable.
6
GeminiFocus
April 2014