An important goal of exoplanet science is
to precisely measure masses and radii for a
large enough sample of low-mass planets so
that robust statistics emerge. Data from the
original Kepler mission have been used to
identify thousands of planet candidates, but
the masses of most of these planets cannot
be measured with existing spectrographs
because the expected radial velocity signals
are too small or the host stars are too faint, or
a combination of both.
Many new transiting planet finder missions
(designed specifically to exploit the synergy
between radial velocity and transit tech-
niques) have recently been approved: the
Kepler spacecraft has been repurposed for
the K2 mission; NASA’s Transiting Exoplanet
Survey Satellite (TESS) mission is scheduled
for launch in March 2018; ESA’s CHaracteris-
ing EXOPlanet Satellite (CHEOPS) mission is
planned for the end of 2018; and ESA’s PLAn-
etary Transits and Oscillations of stars (PLA-
TO) mission is now set for 2026. Therefore,
many new small transiting planets will be
identified over the next decade. This pres-
ents an enormous opportunity to expand
the study of planetary statistics into the re-
gime of planet bulk compositions — if we
can measure the masses of these objects us-
ing the radial velocity method.
The Radial Velocity Method
The techniques for Doppler spectroscopy
have currently progressed to the point
that precisions of 1-2 meters/second (m/s)
are routinely obtained on bright stars (e.g.,
Howard et al., 2011; Lovis et al., 2011), and
precisions of 60-80 centimeters/second
(cm/s) have been obtained in a few select
cases (e.g., Pepe et al., 2011). However, exist-
ing radial velocity instruments become very
inefficient around V = 12th magnitude and
objects with V > 13th magnitude are unat-
tainable for all but the most intense cam-
January 2018 / 2017 Year in Review
paigns. Therefore, 85% of the nearly 5,000
planet candidates from the Kepler mission
that have Kepler magnitudes greater than
13 are essentially out of reach of existing
instruments.
In addition to having substantially improved
precision and reach, the next generation of
radial velocity spectrographs should also
cover longer wavelengths for efficient ob-
servations of very low-mass M dwarfs. One
pathway for studying habitable planets is
focused on the opportunity offered by M
dwarfs; in particular the very lowest-mass M
dwarfs (those with M star < 0.3 M B ). In contrast
to solar-type stars, the habitable zones of M
dwarfs are close-in enough so that planets
in this region have a significant chance of
transiting, making them feasible targets for
transit spectroscopy observations to charac-
terize their atmospheres. The James Webb
Space Telescope (JWST) will be able to make
measurements of the transmission spectra
for such planets (Deming et al., 2009), but we
first have to identify good targets.
Improved reach to fainter stars is important
for the M dwarf science case. The lowest-
mass M dwarfs are intrinsically very faint,
and current instruments can achieve 1 m/s
precision only for the handful of these stars
within a few parsecs (e.g., Proxima Centauri
and Barnard’s Star). To take advantage of
the opportunities offered by transiting low-
mass planets around low-mass M dwarfs,
we need high-precision radial velocity mea-
surements for these stars out to 20 parsecs
(Deming et al., 2009). Only a few percent of
planets in the habitable zones of low-mass
M dwarfs will have the right orbital geome-
try to transit. So a large volume of space has
to be probed to be assured of finding some
that would be ideal targets for atmospheric
studies with JWST. With these conditions in
mind, MAROON-X was conceived and born.
GeminiFocus
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