GeminiFocus January 2018 | Page 18

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 techniques ) 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 CHaracterising EXOPlanet Satellite ( CHEOPS ) mission is planned for the end of 2018 ; and ESA ’ s PLAnetary Transits and Oscillations of stars ( PLATO ) mission is now set for 2026 . Therefore , many new small transiting planets will be identified over the next decade . This presents an enormous opportunity to expand the study of planetary statistics into the regime of planet bulk compositions — if we can measure the masses of these objects using 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 , existing radial velocity instruments become very inefficient around V = 12th magnitude and objects with V > 13th magnitude are unattainable for all but the most intense campaigns . 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 observations 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 characterize 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 lowestmass 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 lowmass planets around low-mass M dwarfs , we need high-precision radial velocity measurements 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 geometry 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 .
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