Determing Radial Velocity Detected Exoplanets' Masses may NOT be Possible With EXO & WFIRST

TRUE MASSES OF RADIAL-VELOCITY EXOPLANETS

Author:

Brown

Abstract:

We study the task of estimating the true masses of known radial-velocity (RV) exoplanets by means of direct astrometry on coronagraphic images to measure the apparent separation between exoplanet and host star. Initially, we assume perfect knowledge of the RV orbital parameters and that all errors are due to photon statistics. We construct design reference missions for four missions currently under study at NASA: EXO-S and WFIRST-S, with external star shades for starlight suppression, EXO-C and WFIRST-C, with internal coronagraphs. These DRMs reveal extreme scheduling constraints due to the combination of solar and anti-solar pointing restrictions, photometric and obscurational completeness, image blurring due to orbital motion, and the "nodal effect," which is the independence of apparent separation and inclination when the planet crosses the plane of the sky through the host star. Next, we address the issue of nonzero uncertainties in RV orbital parameters by investigating their impact on the observations of 21 single-planet systems. Except for two—GJ 676 A b and 16 Cyg B b, which are observable only by the star-shade missions—we find that current uncertainties in orbital parameters generally prevent accurate, unbiased estimation of true planetary mass. For the coronagraphs, WFIRST-C and EXO-C, the most likely number of good estimators of true mass is currently zero. For the star shades, EXO-S and WFIRST-S, the most likely numbers of good estimators are three and four, respectively, including GJ 676 A b and 16 Cyg B b. We expect that uncertain orbital elements currently undermine all potential programs of direct imaging and spectroscopy of RV exoplanets.

Future NASA/NSF Exoplanet Hunting Missions

The last several years have made it clear we are living in a golden era of extrasolar planet studies. Less than 20 years ago, astronomers discovered the first planets orbiting Sun-like stars. Now, the number of exoplanets is in the thousands, including a growing number of “Earth-like” planets, a designation based on some combination of the planets’ mass, radius, and orbit around their stars. Just this past Friday, for example, astronomers reported detecting three planets between 1.5 and 2.4 times the radius of the Earth orbiting a single star, one of which lies in the star’s habitable zone, where liquid water could exist on its surface.

We don’t know, however, if these or other Earth-like planets are really like the Earth in the characteristics that really count: whether they have atmospheres like the Earth, oceans of liquid water like the Earth, and life like the Earth. Those determinations are largely beyond the capabilities of ground- and space-based observatories in operation today. A new generation—arguably, generations—of instruments and telescopes will be needed to determine just how Earth-like these Earth-like worlds really are. And the ability to develop those instruments will depend, at least in part, on the ability of exoplanet scientists to come into agreement on what’s needed to enable that next round of discoveries.

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