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Transit photometry of the M8V dwarf star TRAPPIST-1 (2MASS J23062928-0502285)has revealed the presence of at least seven planets with masses and radiisimilar to that of Earth orbiting at distances that might allow liquid water tobe present on their surfaces. We have been following TRAPPIST-1 since 2011 withthe CAPSCam astrometric camera on the 2.5-m du Pont telescope at the LasCampanas Observatory in Chile. In 2016 we noted that TRAPPIST-1 lies slightlyfarther away than previously thought, at 12.49 pc, rather than 12.1 pc. Here weexamine fifteen epochs of CAPSCam observations of TRAPPIST-1, spanning the fiveyears from 2011 to 2016, and obtain a revised trigonometric distance of $12.56\pm 0.12$ pc. The astrometric data analysis pipeline shows no evidence for along-period astrometric wobble of TRAPPIST-1. After proper motion and parallaxare removed, residuals at the level of $\pm 1.3$ millarcsec (mas) remain. Theamplitude of these residuals constrains the masses of any long-period gas giantplanets in the TRAPPIST-1 system: no planet more massive than $\sim 4.6M_{Jup}$ orbits with a 1 yr period, and no planet more massive than $\sim 1.6M_{Jup}$ orbits with a 5 yr period. Further refinement of the CAPSCam dataanalysis pipeline, combined with continued CAPSCam observations, should eitherdetect any long-period planets, or put an even tighter constraint on these massupper limits.

Astrometric Constraints on the Masses of Long-Period Gas Giant Planets in the TRAPPIST-1 Planetary System