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We present evidence that the recently discovered, directly-imaged planet HD131399 Ab is a background star with non-zero proper motion. From new JHK1L'photometry and spectroscopy obtained with the Gemini Planet Imager, VLT/SPHERE,and Keck/NIRC2, and a reanalysis of the discovery data obtained withVLT/SPHERE, we derive colors, spectra, and astrometry for HD 131399 Ab. Thebroader wavelength coverage and higher data quality allow us to re-investigateits status. Its near-infrared spectral energy distribution excludes spectraltypes later than L0 and is consistent with a K or M dwarf, which are the mostlikely candidates for a background object in this direction at the apparentmagnitude observed. If it were a physically associated object, the projectedvelocity of HD 131399 Ab would exceed escape velocity given the mass anddistance to HD 131399 A. We show that HD 131399 Ab is also not following theexpected track for a stationary background star at infinite distance. Solvingfor the proper motion and parallax required to explain the relative motion ofHD 131399 Ab, we find a proper motion of 12.3 mas/yr. When compared topredicted background objects drawn from a galactic model, we find this propermotion to be high, but consistent with the top 4% fastest-moving backgroundstars. From our analysis we conclude that HD 131399 Ab is a background K or Mdwarf.

Evidence that the Directly-Imaged Planet HD 131399 Ab is a Background Star