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

## TL;DR

The paper provides evidence that the directly-imaged planet HD 131399 Ab is actually a background star, not a bound planet, based on new photometry, spectroscopy, and astrometric analysis indicating its proper motion and spectral type.

## Contribution

The study reanalyzes existing data with higher quality observations to conclusively identify HD 131399 Ab as a background star rather than a planet.

## Key findings

- HD 131399 Ab's spectral energy distribution matches a K or M dwarf.
- Its proper motion is consistent with a high-velocity background star.
- It is not gravitationally bound to HD 131399 A.

## Abstract

We present evidence that the recently discovered, directly-imaged planet HD 131399 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 with VLT/SPHERE, we derive colors, spectra, and astrometry for HD 131399 Ab. The broader wavelength coverage and higher data quality allow us to re-investigate its status. Its near-infrared spectral energy distribution excludes spectral types later than L0 and is consistent with a K or M dwarf, which are the most likely candidates for a background object in this direction at the apparent magnitude observed. If it were a physically associated object, the projected velocity of HD 131399 Ab would exceed escape velocity given the mass and distance to HD 131399 A. We show that HD 131399 Ab is also not following the expected track for a stationary background star at infinite distance. Solving for the proper motion and parallax required to explain the relative motion of HD 131399 Ab, we find a proper motion of 12.3 mas/yr. When compared to predicted background objects drawn from a galactic model, we find this proper motion to be high, but consistent with the top 4% fastest-moving background stars. From our analysis we conclude that HD 131399 Ab is a background K or M dwarf.