Infrared Non-detection of Fomalhaut b -- Implications for the Planet Interpretation

TL;DR

Deep infrared observations of Fomalhaut with Spitzer/IRAC challenge the planet hypothesis for Fomalhaut b, suggesting the observed source is more likely scattered light from dust rather than a giant planet.

Contribution

The study introduces a novel PSF subtraction technique that significantly improves infrared contrast, providing stringent flux limits that question the planetary nature of Fomalhaut b.

Findings

Infrared flux limits exclude a giant planet surface contribution.

The observed source is more consistent with scattered light from dust.

The planet hypothesis for Fomalhaut b is highly unlikely based on new data.

Abstract

The nearby A4-type star Fomalhaut hosts a debris belt in the form of an eccentric ring, which is thought to be caused by dynamical influence from a giant planet companion. In 2008, a detection of a point-source inside the inner edge of the ring was reported and was interpreted as a direct image of the planet, named Fomalhaut b. The detection was made at ~600--800 nm, but no corresponding signatures were found in the near-infrared range, where the bulk emission of such a planet should be expected. Here we present deep observations of Fomalhaut with Spitzer/IRAC at 4.5 um, using a novel PSF subtraction technique based on ADI and LOCI, in order to substantially improve the Spitzer contrast at small separations. The results provide more than an order of magnitude improvement in the upper flux limit of Fomalhaut b and exclude the possibility that any flux from a giant planet surface contributes to the observed flux at visible wavelengths. This renders any direct connection between the observed light source and the dynamically inferred giant planet highly unlikely. We discuss several possible interpretations of the total body of observations of the Fomalhaut system, and find that the interpretation that best matches the available data for the observed source is scattered light from transient or semi-transient dust cloud.