Tests of the planetary hypothesis for PTFO 8-8695b

TL;DR

This study tests the planetary hypothesis for PTFO 8-8695b through multiple observations, finding no supporting evidence and suggesting stellar activity or dust as alternative explanations for the observed fading events.

Contribution

The paper provides the first comprehensive multi-method test of the planetary hypothesis for PTFO 8-8695b, challenging previous interpretations.

Findings

No secular changes in light-curve morphology were observed.

Infrared observations did not detect the expected planetary radiation.

No Rossiter-McLaughlin effect was detected during fading events.

Abstract

The T Tauri star PTFO 8-8695 exhibits periodic fading events that have been interpreted as the transits of a giant planet on a precessing orbit. Here we present three tests of the planet hypothesis. First, we sought evidence for the secular changes in light-curve morphology that are predicted to be a consequence of orbital precession. We observed 28 fading events spread over several years, and did not see the expected changes. Instead we found that the fading events are not strictly periodic. Second, we attempted to detect the planet's radiation, based on infrared observations spanning the predicted times of occultations. We ruled out a signal of the expected amplitude. Third, we attempted to detect the Rossiter-McLaughlin effect by performing high-resolution spectroscopy throughout a fading event. No effect was seen at the expected level, ruling out most (but not all) possible orientations for the hypothetical planetary orbit. Our spectroscopy also revealed strong, time-variable, high-velocity H{\alpha} and Ca H & K emission features. All these observations cast doubt on the planetary hypothesis, and suggest instead that the fading events represent starspots, eclipses by circumstellar dust, or occultations of an accretion hotspot.