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Fomalhaut has hosted a point source inside its ring-shaped disk that vas visible in 2004-2014. The source has been shown to have expanded and faded over time, indicating that it is a dust cloud originating from the disruption of a ~100 km-sized planetesimal. A natural explanation for this phenomenon is a collision with another planetesimal, where both bodies were excited to high eccentricities by a shepherding planet inside the ring. However, another pathway to destruction of the planetesimal is through tidal disruption in the gravitational field of the planet that excited it in the first place. In this talk, I will discuss the relative rates of planetesimal collisions and tidal disruption in the Fomalhaut system, and how to distinguish the two possible scenarios observationally. In particular, I will note that the tidal disruption hypothesis leads to a direct prediction for the ephemerides of the disrupting planet, which is expected to be bright enough to be detectable by JWST soon after its launch. In this framework, the cloud therefore acts as a smoke signal, pointing out the precise location of a young sub-Saturnian planet. The presence or absence of this planet at its predicted location is in turn a distinctive criterion for distinguishing the origins for the cloud.

Markus Janson: Smoke signals from Fomalhaut