The fact that young planets migrate as a result of interaction with their natal disc is well established theoretically and is widely applied when explaining the orbital properties of exoplanets. To date however the very long timescales for planetary migration have meant that there is no direct evidence for this. Moreover theoretical estimates for the rate of planetary migration are rather uncertain.
We have proposed the first potential observational test of planetary migration, pointing out, on the basis of our hydrodynamical simulations of dust and gas in discs, that a migrating planet has a different signature from a stationary planet in terms of the way that it sculpts the disc dust. We have modeled the case of the young disc Elias 24 where the Atacama Large Millimetre Array (ALMA) has demonstrated a pronounced gap (presumed to contain a planet) and a bright emission ring outside. We have shown that the dust emission profile of the disc is better fit by models where the planet migrates . In order to test this hypothesis further we are seeking to acquire data at a range of wavelengths that are sensitive to a range of dust grain sizes. We predict that smaller grains (which are probed by shorter wavelength observations) should be preferentially concentrated in a ring interior to the planet whereas larger grains should instead congregate outside the planet; this therefore changes the emissivity profile at different wavelengths. The predicted profiles depend on how fast dust grains drift inwards relative to how fast the planet is migrating. Thus multi-wavelength observations provide a way to estimate the planet’s migration rate.