A search for debris discs around stars with giant planets

Eight nearby stars with known giant planets have been searched for thermal emission in the submillimetre arising from dust debris. The null results imply quantities of dust typically less than 0.02 Earth masses per star. Conversely, literature data for 20 Sun-like stars with debris discs show that less than or equal to 5 per cent have gas giants inside a few astronomical units - but the dust distribution suggests that nearly all have more distant planets. The lack of overlap in these systems - i.e. few stars possess both inner planets and a disc - indicates that these phenomena either are not connected or are mutually exclusive. Comparison with an evolutionary model shows that debris masses are predicted to be low by the stellar ages of 2-8 Gyr (unless the colliding parent bodies are quite distant, located beyond 100-200 au), but it remains to be explained why stars that do have debris should preferentially only have distant planets. A simple idea is proposed that could produce these largely different systems, invoking a difference in the primordial disc mass. Large masses promote fast gas giant growth and inwards migration, whereas small masses imply slow evolution, low-mass gas giants and outwards migration that increases the collision rate of Kuiper Belt-like objects. This explanation neglects other sources of diversity between discs (such as density and planetesimal composition and orbits), but it does have the merit of matching the observational results.