  We conduct gas and dust hydrodynamical simulations of protoplanetary discs
with one and two embedded planets to determine the impact that a second planet
located further out in the disc has on the potential for subsequent planet
formation in the region locally exterior to the inner planet. We show how the
presence of a second planet has a strong influence on the collection of solid
material near the inner planet, particularly when the outer planet is massive
enough to generate a maximum in the disc's pressure profile. This effect in
general acts to reduce the amount of material that can collect in a pressure
bump generated by the inner planet. When viewing the inner pressure bump as a
location for potential subsequent planet formation of a third planet, we
therefore expect that the mass of such a planet will be smaller than it would
be in the case without the outer planet, resulting in a small planet being
sandwiched between its neighbours - this is in contrast to the expected trend
of increasing planet mass with radial distance from the host star. We show that
several planetary systems have been observed that do not show this trend but
instead have a smaller planet sandwiched in between two more massive planets.
We present the idea that such an architecture could be the result of the
subsequent formation of a middle planet after its two neighbours formed at some
earlier stage.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics