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What Planetary Nebulae Can Tell Us about Planetary Systems

We derive the maximal orbital separations allowed for brown dwarfs and massive planets in order to tidally spin up progenitors of planetary nebulae (PNs). We find the maximal orbital separation to be ~5 AU. We combine this finding with our basic assumption that the axisymmetric structure of elliptical PNs is due to the interaction of their progenitors with binary companions, being stellar or substellar. This leads us to conclude that substellar objects (brown dwarfs or gas-giant planets) are commonly present within several AU around main-sequence stars. For a substellar object to have a high probability of being present within this orbital radius, on average several substellar objects must be present around most main-sequence stars of masses ≲5 M☉. Our arguments suggest that the presence of four gas-giant planets in the solar system is the generality rather than the exception.