Abstracl.We prove the existence of a new type of resonance formation in highly doubly excited atoms and ions. The resonant states represent the quantum analogue of a stable planetary atom configuration discovered recently in classical mechanics. T h e energies and total decay widths of the resonances are calculated quantum mechanically by solving the full three-body Coulomb problem without any approximation. The resonmm energies and the structure of the associated wavefunctions coincide with expectations derived from simple semiclassical models. We discuss excitation schemes which should allow an experimental observation of these strongly correlated electron states.The structure and the formation of highly correlated electronic states in doubly excited atoms and ions is of topical interest in spectroscopy (Camus et a/ 1989, Eichmann et al 1990, Kilgus et al 1990, Hams et a1 1990, Domcke et a/ 1991) and atomic physics (Fano 1983, Herrick 1983. Nevertheless, a global understanding and characterization of these states is still lacking because of the non-separability of the problem. The search for approximate symmetries for the three-body Coulomb problem using grouptheoretical or adiabatic methods was partially successful for doubly excited intrashell resonances (Herrick 1983, Feagin and Briggs 1986, 1988, Rost et al 1991a) and approximate quantum numbers were introduced to classify the multitude of resonances occurring (for a recent review see Rost and Briggs 1991). However, the first stringent test of such models on the basis of accurate ab initio calculations were performed only recently and only for a certain class of states (Rost et a1 1991b, Ezra et a1 1991). In this letter we report on large scale ab initio calculations for planetary helium states (Leopold and Percival 1980), where both electrons are highly excited. We focus on a new class of extremely correlated states, for which the existing classification schemes turn out to be inapplicable. The existence of such states was predicted recently on classical grounds and represents the quantum analogue of a stable classical planetary atomic configuration (Richter and Wintgen 1990). The general mechanism for the formation of this type of long-lived resonance is the existence of a stable periodic orbit, which allows for an (approximate) torus quantization of the nearby phase space region (Miller 1975).The non-relativistic Hamiltonian of a two-electron atom (or ion) is given by (atomic units used)