The first extrasolar planet was detected a decade ago; more than 150 have been catalogued since. But there are significant differences between all of these systems and our own Solar System, particularly in the orbits that the planets follow around their star. The semimajor orbital axes of extrasolar planets are usually comparatively small, often less than 0.03 AU — or less than a tenth of Mercury’s distance from the Sun. Their eccentricities are also often far greater than those of Solar System planets, something that the standard theory of planetary formation cannot explain.

Writing in the Astronomical Journal, Fathi Namouni1 examines whether processes imparting a small relative acceleration to the star–planet system — such as stellar jets and star-disk winds — could produce large orbital eccentricities. In the standard theory, as a planet moves around a star through the disk of material from which it was born, drag forces are thought to act to circularize its orbit and to cause it to migrate towards the host star. Many explanations have been proposed, for example, perturbations by distant companion bodies or by passing stars; or resonant interactions within many-body systems. But, although each of these hypotheses can account for the characteristics of one or more of the planetary systems detected, no single theory has yet provided a satisfactory framework within which all observations can be understood.

Namouni’s model successfully recreates the range of eccentricities observed, and has other advantages. For instance, if the relative acceleration lasts long enough, planets outside of a critical radius will reach high eccentricities and run the risk of being ejected from the host star — which could explain the large number of single-planet systems observed so far. The low eccentricities of the Solar System planets can also be reproduced using Namouni’s model.