Asrophys. J. 702, 716–723 (2009)

The near-circular orbits followed by the planets in the Solar System are probably not the norm. Many planets travelling around other stars appear to follow an oblong or 'eccentric' path.

In some cases, that apparent eccentricity may be caused by the presence of a second, undetected planet. Timothy Rodigas and Philip Hinz at the University of Arizona in Tucson modelled a range of solar systems. They found that about 4% of the time, a hidden planet far from the star could create the illusion that the closer planet was in a highly eccentric orbit. The effect was much more pronounced for planets with moderately oblong orbits.

The team says that their study could help to explain the apparent abundance of eccentric exoplanets. It could also help researchers to find hidden planets.