Why are we not finding as many multi-planet systems as we should? For a long time this question puzzled scientists, but a change to a small assumption used in modeling planetary systems now makes sense of the data we have received from the Kepler Space Telescope. Ironically, the assumption that works involves systems being more similar to our own.
The vast wealth of data produced by the Kepler Space Telescope has given us a chance to build statistical answers to the question of how like our Solar System other systems are. Early efforts concluded we're not finding as many multi-planet systems as we should. To explain this, a theory known as the Kepler dichotomy was produced, which assumed there are two sorts of planetary systems – one rich in planets and the other with only one or two. Even those who proposed this idea couldn't explain why there wouldn't be plenty of systems with intermediary numbers.
However, in a paper accepted by the Monthly Notices of the Royal Astronomical Society (preprint on arXiv), Australian National University PhD student Tim Bovaird and his supervisor Dr Charley Lineweaver claim the dichotomy is false. Instead, the results Kepler is giving us can be explained by rethinking the relationships between planetary orbits. Oddly enough, Bovaird and Lineweaver argue the problem has come from an assumption that most star systems differ from our own.
A common simplification when describing the Solar System, particularly in artistic renderings, has all the orbits in exactly the same plane. Although the planetary orbits are actually very close to co-planar (unlike many comets, for example), the alignment is not perfect. If it was, we would see transits of Venus every year or two, instead of a couple of times a century.
Planetary orbits can differ from being perfectly co-planar in two ways. Bovaird and Lineweaver refer to these as “flat” and “flared”, based on whether the distribution of orbital inclinations depends on a planet's distance from the Sun or not.