The architecture of planetary systems is an increasingly important factor to exoplanet scientists. This illustration shows the Kepler-11 system where the planets are all roughly the same size and their orbits spaced at roughly the same distances from each other. The the planets are, in the view of scientists involved with the study, “peas in a pod.” (NASA)
Before the discovery of the first exoplanet that orbits a star like ours, 51 Pegasi b, the assumption of solar system scientists was that others planetary systems that might exist were likely to be like ours. Small rocky planets in the inner solar system, big gas giants like Jupiter, Saturn and Neptune beyond and, back then, Pluto bringing up the rear
But 51 Peg b broke every solar system rule imaginable. It was a giant and hot Jupiter-size planet, and it was so close to its star that it orbited in a little over 4 days. Our Jupiter takes 12 years to complete an orbit.
This was the “everything we knew about solar systems is wrong” period, and twenty years later thinking about the nature and logic of solar system architecture remains very much in flux.
But progress is being made, even ...