Dr. Lokesh Mishra and his colleagues from the University of Bern and Geneva Observatory propose that the space of planetary system architectures be partitioned into four classes: similar, mixed, anti-ordered, and ordered.

“More than a decade ago, astronomers noticed, based on observations with NASA’s Kepler space telescope, that planets in other systems usually resemble their respective neighbors in size and mass — like peas in a pod,” Dr. Mishra said.

“But for a long time it was unclear whether this finding was due to limitations of observational methods.”

“It was not possible to determine whether the planets in any individual system were similar enough to fall into the class of the ‘peas in a pod’ systems, or whether they were rather different — just like in our Solar System.”

In their research, Dr. Mishra and co-authors aimed to develop a framework to determine the differences and similarities between planets of the same systems.

And in doing so, they discovered that there are not two, but four such system architectures.

“We call these four classes ‘similar,’ ‘ordered,’ ‘anti-ordered’ and ‘mixed’,” Dr. Mishra said.

“Planetary systems in which the masses of neighboring planets are similar to each other, have similar architecture.”

“Ordered planetary systems are those, in which the mass of the planets tends to increase with distance from the star — just as in the Solar System.”

“If, on the other hand, the mass of the planets roughly decreases with distance from the star, we speak of an anti-ordered architecture of the system.”

“And mixed architectures occur, when the planetary masses in a system vary greatly from planet to planet.”

“This framework can also be applied to any other measurements, such as radius, density or water fractions,” said University of Bern’s Professor Yann Alibert.

“Now, for the first time, we have a tool to study planetary systems as a whole and compare them with other systems.”

“Our results show that ‘similar’ planetary systems are the most common type of architecture,” Dr. Mishra said.

“About eight out of ten planetary systems around stars visible in the night sky have a ‘similar’ architecture.”

“This also explains why evidence of this architecture was found in the first few months of the Kepler mission.”

What surprised the team was that the ‘ordered’ architecture — the one that also includes the Solar System — seems to be the rarest class.

There are indications that both the mass of the gas and dust disk from which the planets emerge, as well as the abundance of heavy elements in the respective star play a role.

“From rather small, low-mass disks and stars with few heavy elements, ‘similar’ planetary systems emerge,” Dr. Mishra said.

“Large, massive disks with many heavy elements in the star give rise to more ordered and anti-ordered systems.”

“Mixed systems emerge from medium-sized disks. Dynamic interactions between planets — such as collisions or ejections — influence the final architecture.”

“A remarkable aspect of these results is that it links the initial conditions of planetary and stellar formation to a measurable property: the system architecture.”

“Billions of years of evolution lie in between them,” Professor Alibert said.

“For the first time, we have succeeded in bridging this huge temporal gap and making testable predictions. It will be exciting to see if they will hold up.”