Astronomers have classified planetary systems into four different categories based on the size and position of the planets. Apparently, the architecture of our own solar system is of the rarest kind. Decades of telescopes dedicated to searching for worlds around stars other than our own Sun have so far discovered more than 5,300 of these exoplanets in 3,910 planetary systems. With this amount of data, astronomers were able to group these planets into different groups based on their properties – rocky planets, gas giants, super-Earths, mini-Neptunes, and water worlds, among others.

But can planetary systems themselves be similarly classified? And if so, what does our solar system look like on a cosmic scale? The answers to these questions were the aim of a new study by Swiss scientists who examined data from all 853 systems known to contain multiple planets.

Based on this analysis, the team decided on four main classes, in which planetary systems are divided according to the size and position of their planets: similar, regular, irregular, and mixed. Similar systems, the most common arrangements, are systems in which all planets are roughly the same size – for example the TRAPPIST-1 system, which includes seven Earth-sized rocky planets. Regular systems are systems where the inner planets are small and rocky, giving way to gas and ice giants in the slums. Our own solar system falls into this group, and the team says it’s the rarest configuration.

Anti-order systems are the opposite – larger planets appear closer to the star and get smaller as they move forward. And finally, there are mixed systems that don’t seem to have any rhyme or reason about the placement of their planets.

So how do planetary systems work in these different configurations? As with so many things, the team says it’s a mix of “nature and nurture” – dependent in part on the initial conditions in which the system was born, including the mass and abundance of the planet-forming disk of dust and gas. heavy metals in the host star. It also depends in part on the dynamics of the planets over the lifetime of the system.

“From very small, low-mass disks and stars containing few heavy elements, ‘similar’ planetary systems emerge,” said Lokesh Mishra, lead author of the study. “Large, massive disks with lots of heavy elements in the star give rise to more ordered and disordered systems. Hybrid systems arise from medium-sized disks. Dynamic interactions between planets, such as collisions or launches, affect the final architecture.”

The more we learn about other planetary systems, the better we can understand our place in the universe.