Everything is in order in our solar system. Smaller rocky planets such as Venus, Earth and Mars orbit relatively close to our star. On the other hand, huge gas and ice giants such as Jupiter, Saturn and Neptune travel in wide orbits around the sun. Scientists from the Universities of Bern and Geneva and the National Center of Competence in Research (NCCR) PlanetS show in two studies that our planetary system is unique in this respect.

But not all planetary systems are structured like our solar system. Scientists from the Universities of Bern and Geneva, as well as from the National Center of Competence in Research PlanetS, have now shown for the first time that there are, in fact, four types of planetary systems.

Based on studies with the pioneering Kepler telescope, astronomers discovered more than a decade ago that planets in other systems typically resemble their neighbors in size and mass, like peas in a pod. It was impossible to tell if the planets in any given system were similar enough to be considered “peas in a pod” systems or if they were significantly different, similar to our solar system.

Artist's impression of the four classes of planetary systems
Artist’s impression of the four classes of planetary system architecture. A new architecture framework allows researchers to study an entire planetary system at the system level. If the small planets within a system are close to the star and massive planets further away, such systems have an “ordered” architecture. Conversely, if the masses of the planets in a system tend to decrease with distance from the star, then these systems are ‘Anti-Ordered’. If all planets in a system have similar masses, then the architecture of this system is ‘Similar’. ‘Mixed’ planetary systems are those in which the planetary masses show large variations. Research suggests that planetary systems with the same architecture class share common formation pathways. © NCCR PlanetS, Illustration: Tobias Stierli

Scientists developed a framework to determine the differences and similarities between planets of the same systems. In doing so, they discovered that there are not two, but four such system architectures.

Study lead author Lokesh Mishra, a researcher at the University of Bern and Geneva and the NCCR PlanetS said: “Our results show that ‘similar’ planetary systems are the most common type of architecture. About eight out of ten planetary systems around stars visible in the night sky have a “similar” architecture. This also explains why evidence for this architecture was found in the early months of the Kepler mission.”

“What surprised the team was that the ‘ordered’ architecture – which includes the solar system – appears to be the rarest class.”

“There are indications that both the mass of the disc of gas and dust from which the planets emerge and the abundance of heavy elements in the respective star play a role. From relatively small, low-mass disks and stars with few heavy elements, “similar” planetary systems arise.”

“Large, massive disks with heavy elements in the star make for more ordered and anti-ordered systems. Mixed systems arise from medium-sized drives. Dynamic interactions between planets – such as collisions or ejection – influence the final architecture.”

Co-author Yann Alibert, professor of planetary sciences at the University of Bern and the NCCR PlanetS, said: “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. Between them lie billions of years of evolution. For the first time, we managed to bridge this huge gap in time and make testable predictions. It will be exciting to see if they keep it up.”

Magazine reference:

  1. L. Mishra, Y. Alibert, S. Udry, C. Mordasini, A framework for the architecture of exoplanetary systems. I. Four classes of planetary system architecture, astronomy and astrophysics, accepted December 2022. DOI: 10.1051/0004-6361/202243751
  2. L. Mishra, Y. Alibert, S. Udry, C. Mordasini, A framework for the architecture of exoplanetary systems. II. Nature vs. Nurture: Emerging Formation Pathways of Architecture, Astronomy, and Astrophysics Classes, Accepted December 2022. DOI: 10.1051/0004-6361/202244705
  3. Research Highlight Article in Nature Astronomy: Maltagliati, L. Finding order in planetary architectures. Wet Astron 7.8 (20230). DOI: 10.1038/s41550-023-01895-0