There are, in fact, four types of planetary systems

These four classes are 'similar', 'ordered', 'anti-ordered,' and 'mixed'.

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Everything is in order in our solar system. Smaller rocky planets like Venus, Earth, and Mars orbit our star relatively closely. On the other hand, huge gas and ice giants like Jupiter, Saturn, and Neptune travel in broad orbits around the sun. In two studies, scientists from the Universities of Bern and Geneva and the National Centre of Competence in Research (NCCR) PlanetS show 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 Centre 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 made with the ground-breaking Kepler telescope, astronomers discovered more than ten years 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 sufficiently similar to be considered “peas in a pod” systems or whether they were significantly different, akin to our solar system.

Artist impression of the four classes of planetary system
Artist impression of the four classes of planetary system architecture. A new architecture framework allows researchers to study an entire planetary system at the systems level. If the small planets within a system are close to the star and massive planets further away, such systems have ‘Ordered’ architecture. Conversely, if the mass of the planets in a system tends to decrease with distance to the star 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 which have the same architecture class have common formation pathways. © NCCR PlanetS, Illustration: Tobias Stierli

Scientists developed a framework to determine the differences and similarities between planets of the same systems. While 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 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.”

“Large, massive disks with 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.”

Co-author Yann Alibert, Professor of Planetary Science 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. Billions of years of evolution lie in between them. 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.”

Journal 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 in 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 versus nurture: Emergent formation pathways of architecture classes, Astronomy and Astrophysics, Accepted December 2022. DOI: 10.1051/0004-6361/202244705
  3. Research Highlight Article in Nature Astronomy: Maltagliati, L. Finding order in planetary architectures. Nat Astron 7, 8 (20230). DOI: 10.1038/s41550-023-01895-0
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