In our Solar System the smallest rocky planets – such as Venus, Earth or Mars – orbit relatively close to our star. The large gas and ice giants – such as Jupiter, Saturn or Neptune – on the other hand, move in large orbits around the Sun. But is this true for other planetary systems? Recently, two studies demonstrate that our Solar System is quite unique. A new classification has been recently proposed and maybe it could be part of the next generation of science schoolbooks.
The Institute of Physics of the University of Bern and the Geneva Observatory of the University of Geneva with the study of Mishra et al. [1] explain that the space of planetary system architectures are partitioned into four classes: similar, mixed, anti-ordered, and ordered. They applied their framework to observed and synthetic multi-planetary systems, thereby studying their architectures of mass, radius, density, core mass, and the core water mass fraction.
The new classification [1]
Similar: Planetary systems have a similar architecture when all planets in the system have masses that are approximately similar to each other. This architecture is the most common outcome of planetary formation and is also the most frequent architecture class observed.
Mixed: Planetary systems where the planetary masses (inside-out) show broad increasing and decreasing variations have mixed architecture. These systems are also composed of super- Earths, sub-Neptunes, Neptunes, and Jovian planets.
Anti-ordered: Planetary systems where the planetary mass shows an overall decrease with distance have an anti-ordered architecture. The multiplicity distribution shows that these systems tend to have fewer planets than similar or mixed architecture. This is an indication that the formation pathway of these architectures differs considerably from the other two types of architecture.
Ordered: Planetary systems where the planetary masses show an overall increase with distance have an ordered architecture. Ordered architecture is a rare outcome for the Bern model. Observations are generally biased against discovering small and less massive planets which are farther away from their host star. Such biases, however, make ordered systems the second most common architecture class. Fifteen systems in the actual catalogue exhibit this architecture. Unsurprisingly, the most notable known example of this architecture class is the Solar System.
The study probes novel questions and new parameter spaces for understanding theory and observations. Future studies may utilize the proposed framework to not only constrain the knowledge of individual planets, but also the multi-faceted architecture of an entire planetary system. In the future this could help understanding the role of system architectures in hosting habitable worlds, also thanks to further advances in era of new telescopes.
For more details see the literature below.
[1] Framework for the architecture of exoplanetary systems – I. Four classes of planetary system architecture; Lokesh Mishra, Yann Alibert, Stéphane Udry and Christoph Mordasini; Astronomy & Astrophysics; 670, A68 (2023)
