Astronomers have long suspected that not all planetary systems are constructed like our solar system. A team of scientists from the Universities of Bern and Geneva and the National Research Center (PRN) PlanetS proves for the first time that there are actually four types of planetary systems. This classification will allow astronomers to study these systems as a whole and compare them to others. These results can be discovered in two articles published in the journal Astronomy and Astrophysics. Artist’s rendering of the four classes of planetary system architecture. This new nomenclature will allow scientists to study an entire planetary system as a single entity. © NCCR PlanetS, illustration: Tobias Stierli
Everything seems to be in order in our solar system: small rocky planets such as Venus, Earth or Mars orbit the sun at a relatively small distance. Gas and ice giants such as Jupiter, Saturn or Neptune have larger orbits around the sun. Researchers from the Universities of Bern and Geneva, members of the PRN PlanetS, show in two studies published in the journal Astronomy & Astrophysics that our planetary system is exceptional in this respect.
Like peas in a pod
“Based on observations made more than a decade ago with the then-state-of-the-art Kepler space telescope, astronomers had found that planets in other systems were generally similar in mass and size to their respective neighbors, like peas in a pod.” , explains the study’s lead author, Dr. Lokesh Mishra, researcher at the Universities of Bern and Geneva and member of PRN PlanetS. However, doubts remained for a long time: Was this result real or was it an illusion due to the limitations of the observation methods? “Back then, it was impossible to determine whether the planets in a system were similar enough to fall into the ‘peas in a pod’ category, or whether they were rather different like our solar system,” says the researcher.
Therefore, the scientist has developed a nomenclature to determine the differences and similarities between planets of the same system. Then he discovered that there are four types of architecture for planetary systems, not two.
Four classes of planetary systems
“We call these four classes ‘similar’, ‘ordered’, ‘anti-ordered’ and ‘mixed’,” says Lokesh Mishra. Planetary systems where the masses of neighboring planets are nearly identical fall into the “similar” category. Systems in which the mass of the planets increases with increasing distance from the star fall into the “ordered” category, such as our solar system. If, on the other hand, the mass of the planets decreases with distance from the star, scientists speak of an “anti-ordered” architecture. Finally, the “mixed” type is used to describe a system in which the masses of the planets are significant and vary from planet to planet.
“This nomenclature can also be used for any other type of measurement, such as the radius, density or water content of these planets,” adds study co-author Yann Alibert, Professor of Planetary Sciences at the University of Bern and member of the PRN PlanetS . “Now, for the first time, we have a tool that allows us to study planetary systems as a whole and compare them to other systems.”
These conclusions also lead to new questions: What is the most common architecture? What are the key factors driving the emergence of each architectural type? Which factors have no influence? Researchers already have the answer to some of these questions.
A lineage spanning several billion years
“Our results show that the ‘similar’ planetary system architecture is the most common. About eight out of ten star systems visible in the night sky have such an architecture,” says Lokesh Mishra. “This also explains why evidence of this architecture was already found in the first few months of the Kepler mission.” What surprised the research team: The “ordered” architecture that corresponds to our solar system seems to be the rarest.
According to Lokesh Mishra, the mass of the disks of gas and dust from which the planets form and the abundance of heavy elements in the respective host stars seem to play a key role. “‘Similar’ systems are formed from relatively small, low-mass disks. In contrast, massive disks containing many heavy elements lead mainly to “ordered” and “anti-ordered” systems. Finally, the “mixed” systems are the result of medium-sized discs. However, dynamic interactions between planets such as collisions or ejections also influence the final architecture.
“One of the remarkable aspects of these results is that they link the initial conditions of star and planet formation to a property that can be observed today: the architecture of the planetary system. In between lie billions of years of evolution. For the first time, we have succeeded in establishing a connection despite such a large time interval in order to be able to make verifiable predictions. It will be exciting to see if these predictions hold up over time,” concludes Yann Alibert.