It has long been clear to astronomers that planetary systems are not necessarily structured like our solar system. Researchers from the Universities of Bern and Geneva and the National Centre of Competence in Research PlanetS have now shown for the first time that there are a total of four classes of planetary systems.
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In our solar system, everything seems to be in order: The smaller 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 wide orbits around the sun. In two studies published in the scientific journal Astronomy & Astrophysics, researchers from the Universities of Bern and Geneva and the National Centre of Competence in Research (NCCR) PlanetS show that our planetary system is quite unique in this respect.
Like peas in a pod
"Already more than a decade ago, astronomers noticed, based on observations with the then groundbreaking Kepler telescope, that planets in other systems usually resemble their respective neighbours in size and mass - like peas in a pod," says study lead author Lokesh Mishra, who conducts research at the University of Bern and Geneva, as well as the NCCR PlanetS. But for a long time it was unclear whether this finding was due to limitations in observational methods. "It was impossible to determine whether the planets in a certain system were similar enough to fall into the class of 'pea-in-a-pod' systems, or whether they were rather different - just like in our solar system," says Mishra.
Therefore, the researcher developed a concept to determine the differences and similarities of planets of the same systems. And in doing so, he discovered that there are not two, but four such system architectures.
Four classes of planetary systems
"We call these four classes 'similar', 'ordered', 'anti-ordered' and 'mixed'," says Mishra. Planetary systems in which the masses of neighbouring planets are similar to each other have similar architecture. Ordered planetary systems are those in which the mass of the planets tends to increase with distance from the star - just as in our solar system. If, on the other hand, the mass of the planets decreases with distance from the star, researchers speak of an anti-ordered architecture of the system. And mixed architectures occur when the planetary masses in a system vary greatly from planet to planet.
"This concept can also be applied to any other measurand, such as radius, density or water fractions," says study co-author Yann Alibert, who conducts research at the University of Bern and the NCCR PlanetS. "Now, for the first time, we have a tool to study planetary systems as a whole and compare them with other systems."
The findings also raise questions: Which architecture is the most common? Which factors control the emergence of an architecture type? Which factors do not play a role? The researchers can answer some of these.
A bridge over billions of years
"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 such 'similar' architecture," says Mishra. "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 - that is, the one that includes the solar system - seems to be the rarest class.
According to Mishra, there are indications that both the mass of the gas and dust disk from which the planets emerge and the abundance of heavy elements in the respective star play a role. "From rather small, less massive disks and stars with few heavy elements, 'similar' planetary systems emerge. Large, massive disks with many 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," Mishra explains.
"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. In between lie billions of years of evolution. We have succeeded for the first time in bridging this huge temporal gap and making testable predictions. It will be exciting to see whether they will stand up," Alibert sums up.
Publication details:
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
https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/202243751
DOI: 10.1051/0004-6361/202243751
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
https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/202244705
DOI: 10.1051/0004-6361/202244705
14.02.2023