Four planetary system architectures discovered by researchers

Through the development of a framework designed to determine the differences and similarities between planets of the same systems, researchers have discovered that there are four planetary system architectures.

Our solar system appears to be in order, with the smaller rocky planets Venus, Earth, and Mars orbiting close to the Sun. On the other hand, the large gas and ice giants, such as Jupiter, Saturn, and Neptune, move in wide orbits around our star. Researchers from the universities of Bern and Geneva, and the National Centre of Competence in Research (NCCR) PlanetS have now revealed that, in this respect, our planetary system may be quite unique.

The findings were published in the journal Astronomy & Astrophysics.

A framework to understand how planets of the same systems vary

“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,” said study lead author Lokesh Mishra, researcher at the University of Bern and Geneva, as well as the NCCR PlanetS.

However, it has been unclear for a long time whether this discovery was simply due to the limitations of observational methods.

Artist impression of the four classes of planetary system architecture. © NCCR PlanetS, Illustration: Tobias Stierli

“It was not possible to determine whether the planets in any individual system were similar enough to fall into the class of the ‘peas in a pod’ systems, or whether they were rather different – just like in our solar system,” said Mishra.

Because of this, Mishra developed a framework to pinpoint the differences and similarities between planets of the same systems. Through the development of the framework, he found that there are four planetary system architectures.

What are the four planetary system architectures?

“We call these four classes ‘similar’, ‘ordered’, ‘anti-ordered’ and ‘mixed’,” said Mishra.

A planetary system that has neighbouring planets that are comparable to each other has similar architecture. An ordered planetary system, like our solar system, is one where the mass of the planets increases with distance from the star.

Alternatively, if the mass of the planets decreases with distance from the star, the architecture of the system is regarded as anti-ordered. A mixed structure occurs when the planetary masses in a system vary greatly from planet to planet.

© Nevio Heimberg, University of Bern

“This framework can also be applied to any other measurements, such as radius, density or water fractions,” said study co-author Yann Alibert, Professor of Planetary Science 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 raise questions, such as ‘which planetary structure is the most common?’ and ‘which factors control the emergence of an architecture type?’ These can be answered through the researchers’ results.

The results of the study reveal the rarity of our solar system

“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,” says Mishra. “This also explains why evidence of this architecture was found in the first few months of the Kepler mission.”

The team were surprised that the ‘ordered’ architecture, which includes our solar system, appeared to be the rarest planetary system. There are indications that the mass of the gas, the dust disk from which the planets emerge, and the abundance of heavy elements in the star all play a role in the formation of an ordered planetary system.

“From rather small, low-mass 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 explained.

“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,” Alibert concluded.

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