Everything seems to be fine in our solar system: Smaller rocky planets such as Venus, Earth or Mars orbit relatively close to our star. 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, scientists from the Universities of Bern and Geneva and the National Center of Competence in Research (NCCR) Planets show that our planetary system is quite unique in this regard.
Like a pea in a pod
Mishra says “More than a decade ago, astronomers noticed from observations by the then-groundbreaking Kepler telescope that planets in other systems tend to be similar in size and mass to their neighbors—like peas in a pod,” says study lead author Lokesh Mishra, a researcher. at the University of Bern and Geneva and also at NCCR PlanetS. However, it was not clear for a long time whether this finding was due to the limitations of the observational methods. “It was not possible to determine whether the planets in any individual system were similar enough to fall into the ‘pod game’ class of systems, or whether they were somewhat different—just like in our solar system,”
Therefore, the researcher developed a framework for determining the differences and similarities between planets of the same systems. And in the process 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 neighboring planets are similar have similar architecture. Ordered planetary systems are those in which the mass of the planets tends to increase with distance from the star – just like in our solar system. If, on the other hand, the mass of the planets roughly decreases with the distance from the star, scientists speak of an anti-ordered architecture of the system. And a mixed architecture occurs when the planetary masses in a system vary greatly from planet to planet.
“This framework can also be applied to any other measurements, such as radius, density or water fraction,” says study co-author Yann Alibert, professor of planetary science at the University of Bern and NCCR PlanetS. “Now, for the first time, we have a tool to study planetary systems as a whole and compare them to other systems.”
The findings also raise questions: Which architecture is most common? What factors govern the emergence of a type of architecture? Which factors do not play a role? Some of these can be answered by researchers.
A bridge spanning 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 a ‘similar’ architecture,” says Mishra. “This also explains why evidence of this architecture was found in the early months of the Kepler mission.” To the team’s surprise, “ordered” architecture—one that includes the solar system—appears to be the rarest class.
According to Mishra, there are indications that both the mass of the disk of gas and dust from which the planets emerge, and the amount of heavy elements in the star in question, play a role. “From relatively small, low-mass disks and stars with few heavy elements, ‘similar’ planetary systems
surface. Large, massive disks with many heavy elements in the star give rise to more ordered and anti-ordered systems. Mixed systems are created from medium-sized disks. Dynamic interactions between planets – such as collisions or ejections – affect the final architecture,” explains Mishra.
“A remarkable aspect of these results is that they link the initial conditions for the formation of planets and stars to a measurable property: the architecture of the system. Between them lie billions of years of evolution. For the first time, we were able to bridge this huge time gap and make testable predictions. It will be exciting to see if it holds up,” concludes Alibert.
