In our solar system everything seems to be meticulously well ordered: the smaller rocky planets, such as Venus, Earth or Mars, orbit relatively close to our star while the large gas and ice giants, such as Jupiter, Saturn or Neptune, move much farther away, tracing wide orbits around the Sun. But ‘out there’ that apparently immutable architecture does not seem to be the general rule. Quite the contrary.
At the time of writing these lines, NASA’s Exoplanets website counted 3,921 different planetary systems identified so far by astronomers. And it turns out that in most of them things do not work in the same way as in our solar system.
In two studies that appeared in ‘Astronomy & Astrophysics’ (here and here), in fact, researchers from the Universities of Bern and Geneva and the National Center for Research Competence (NCCR) show that in a certain sense our planetary system is unique, Or at least the rarest.
“More than a decade ago – explains Lokesh Mishra, lead author of the study – astronomers noticed, based on observations with the then-innovative Kepler telescope, that planets in other systems generally resemble their respective neighbors in size and mass. , like peas in a pod. But for a long time it was not clear if this finding was due to the limitations of the observational methods. It was then not possible to determine whether the planets in any system were similar enough to fall into the ‘peas in a pod’ type, or were instead quite different, as is the case with our own solar system.”
four different types
In order to find out, Mishra developed a framework for determining the differences and similarities between planets in the same systems. And in doing so, he discovered that there are not two, but four possible architectures of planetary systems: similar, ordered, anti-ordered, and mixed. In the words of the scientist, the first are those in which “the masses of the neighboring planets are similar to each other.”
For their part, “ordered planetary systems are those in which the mass of the planets tends to increase with distance from the star, as happens in our solar system.” If, on the contrary, the mass of the planets decreases approximately with the distance from the star, the researchers speak of an anti-ordered architecture of the system. And finally, there are mixed architectures, when the planetary masses of a system vary greatly from one planet to another.
“This framework -says Yann Alibert, co-author of the study- can also be applied to any other planetary measurement, such as radius, density or water fractions. Now, for the first time, we have a tool to study planetary systems as a whole and compare them with other systems.”
new questions
The possibility of establishing comparisons between some systems and others, however, also raises new questions. What architecture is the most common? What factors control the emergence of a particular type of architecture? Some of them are answered in the study itself.
In Mishra’s words, “Our results show that ‘similar’ planetary systems are the most common type of architecture. Approximately eight out of ten planetary systems around stars visible in the night sky have ‘similar’ architecture.” The surprise came with ‘ordered’ architecture, which also includes our solar system, which seems to be the rarest kind.
And what does it depend on whether the distribution of planets in a system is one way or another? According to Mishra, there are indications that both the mass of the disk of gas and dust from which the planets emerge, and the abundance of heavy elements in the respective star play a role in determining the architecture of a solar system. “From fairly small, low-mass disks and stars with few heavy elements, ‘similar’ planetary systems emerge,” Mishra explains. Large, massive disks with many heavy elements in the star give rise to more ordered and anti-ordered systems. And mixed systems arise from medium-sized disks. Dynamic interactions between planets, such as collisions or ejections, also influence the final architecture.”
“A notable aspect of these results -Alibert concludes- is that they link the initial conditions of planetary and stellar formation to a measurable property: the architecture of the system. Between one and the other mediate billions of years of evolution. For the first time, we have managed to bridge this huge time gap and make testable predictions. It will be exciting to see if they are fulfilled ».