FEB 21, 2023 8:30 PM PST

Astronomers Find Four Classes of Planetary Systems

An artist's impression of the four classes of planetary system architecture. Credit: NCCR PlanetS, Tobias Stierli

NASA’s Kepler Mission, launched in 2009, revolutionized the field of exoplanet science. It provided a vast amount of information about planets orbiting stars other than the Sun and the planetary systems they reside in. The data obtained by Kepler appeared to tell astronomers that planets in other systems usually resemble their respective neighbors in size and mass, much like peas in a pod. This is much different than the architecture of our own Solar System, where planets grow in size as their orbital distance from the Sun increases. It has been unclear whether this trend was the result of an observational bias of the mission until now.

Researchers from the Universities of Bern and Geneva, as well as from the National Center of Competence in Research PlanetS, have now shown that our planetary system is unique in the way that it is organized. In a series of papers published in the journal Astronomy & Astrophysics, the researchers have shown that there are four distinct types of planetary systems (Four Classes of Planetary System Architecture, Emergent Formation Pathways of Architecture Classes).

The team developed a framework to categorize the differences and similarities between planets within the same planetary system. Within this framework, they defined four classes of planetary systems: similar, ordered, anti-ordered, and mixed. In a ‘similar’ system, the masses of neighboring planets are similar to each other. An ‘ordered’ system is composed of planets where the mass of the planets tends to increase with distance from the star. This is the category in which our Solar System falls into. An ‘anti-ordered’ system is composed of planets where the mass of the planets roughly decreases with distance from the star. The last category is a ‘mixed’ system, which is a system in which the planetary masses vary vastly from planet to planet.

The researchers found that ‘similar’ planetary systems are the most common type of architecture, making up approximately 8 out of every 10 planetary systems. ‘Ordered’ systems, like our Solar System, seem to be the rarest class.

The study also indicates that the emergent planetary system architecture depends on the initial conditions of stellar and planetary formation. The team found evidence that the mass of the gas and dust disk from which the planetary system emerges and the abundance of heavy elements in the central star(s) are important factors. ‘Similar’ planetary systems seem to emerge from small, low-mass disks and stars composed of few heavy elements. ‘Ordered’ and ‘anti-ordered’ systems tend to emerge from large, massive disks and stars composed of many heavy elements, while ‘mixed’ systems emerge from medium-sized disks.

There are sure to be many other factors that influence planetary system architecture, such as dynamic interactions between planets within a system, but these results serve as a good stepping stone to help astronomers understand how various factors play a role. It will be interesting to see if these hypotheses hold up as astronomers learn even more about exoplanets and planetary systems with data obtained by NASA’s Transiting Exoplanet Survey Satellite and NASA’s James Webb Space Telescope.

Source: University of Bern

About the Author
Doctorate (PhD)
I'm a stellar astrophysicist by training with a passion for formal and informal education and diversity and inclusion in STEM. I love to take a humanistic approach to my work and firmly believe that all of humanity is united under one sky.
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