When an exoplanet is discovered, scientists are quick to describe it and explain its properties. Now, we know of thousands of them, many of which are members of a planetary system, like the well-known TRAPPIST-1 family of planets.
Patterns are starting to emerge in these exoplanetary systems, and in new research, a team of scientists says it’s time to start classifying exoplanet systems rather than just individual planets.
The paper is “Architecture Classification for Extrasolar Planetary Systems,” and it’s available on the pre-print site arxiv.org. The lead author is Alex Howe from NASA’s Goddard Space Flight Center. The authors say it’s time to develop and implement a classification framework for exoplanet systems based on our entire catalogue of exoplanets.
“With nearly 6000 confirmed exoplanets discovered, including more than 300 multiplanet systems with three or more planets, the current observational sample has reached the point where it is both feasible and useful to build a classification system that divides the observed population into meaningful categories,” they write.
The authors explain that it’s time for a systemic approach to identifying patterns in exoplanet systems. With almost 6,000 exoplanets discovered, scientists now have the data to make this proposition worthwhile.
What categories do the authors propose?
The first step is necessarily broad. “The core of our classification system comes down to three questions for any given system (although, in several cases, we add additional subcategories). Does the system have distinct inner and outer planets?” the authors write.
Next comes the question of Jupiters. “Do the inner planets include one or more Jupiters?” After that, they ask if the inner planets contain any gaps with a period ratio greater than 5. That means if within the gaps between the inner planets, are there any instances where the ratio of the orbital periods of two hypothetical planets occupying those gaps would exceed 5? Basically, that boils down to asking if the absence of planets in specific regions in the inner solar system is related to unstable orbits.
These three questions are sufficient to classify nearly all of the exoplanet systems we’ve discovered.
“We find that these three questions are sufficient to classify ~97% of multiplanet systems with N ?3 planets with minimal ambiguity, to which we then add useful subcategories, such as where any large gaps occur and whether or not a hot Jupiter is present,” the authors write.
The result is a classification scheme that divides exoplanet systems into inner and outer regimes and then divides the inner regimes into dynamical classes. Those classes are:
- Peas-in-a-pod systems where the planets are uniformly small
- Warm Jupiter systems containing a mix of large and small planets
- Closely-space systems
- Gapped systems
There are further subdivisions based on gap locations and other features.
“This framework allows us to make a partial classification of one- and two-planet systems and a nearly complete classification of known systems with three or more planets, with a very few exceptions with unusual dynamical structures,” the authors explain.
In summary, the classification scheme first divides systems into inner and outer planets (if both are detected). Systems with more than three inner planets are then classified based on whether their inner planets include any Jupiters and whether (and if so, where) their inner planets include large gaps with a period ratio >5. Some systems have other dynamical features that are addressed separately from the overall classification system.
6 R?, red), Neptunes (3.5-6 R?, gold), Sub-Neptunes (1.75-3.5 R?, blue), and Earths (