Our Solar System Is Even Stranger Than We Thought

Our Solar System Is Even Stranger Than We Thought

Lauren Weiss

In addition to having similar sizes, planets in these peas-in-a-pod-like systems also have regular orbital spacing. We found that the orbital distance between the first pair of planets is a good predictor of the orbital distance of the third planet, and the fourth, and so on. (Regular spacing also exists in our solar system out to Uranus and is called the Titius-Bode Law, but Neptune and Pluto do not follow the pattern). Furthermore, there is a link between the sizes of the planets and their spacing: the systems with the smallest planet sizes also have the closest orbital spacings.

What do these patterns mean? Planet formation is surely governed by the laws of physics, but we do not have a clear narrative of how these laws manifest in the messy environment of planetary birth. Theories of planet formation were mostly written before the discovery of the first exoplanet; their purpose was to explain the emergence of our own solar system from a disk of gas and dust. A widely accepted (but unconfirmed) theory about planet formation involves the rise of so-called “oligarchs,” young precursors of planets that each influence a swath of fixed width within the disk around the star. (Pluto is no longer considered a planet because Pluto was never big enough to be an oligarch.)

Oligarchic theory predicts roughly equal-mass oligarchs spaced at regular intervals, with the size of the oligarch dependent on the width of its influence. However, because our solar system is not a system of equal-mass planets at regular spacing, the rise of oligarchs is considered a mere chapter in our solar system’s history, an early pattern that was later overwritten by violent impacts that formed our very dissimilar terrestrial planets.