We are so familiar with our solar system that we often presume it is generally how star systems are built. Four little planets close to the star, four large gas planets farther away, and all with roughly circular orbits. But as we have found ever more exoplanets, we’ve come to understand just how unusual the solar system is. Large planets often orbit close to their star, small planets are much more common than larger ones, and as a new study shows, orbits aren’t always circular.
Published in the Proceedings of the National Academy of Sciences, the study looks at the correlation between the size of an exoplanet and the shape of its orbit. This isn’t an easy thing to do. Most exoplanets are discovered using the transit method, where a planet passes in front of its star with each orbit. We don’t see the planet itself, but rather the dimming of the star as the planet blocks some of the light. The time between transits gives you the period of an exoplanet’s orbit, but it doesn’t tell you anything about the shape of its orbit.
To determine that, you need to look at how the star dims with each transit, what’s known as the light curve. The shape of the light curve tells you all sorts of things, such as whether the planet passes in front of the center of the star or off to one side, and how quickly it’s moving as it passes. To determine whether an exoplanet has a circular or elliptical orbit, astronomers have to carefully model the light curve, comparing it to orbital simulations. For this study, the team looked at the transits of more than 1,600 exoplanets to determine orbital shapes, then correlated orbits to the size of each exoplanet.
Eccentricity changes with planet size. Credit: Greg Gilbert
They found that while small exoplanets almost always have circular orbits, planets larger than about Neptune more commonly have elliptical orbits. This is true regardless of whether the large planet orbits close to the star or more distantly, and it suggests that large and small planets form in different ways. All planets form by accumulating gas and dust from the disk of material surrounding a young star, but large planets need a metal-rich environment to gain mass quickly, and they must capture hydrogen and helium to grow to a larger size. This more rapid growth likely leads to more chaotic formation rates and more elliptical orbits.
This study also has implications for the evolution of stellar systems. Planets with elliptical orbits are more likely to disrupt the orbits of their neighbors, which can lead to bombardment periods and collisions between terrestrial planets, similar to the collision that produced the Earth-Moon system. The early period of a star system is probably more turbulent than we first suspected.
Reference: Gilbert, Gregory J., Erik A. Petigura, and Paige M. Entrican. “Planets larger than Neptune have elevated eccentricities.” Proceedings of the National Academy of Sciences 122.11 (2025): e2405295122.