Why Giant Planets Often Have Slightly Stretched Orbits express
A planet's orbit is its path around its star. If the path is perfectly circular, the planet always stays at the same distance from the star. But many planets have slightly stretched paths, like a running track in a stadium. Astronomers studied hundreds of giant planets around other stars and found a curious pattern.
The most massive planets, those several times heavier than Jupiter, most often move in slightly elongated orbits. Elongation is measured on a scale from 0 to 1: zero means a circle, one means a highly stretched oval. For such heavyweight planets, the elongation is most often around 0.3. It's as if a runner on a track were running not in a circle but in a slightly flattened oval.
Where does this elongation come from? It seems that in their youth, these planets experienced violent events. Possibly, they collided with other large bodies, like when our Moon formed, when a Mars-sized object slammed into the young Earth. Such impacts could have slightly "stretched" the orbits of giant planets.
This discovery helps us understand how planetary systems are born and evolve. If a star has a massive planet on a slightly elongated orbit, it could be a trace of an ancient cosmic accident. So by studying orbits, we read the history of distant worlds.
Original abstract
as written by the authors arXiv:2601.18877
In this study, we compute completeness-corrected occurrence rates of giant exoplanets as a function of mass, semimajor axis, and eccentricity, using the approximately uniform California Legacy Survey sample of RV-discovered planets published in Rosenthal et al. 2021. We recover the previously-detected rise in occurrence with semimajor axis for both lower- and higher-mass subsets of the population out to $$\sim$$5 au. When restricting to planets with semimajor axes between 0.1 and 4.5 au (roughly speaking, the "peak" of giant planet occurrence), we find evidence for distinct eccentricity distributions for each of two mass sub-populations. Most strikingly, we observe a peak in the eccentricity distribution of super-Jovian planets (3-20~M$$_{\rm J}$$) at 0.3, which is apparent using two different parameterizations of the eccentricity distribution model. A hierarchical histogram model reveals that $$\sim$$92% of posterior samples indicate an elevated occurrence rate of super-Jupiters with modest eccentricities (0.2-0.4) compared to lower or higher eccentricities (i.e. evidence for a moderate eccentricity "peak"), and 99% of samples indicate super-Jupiters with modest eccentricities are more common than those with lower eccentricities (i.e. evidence that moderate eccentricities are more common than low eccentricities). We use a truncated Gaussian model fit to pinpoint the location of the super-Jupiter eccentricity peak with more precision, finding a maximum a posterior (MAP) peak location of $$e=0.3$$. This low but elevated characteristic eccentricity could be the result of dynamically hot histories, perhaps involving a giant impacts phase. All analysis code for this project is publicly available on Zenodo (https://zenodo.org/records/18089157) and GitHub (github.com/sblunt/eccentricities).
Earth's orbital elongation is only 0.017—almost a perfect circle. Pluto's is 0.25, which is why Pluto sometimes comes closer to the Sun than Neptune.