TESS reveals how planet size shapes orbital eccentricity for warm single exoplanets

Star system: HD 191785 | Distance from the Sun: 66.7 light years

Astronomers have used data from NASA’s TESS mission to study 347 single exoplanets with orbital periods between 8 and 200 days. These planets, called 'warm' because they are neither too close nor too far from their stars, show a surprising link between their size and the shape of their orbits.

The team analyzed the planets by grouping them into size categories: sub-Neptunes, sub-Saturns, and Jovians. They found that most small and medium-sized planets have nearly circular orbits, while larger gas giants are much more likely to have stretched, or eccentric, orbits. The transition between these two behaviors happens at a planet radius of about 9.8 times that of Earth. This means that warm sub-Saturns tend to follow the same low-eccentricity pattern as sub-Neptunes, but warm Jovians are often found on highly eccentric paths.

The study also found a small group of sub-Neptunes with very eccentric orbits, likely influenced by unseen companion planets. These results suggest that warm gas giants are shaped by two different evolutionary paths, or by a single process that later increases their orbital eccentricity.

Key findings
- Most warm sub-Neptunes and sub-Saturns have nearly circular orbits
- Warm Jovian planets are frequently found on highly eccentric orbits
- The transition between low and high eccentricity occurs at about 9.8 Earth radii
- About 59% of warm Jovians belong to the high-eccentricity group
- A small fraction of sub-Neptunes also have highly eccentric orbits, likely due to external companions

Scientific study

The Orbital Eccentricity--Radius Distribution for Warm, Single Planets in TESS (external link)