While our solar system sports one Jupiter-like planet, exoplanet discoveries have revealed that giant planets come in a few different forms in systems with varying architectures. A recent study has explored how the mass–eccentricity relationship in warm Jupiters is shaped by dynamical interactions between planets.

Jupiter-like Planet Eccentricities

Exoplanet surveys have uncovered three different Jupiter-like planet populations across the galaxy: hot, warm, and cold Jupiters. Much like our own, cold Jupiters orbit on long periods, far away from their host stars. Conversely, hot Jupiters zip around their host stars on extremely short-period orbits. Somewhere in between, warm Jupiters orbit their host stars with periods from 10 to 365 days.

These giant planets have proven to be deeply entwined with the makeup of their host systems, and their orbital properties have been used to test and trace planet formation pathways. For example, long-period cold Jupiters discovered in radial velocity surveys have exhibited a positive correlation between planetary mass and orbital eccentricity. This mass–eccentricity relationship, as previous studies have suggested, is likely the result of a process known as planet–planet scattering, in which dynamical interactions between planets in the same system cause significant and chaotic changes to their orbits.

Does this relationship extend to shorter-period giant planets as well? Hot Jupiters are heavily impacted by tidal interactions with their host stars, circularizing their orbits and erasing dynamical imprints on their eccentricities. Warm Jupiters, on the other hand, orbit far enough away for their eccentricities to preserve clues about their energetic histories. Recently, thanks to the Transiting Exoplanet Survey Satellite (TESS), astronomers have identified a positive correlation between planetary mass and eccentricity in a growing sample of warm Jupiters, but the exact driver of this trend has yet to be uncovered.

Planets Scattering Planets

To test if the positive mass–eccentricity correlation in warm Jupiters is consistent with planet–planet scattering, Jiayin Dong (University of Illinois at Urbana-Champaign) constructed a sample of 85 warm Jupiters from the NASA Exoplanet Archive. This carefully selected sample, dominated by discoveries from TESS and Kepler, included planets with masses between 0.3 and 15 Jupiter masses and orbital periods between 10 and 365 days. In conjunction with this sample, the authors built a planet–planet scattering framework to assess the range of properties anticipated for scattered warm Jupiter systems.

Comparing the observed trends in their sample with their theory-based expectations, Dong and team showed that planet–planet scattering naturally explains the mass–eccentricity relation of warm Jupiters. In their framework, warm Jupiters originate in compact, multiplanet systems, and as the systems evolve, gravitational interactions between planets excite orbital eccentricities and inclinations, leading to close encounters. These encounters can cause planets to collide or to be ejected from the system entirely.

The mass–eccentricity relation of warm Jupiters, the authors suggested, represents a transition from collision-dominated interactions among low-mass planets to ejection-dominated interactions among high-mass planets. For massive enough systems, scattering can excite eccentricities high enough to trigger the high-eccentricity migration responsible for moving hot Jupiters to their close-in orbits. Overall, this study suggests that warm Jupiters are not a distinct planet species, but instead may represent the shorter-period counterparts of cold Jupiters whose orbits are shaped by the same dynamical processes. Future observations expanding the sample of warm Jupiters and discoveries of additional planet companions will further uncover the links between hot, warm, and cold Jupiter systems.

Citation

“Planet–Planet Scattering Explains the Mass–Eccentricity Relation of Warm Jupiters,” Jiayin Dong et al 2026 ApJL 1007 L29. doi:10.3847/2041-8213/ae930e

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Title: The Gap–Giant Association: Are Planets Hiding in the Gaps?
Authors: Caleb Lammers and Joshua Winn
Authors’ Institutions: Princeton University
Status: Published in ApJ

The Kepler space telescope played an almost decade-long game of hide-and-seek. After nine and a half years of operation, Kepler detected more than 2,700 planets outside of our solar system primarily by using the transit method, which is particularly efficient at detecting planets that orbit close to their host stars. Astronomers have also been detecting planets with the radial velocity technique, which is better at finding larger planets at longer orbital periods. By looking for planets with both the transit and radial velocity techniques, astronomers can look for both close-in and far-out planets to paint a more complete picture of other solar systems in our galaxy.

Using the Kepler Giant Planet Survey, the authors of today’s article found that among the 26 systems with three or more transiting planets, four were found to also have an outer giant planet. They quickly noticed that instead of an evenly spaced out inner system of planets, every single one of these four systems with the outer giant planets revealed a notable gap between two of their inner planets. They dubbed this phenomenon the “gap–giant association” — in other words, when an outer giant planet exists in a planetary system, there is evidence of a large gap between two of the inner planets, as shown in Figure 1. This pattern has been observed before, but there has been little theoretical work to explain the effects of outer giants on orbital spacings. In today’s article, the authors ask the question: are there planets inside the gap hiding from us? To address this, they conduct simulations to see if (a) these systems can host a hiding planet in their gaps without becoming dynamically unstable and (b) if such a planet can remain hidden when looked for with the transit method.

Close Your Eyes and Count to N

The authors of today’s article model these four systems (Kepler-48, Kepler-65, Kepler-90, and Kepler-139) with a planet added to their gaps and use an N-body simulation (i.e., fancy physics calculations that track how planets gravitationally interact) to evolve the systems over time and evaluate their long-term stability. They find that each of these injected planets has a high survival rate, which means that each of these four systems could stably host an additional ~2–20 Earth-mass planet in its gap for billions of years without falling apart. The authors then calculated whether the outer giants could “hide” these theoretical gap planets by tilting the planets’ orbits enough so that they would not be detectable with the transit method. In order to do this, the outer giant would have to exert a large enough gravitational force on the gap planet to cause it to precess at a rate independent of its neighbors, allowing its orbital inclination to grow. However, they found that the outer giant planets are either too far away or not massive enough to do the job.

Only one system, Kepler-90, has an outer giant that could potentially tilt the gap planet’s orbit enough for it to fly under the radar if the giant were also on a modestly inclined orbit. However, previous detections of Kepler-90’s outer giant suggest that its orbit is well-aligned with those of the inner system, making this hypothesis implausible. They therefore concluded that it is unlikely that planets between ~2 and 20 Earth masses are hiding within these gaps.

Seeking Out Another Option

Before completely abandoning this hypothesis, the authors propose that maybe the gaps do contain planets, but they’re just too small for Kepler to detect. They calculate the detection efficiencies as a function of planet radius and orbital period for each of the four systems and find that planets smaller than about ~0.5–1 Earth radius could have gone unnoticed (as shown in Figure 2). While this would make these systems slightly unusual (most multiplanetary systems have relatively uniform planet sizes), it’s not completely impossible. However, they don’t rule out the possibility that these gaps are truly empty, and that the presence of an outer giant could prevent planets from forming in certain regions or disrupt their orbits after formation. While it is hard to draw any firm conclusions with a sample size of four systems, this study highlights how much we still don’t understand about planetary system architecture, and therefore planetary formation and evolution.

Though the Kepler mission has come to an end, ongoing radial velocity surveys will continue to expand the sample of systems like these four so astronomers can get a clearer picture of what’s behind this gap–giant association.

Original astrobite edited by Annelia Anderson.

About the author, Tori Bonidie:

I am a 5th-year PhD candidate studying exoplanet atmospheres at the University of Pittsburgh. Prior to this, I earned my BA in astrophysics at Franklin and Marshall College where I worked on pulsar detection as a member of NANOGrav. In my free time you can find me cooking, napping with my cat, or reading STEMinist romcoms!

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