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Title: Dynamics of Planetary Rings Under Thermal Forces
Authors: Wen-Han Zhou et al.
First Author’s Institution: The University of Tokyo
Status: Published in ApJL

If you ask anyone what their favourite planet is, the answer you’ll most likely hear is Saturn. Why? Why else than the beautiful and intricate ring system surrounding the gas giant. The other gas and ice giants in our solar system — Jupiter, Uranus, and Neptune — have ring systems themselves but none quite as striking as Saturn’s. Would it surprise you to learn that astronomers’ best models have not yet totally explained why Saturn’s rings look the way they do?

In contrast, it may not surprise you to hear that people have been trying to explain the rings for as long as we have seen them with telescopes. We now know that planetary rings are collections of relatively small particles (think micrometre up to metre sized), most likely having once been the material of a larger body that was disrupted either by collisions or tidal forces. This material, due to the gravity of its local planet, is sculpted into a flat disc where more subtle interactions then lead to substructure forming within the disc such as gaps and ringlets. Many of these substructures are explained by well-understood physics — for example gaps being carved by embedded moonlets or resonances sculpting ring edges — though there remain some outstanding problems in our understanding.

The authors of today’s article set their sights on the problematic inner edge of Saturn’s A ring (Figure 1). They describe some mechanisms — namely the collisions of micrometeoroids within the rings — by which a sharp ring edge can be maintained, but there exists a gap in the understanding of how such an edge can form in the first place. All hope is not lost, though, as today’s authors reintroduce a physical process they call the “eclipse–Yarkovsky” effect, which seems to explain these phenomena.

Billions of Rocket-Powered Bumper Cars

The idea behind this process revolves entirely around light. When sunlight hits a particle within a planetary ring, non-absorbed sunlight imparts a little “bump” onto a particle, ever so slightly altering its trajectory. At the same time, some of the sunlight is absorbed into the particle, which heats it up; light is soon re-emitted via thermal radiation, which, again, can slightly change the trajectory of these tiny particles. From a point on the surface of each ring particle (see rightmost side of Figure 2), the photons from this thermal radiation are all emitted in random directions; however, across the whole surface of the particle there is a net force! These tiny particles are spinning, and so the side just near “sunset” is hottest and emitting the most thermal photons. In this way, the pressure from the sunlight plus the thermal radiation of each particle imparts a net torque on the ring itself which changes the angular momentum of the ring.

What we described just then is essentially solar radiation pressure plus the Yarkovsky effect. It turns out that the net force from this process typically averages out to zero as the ring particles orbit around the planet. This assumes, though, that the sunlight is constantly shining on the ring. Stunning imagery of Saturn tells us that this isn’t the case, so what happens when we take into account the shadow cast on the ring from the planet? Today’s authors find that the net effect induces a positive change in angular momentum of the ring particles, which they call the eclipse–Yarkovsky effect.

After today’s authors detailed all of the math involved in this process (and there is a lot), they put it into practice to try to help explain Saturn’s curious rings. Including the eclipse–Yarkovsky effect together with other known effects that drive ring evolution allowed them to reproduce the optical depth profile (how thick the ring looks) of Saturn’s A ring better than ever before (notably the sharp inner edge in Figure 3). On top of this, the effect provides another avenue for moonlet formation in the outer edges of ring systems as the positive torque from the effect drives material out toward and away from the Roche limit.

Undeniably useful for Saturn, the eclipse–Yarkovsky effect may also explain some other conundra in the solar system. Mars may have once had rings, which are thought to have eventually clumped together to form the inner moon Phobos. This idea is problematic, though, in that current models suggest there should still be some residual ring system around Mars even after Phobos formed. Enter the eclipse–Yarkovsky effect: being 100 times stronger for Mars than Saturn (due to its more intense incident sunlight), the effect may have driven out and dispersed that residual ring altogether. While the authors are currently looking into this possibility, the reintroduction of the eclipse–Yarkovsky effect has already shown great promise for our most beautiful ringed planetary neighbour.

Original astrobite edited by Wasi Naqvi.

About the author, Ryan White:

I am a first-year PhD student at Macquarie University in Australia, working mainly on binary/multiple systems with massive stars (Wolf–Rayets in particular!). Outside of study, I’m probably drinking coffee, baking, reading, or going for a run. You can also find me procrastinating on Bluesky @astroryan.bsky.social.

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Each Jupiter-size planet in the galaxy falls into one of three distinct categories: hot, warm, or cold. A new study suggests that despite the apparent differences between these populations, they may have all formed from the same underlying dynamical process: a game of pool played at planetary scales.

A Diversity of Jupiters

Though our solar system has only one Jupiter-size planet, elsewhere in the galaxy we have found three different species of these massive gas giants. Cold Jupiters closely resemble their namesake and orbit far from their host stars; hot Jupiters are the opposite and are found whipping around their stars on extremely close-in orbits. In between these are the warm Jupiters, which tend to orbit in the intermediate space between 0.1 and 1.0 au.

Though these three populations are defined by their orbital distances, they differ from each other in other ways as well. For example, hot Jupiters almost never have nearby companions; if there are any other planets circling the same star, they’re usually far-out cold Jupiters. They can also orbit in pretty much any direction, including opposite the direction of their star’s spin, and are usually on perfectly circular orbits. In contrast, warm Jupiters often have friends nearby, are much more aligned with their stars’ spins, and can have modest eccentricities.

Given these differences, it’s often thought that each of these populations arrived at its current location through different dynamical processes and that the history of the warm Jupiters is likely quite different from the history of the hot Jupiters. However, a new study led by Julia Esposito (Georgia Institute of Technology) has proposed an alternative view. Maybe these populations, though they appear different now, were all created by the same process: planet–planet scattering.

Virtual Planetary Billiards

Esposito and collaborators set up 1,500 virtual planetary systems with three massive planets each, then simulated how the orbits evolved before probing the final configurations. In crucial contrast to previous simulation studies, the team initialized their Jupiters across a range of different distances and included the effects of tides sapping energy from orbits of planets that got too close to their host stars.

At the end of the simulation, the team surveyed the digital carnage. Almost every virtual system ended with only two planets after either ejecting one away from the star or having two planets collide. But, remarkably, the remaining two-planet systems looked tantalizingly similar to what we actually observe, with a mix of hot, warm, and cold Jupiters. Even more exciting, the end populations were highly correlated to where the violent scattering event took place.

For example, the warm Jupiters were almost all produced by “warm scattering” simulations, where the scattering took place between 0.1 and 1.0 au. The planets that survived the simulations and ended up as warm Jupiters matched all of the properties of the real warm Jupiter population: they had nearby companions, were moderately eccentric, and were mostly aligned with their stars. The hot Jupiters, meanwhile, were almost all produced by “cold scattering” events where the flybys happened far from the star and resulted in one planet hurtling inwards. These also matched all of the observed properties of real hot Jupiters.

The researchers concluded that planet–planet scattering can produce both the warm and hot Jupiter populations so long as you let the planets scatter from a variety of different distances. This exciting theoretical insight, if correct, would mean that astronomers could stop searching for different pathways to create each population. Happily, this model also provides testable predictions, and the authors lay out how the theory could be supported or disproven with additional data. Through virtual experiments like these, astronomers continue to build up an understanding of how the wide range of planetary architectures observed across the galaxy came to be.

Citation

“Unified Formation Channel of Hot and Warm Jupiters via Planet–Planet Scattering,” Julia Esposito et al 2026 ApJL 1003 L3. doi:10.3847/2041-8213/ae61b0

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What would happen to a gaseous exoplanet if it was struck by a protoplanet? Recent research suggests that it would ring like a bell, and that just maybe, we’d be able to observe this ringing back on Earth.

Heavy Metal Mystery

Previous studies have hinted that giant planets have more heavy elements in their cores than we’d expect if they had formed from the exact same stew of gas and dust as their host stars. How exactly they end up with these enriched compositions is something of a mystery, though. Astronomers have proposed two potential theories: one somewhat dull, and one that involves dramatic collisions between the massive protoplanets.

In the first case, perhaps growing giant planets clear out the gas along their orbits, then steadily and quietly accrete the dust that spills over into the gaps. In the latter case, theorists have conjured a model in which growing gas giant planets repeatedly collide and merge with large and somewhat rocky protoplanets. Both of these mechanisms would produce the enhanced heavy element contents observed today, so how can we tell these two paths apart? Recent work by a team led by J.J. Zanazzi (University of California, Berkeley) proposes one such test.

Ringing Planets

Through mathematical derivations, the researchers demonstrate that if a gas giant were to collide with a protoplanet, the impact would cause the planet’s surface to physically oscillate inwards and outwards, and for its surface temperature to vary. In other words, the planet would “ring” like a bell struck with a hammer, and as it physically convulses in the aftermath of the merger it would grow brighter and fainter in a repeating, periodic pattern. In principle, if the ringing lasts for long enough and the oscillations in brightness were large enough, observers back on Earth could record the amplitude and period of the changes and back out some constraints on what had happened and when.

Excitingly, the researchers demonstrate that not only would the oscillations last for millions of years, they would also be bright enough for JWST to detect for one specific planet named Beta Pictoris b. Should this world have undergone a merger in the last 18 million years, just a few hours of observations with JWST’s Near-Infrared Camera would reveal 45-minute pulsations in which the planet’s brightness changes by about 1%. Since the planet is too far away from its host star for these oscillations to be caused by any gravitational interactions, detecting any periodic signal would be a slam-dunk for the major merger theory.

Beta Pictoris b is a young planet in a system that’s still forming, and it orbits its host star on an eccentric path that takes it through dense regions of the disk. Hopefully, future observations will test whether this treacherous path produced any massive collisions in the relatively recent past. Until then, though, we can all enjoy charming visions of massive planets colliding with one another and ringing through the night.

Citation

“Seismic Oscillations Excited by Giant Impacts in Directly Imaged Giant Planets,” J. J. Zanazzi et al 2025 ApJ 993 3. doi:10.3847/1538-4357/ae04ec

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By pushing JWST to its limits, astronomers may have just spotted a planet around the nearest Sun-like star, Alpha Centauri A. Though another look at the star is needed next year to confirm the finding, if the detection holds up, this discovery easily earns several superlatives: the planet would be the nearest, oldest, coldest, and lowest-mass world ever directly imaged beyond our solar system.

Hi, Neighbor!

The scale of our Milky Way galaxy is hard to comprehend with our Earth-bound minds that are more attuned to measuring distances in miles than in light-years. However, even though our suburban stellar neighborhood is tens of thousands of light-years from the galactic downtown, it is not empty: there are a handful of nearby systems that offer unique opportunities to study stars and their potential planet children “up close.”

The nearest star system to the Sun at just over 4 light-years away is called Alpha Centauri. It’s a star “system” since what appears to the naked eye to be one star is actually three objects orbiting each other: Alpha Centauri A and Alpha Centauri B are two Sun-like stars that circle each other every 80 years or so, while Alpha Centauri C, better known as Proxima Centauri, circles the inner pair on a much wider orbit.

Though Proxima Centauri is the most famous of the trio, since it’s technically the closest star to the Sun, its larger siblings are arguably better targets for searching for planets. Since these stars are more massive, their habitable zones are large enough that astronomers can block out the star itself and search the surrounding area for planets using a tool called a coronagraph.

Challenging Observations

Back in 2019, a team of astronomers using a coronagraph on the Very Large Telescope uncovered tantalizing evidence of a giant planet orbiting Alpha Centauri A. However, confirming or rejecting this initial peek using telescopes on Earth’s surface was challenging. When JWST finally launched in 2021, following up on this hint was a top priority. Astronomers first attempted to point JWST toward the Alpha Centauri system in 2023, but unfortunately, the observations from that attempt were unusable.

Though one might think aiming a telescope at a nearby star is easier than taking a look at something more distant, this is unfortunately not the case. For one thing, the stars of the Alpha Centauri system are bright enough to overwhelm JWST’s sensitive detectors without careful calibration. For another, keeping the telescope locked on target and the star behind the coronagraph requires continuous tweaks to account for the Sun’s motion relative to Alpha Centauri, JWST’s motion relative to the Sun, and Alpha Centauri A’s motion relative to Alpha Centauri B. The team regrouped, tested a new observing strategy in mid-2024, then took three long observations in August 2024, February 2025, and April 2025.

A Planet Candidate

This time, the observations went off nearly without a hitch. After careful data processing, the team led by Charles Beichman and Aniket Sanghi of the California Institute of Technology found a bright blob positioned right beside the star in their August data. Interestingly, the blob disappeared in the next two observations. Working on the hunch that this blob was the same object as the planet candidate seen by the Very Large Telescope, the team realized that most allowed orbits would place the planet too close to the star to be visible in those later observations. In other words, the planet might be playing hide-and-seek, peeking out from behind the star in one month only to vanish in the next.

The authors estimated that the source is a 1–1.1 Jupiter radius planet with a mass of 90–150 Earth masses (0.28–0.47 Jupiter mass) on a 2–3-year orbit. The team stressed that with just one detection, this should still be regarded as a planet candidate, not a confirmed discovery. If the planet is eventually verified, however, it stands out among its peers as the first mature, cold planet ever imaged within its star’s habitable zone. Future detectors may be able to collect spectra of the planet itself, unlocking new insights into giant planet evolution and composition in multiple-star systems with nested orbits, like the Alpha Centauri system.

Now that JWST has a proven strategy for observing this challenging system, it should be possible to take follow-up images. If the planet is real and traveling on its expected orbit, it should reappear beside Alpha Centauri A in August 2026. Hopefully, JWST will be looking when it does, and we can finally confirm that we have a planetary neighbor next door.

Citation

“Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of α Cen A. I. Observations, Orbital and Physical Properties, and Exozodi Upper Limits,” Charles Beichman et al 2025 ApJL 989 L22. doi:10.3847/2041-8213/adf53f

“Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of α Cen A. II. Binary Star Modeling, Planet and Exozodi Search, and Sensitivity Analysis,” Aniket Sanghi et al 2025 ApJL 989 L23. doi:10.3847/2041-8213/adf53e

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