Wen-Han Zhou

Wen-Han Zhou

postdoc@UTokyo

Solar system small bodies

My research includes:

Eclipse-Yarkovsky effect on planetary rings (Zhou et al., 2026a)

Planetary rings provide natural laboratories for studying the fundamental processes that govern the evolution of planetary systems. However, several key features, such as the sharp inner edges of Saturn’s rings remain unresolved. In this work, we introduce and quantify the Eclipse–Yarkovsky (EY) effect, a thermal torque arising from asymmetric thermal emission of particles during planetary eclipses, which is effective for particles larger than millimeters in size. We formulate this effect within a continuum framework appropriate for collisionally coupled planetary rings and derive the continuum evolution equation that includes the EY torque and viscous diffusion (Eq.26), constraining its magnitude using ring particle spin distributions obtained from $N$‑body simulations.

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Schematics of eclipse Yarkovsky effect on a planetary ring.

We find that the EY effect systematically produces a positive angular momentum flux that could overcome the viscous torque, driving ring material outward and leading to long‑term decretion. The total EY torque principally depends on the optical depth, in which we identify three dynamical regimes: dense, transitional, and tenuous regimes, each exhibiting distinct evolutionary pathways. In the dense or transition regimes, the EY torque can produce a sharp inner edge such as that of Saturn’s A ring. In the tenuous regime, it can drive an entire ring outward while preserving shape. This outward transport may also facilitate satellite formation beyond the Roche limit. We also quantitatively show that planetary thermal radiation exerts an opposing torque, namely planetary-Yarkovsky effect, whose importance depends on planetary emissivity and ring-particle albedo, and may lead to inward transport in Saturn’s close-in rings.

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Saturn's A ring evolution with our model with only the viscous and EY effect included, in comparison with observation data. The evolutionary time is 81 Myrs.

Binary Yarkovsky effect on binary asteroids (Zhou et al., 2024a; Zhou, 2024)

We discovered how the Yarkovsky effect affects the binary asteroids.

(1) The eclipse-induced thermal perturbation produces a net thermal force that moves the component towards the synchronous orbit where the spin period equals the orbital period.

(2) The radiation from the other component produces a net thermal force moves the component towards the opposite direction of the synchronous orbit.

For nearly co-planar binaries, for which the mutual orbit aligns with the heliocentric orbit, the (1) always dominates over (2). We propose this effect could account for the synchronization process of small binary asteroids in main belt asteroids and near Earth asteroids.

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Schematics of binary Yarkovsky effect.
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The temperature distribution of binary asteroids.

With this newly discovered effect, more discoveries on binary asteroid dynamics to come soon…

Crater-induced YORP (CYORP) effect on main belt asteroids (Zhou et al. 2022; Zhou & Michel 2024)

We developed a semi-analytical method to calculate the temperature distribution of a crater and the produced thermal torque, namely the CYORP torque. We found that roughly speaking, a crater with the size 1/3 of the asteroid could produce a CYORP torque comparable to the YORP torque. Based on this tool, we can estimate the YORP torque change brought by a sub-catastrophic impact, and study the asteroid rotational evolution under collisions and YORP.

Image 1
Temperature of Bennu's surface over a day (numerical).
Image 2
Temperature of a crater over a day (analytical).
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Evolution of the spin rate (left panel) and the obliquity (right panel) of a 10 km synthetic asteroid. In the presence of the static YORP torque (blue line), the asteroid gradually decelerates until it reaches a quasi-non-rotational state. Subsequently, we impose a new rotational state on the asteroid by assigning random values of rotational speed and obliquity. Conversely, when incorporating the CYORP torque (red line), the asteroid follows a different path, exhibiting random fluctuations in its spin rate due to the occurrence of impacts, creating new craters that lead to changes in the CYORP torque. As a result, the asteroid experiences intermittent transitions between spin up and spin down.

With CYORP, it’s time to estimate the stochastic YORP and Yarkovsky effect…

Rotational evolution of main belt asteroids (Zhou et al., 2025, Nat. Astro.)

There are three mysteries regarding the slowly rotating asteroids in the main belt:

  1. Excess of slow rotators. The overabundance of asteroids rotating slowly was realized about 40 years ago. Collisions typically produce a Maxwell distribution in the spin rate and the classic YORP theory produces a uniform distribution. The existing theory cannot explain the excess of slow rotators.

  2. Gap. Recent Gaia observation reveals a visible gap in the period-diameter distribution of asteroids, which separates the slow rotators from faster rotators. Classic theory only predicts smooth distributions without any kind of gap.

  3. Tumbler distribution. The distribution of tumbling asteroids in the period-diameter is not well explained.

This paper attempted to solve the above three problems by constructing a novel comprehensive rotation evolution model and fitting it to Gaia observation.

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Period-diameter distribution of main belt asteroids from Gaia data (left) and simulation (right).