Research | Xuan Ji

Atmospheric Retention

Which planets have atmosphere and why?

The Cosmic Shoreline Informed by Stellar Evolution and XUV-driven Hydrodynamic Escape

Cosmic shoreline under stellar evolution and hydrodynamic escape

Atmospheric escape responds nonlinearly to XUV flux because of the rich heating and cooling processes involved in the upper atmosphere. Then how does the cosmic shoreline change with more realistic escape physics? Our updated shoreline incorporate:

Stellar bolometric evolution.
Stellar X-ray and EUV evolution .
Hydrodynamic escape results for different atmospheric compositions: CO₂: ; N₂: and ; and H₂O:

Our results suggest that more massive rocky planets are more resistant to atmospheric loss than predicted by the traditional cosmic shoreline, with encouraging implications for 55 Cancri e and other super-Earths, and motivate specific additional hydrodynamic simulations of secondary atmospheres.

Puffy Planets: Trend of Ratioed Density Shaped by Cosmic Shoreline

A thick atmosphere inflates a planet's transit radius, lowering its measured density. By normalizing observed densities against an Earth-like composition model (the ratioed density 蟻/蟻鈯�), we can test the cosmic shoreline statistically: highly irradiated, low-mass planets lose their atmospheres and approach bare-rock densities, while larger, less-irradiated planets retain atmospheres and appear less dense. Monte Carlo simulations confirm that this trend: ratioed density increasing with instellation and decreasing with planetary mass, which traces the shoreline's shape. Detecting it at 90% confidence with a sample of ~1000 planets requires reducing radius measurement uncertainties to ~1% of today's values.

Atmosphere Erosion by Impacts

Placeholder figure for impact erosion model framework

The atmospheric erosion process in this paper includes: (1) cumulative planetesimal impacts over each N-body output interval, and (2) discrete giant impacts. For planetesimals, I sample a size distribution that matches the total delivered mass in each interval, convert to impactor masses, and compute loss per impact with the prescription from . For giant impacts, I use a prescription from .

You can use this tool to calculate the atmospheric loss:

Climate under Orbital Forcing

How climate evolves with evolving orbits.

Inner Habitable Zone (IHZ) as a Function of Eccentricity

Orbital-Mean Stellar Flux Approximation Holds for Earth-like Planets

For planets with strong seasonal variations in incoming stellar flux, our scaling analysis shows that, with a 10-m ocean, the surface temperature responds slowly enough that it changes little over an orbit. As a result, the climate is mainly set by the orbit-averaged incoming energy. This gives a simple way to estimate whether the planet stays below the runaway greenhouse limit (the upper solid black line in the left figure):

S_a/S^* <(1 - e²)^1/2

where S_a is stellar flux at semi-major axis a, S^* is the critical stellar flux for a circular orbit.

3D climate extension of IHZ under high eccentricity

Summer Cloud Loss at High Eccentricity Warms the Planet

ExoCAM 3D GCM results show that the orbital-mean flux approximation works well up to about e=0.3. At lower stellar flux, some adjustment is needed because sea ice increases the albedo. For the more extreme case of e=0.6 (yellow line), the surface temperature is noticeably higher, which we trace to a lower annual-mean albedo. The reason is the loss of summer cloud cover: strong shortwave absorption heats the atmosphere, shuts down convection, and reduces clouds, allowing the planet to absorb more incoming radiation.

Ji and Abbot (submitted to PSJ)

Snowball Bistabiity Disappear with e>0.2

On Earth, Snowball episodes may help with rising oxygen and complex life. We show that for exoplanets on eccentric orbits, the climate can lose the usual back-and-forth between a fully frozen Snowball state and a warmer state with open ocean: once seasonality becomes strong enough, summer melting outpaces winter ice growth, so the Snowball state disappears more easily than we used to think.

Clouds and Seasonality with Varying Rotation Rates

Cloud seasonality under varying rotation rates

As the rotation rate changes, clouds produce a local minimum in planetary albedo at 1/8 of Earth鈥檚 rotation rate. This appears to be a transition point: below it, the planet behaves like an 鈥渁ll tropics鈥� world, with ITCZ clouds migrating nearly to the poles. Because the cloud response is not monotonic, surface temperature and precipitation also peak near 1/8 of Earth鈥檚 rotation rate. In addition, the seasonal transition from summer toward winter becomes later at faster rotation, and clouds make this seasonal lag even stronger.

Orbital Evolution

How system architecture sets long-term orbital evolution.

Sweeping Secular Resonances in Cold-Jupiter鈥揝uper-Earth Systems

In planetary systems containing both cold Jupiters and super-Earths, with the disk dissipating, sweeping secular resonances (SSR) can excite inner embryos, trigger collisions, and shape rocky-planet assembly. I also built an analytical approximation tool to diagnose where and when SSR occurs.

[Ji, Zheng, Lin (in prep)]

桔子视频

How planets evolve into habitable worlds.